Environmental chemistry (notes)12IEEM BY Muhammad Fahad Ansari 12IEEM14

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Muhammad Fahad Ansari 12IEEM14

Muhammad Fahad Ansari 12IEEM14

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  • 1. Environmental Chemistry 1 Environmental ChemistryWhat is Chemistry?The science that deals with matter; its composition; its properties; the changes in composition that itwill undergo; its relationship to energy; and the laws, principles, theories, and concepts that describe,interpret, and predict its behavior and basic nature.Periodic TableMetals: They are good conductors of electricity; they contain only one or two weakly electrons in theirouter energy levels.Non-metals: They have relatively strong attraction for the electrons in their outer energy levels.Metalloids: The elements whose properties are intermediate between those of the metallic and non-metallic elements.Bonding between Elements:Ionic Bond: The electrostatic attraction that exists between positive ions formed when one atom loseselectrons and negative ions formed by the atoms that gain the electrons.Covalent Bond (Coordinate Covalent Bond): A bond between two atoms in which they are overlappingorbitals and sharing a pair of electrons, both of which originally belonged to only one of the atoms.The Polar Covalent BondWhen, in a covalent bond, certain atoms have a partial positive charge and others have a partialnegative charge, we say that the covalent bond is polar. This type of bond is called a polar covalentbond. (One atom “hogs” the electrons; the atom that “hogs” is the one with greater electronegativity).The Metallic BondA metallic bond results when two metals bond. In metallic bonding, the metal atoms donate valenceelectrons to become cations. These valence electrons are not directly transferred to another atom asthey are in ionic bonding. Instead, they move about freely throughout the sample, producing anattractive force that keeps the metal cations in place. Often, the behavior of these electrons is referredto as a “sea of mobile electrons.” Because of the motions of the free electrons, metals arecharacteristically good conductors of electricity and heat.• Ionic bond with: exchange of electrons to form stable octet; bond between a metal and nonmetal; example: NaCl, CaF2• Covalent bond with: sharing of electrons to form stable octet; bond between nonmetals; example: Cl2, N2• Polar covalent bond with: unequal sharing of electrons to form stable octet; example: H 2O, NH3• Metallic bond with: sea of mobile electrons; bond between metals; example: Cu, AgMuhammad Fahad Ansari 12IEEM14
  • 2. Environmental Chemistry 2Molecules can also be polar or Non-polar• Any diatomic molecule that has a non-polar bond is non-polar. Example: All diatomic molecules: Cl 2, N2, O2, etc… Otherwise, it will generally be polar.• Molecules that consist of 3 or more atoms are generally polar unless the following condition is met: if the central atom has no lone pairs and is surrounded by atoms of one element, then the molecule will be non-polar, for example, CO2. In these cases, individual bond polarities cancel each other.Electro-negativity: (Table)• The bonds between two elements can be purely ionic, purely covalent, or anywhere in between depending on the elements involved.• The electro-negativity expresses the power of an atom in a molecule to attract shared electrons.• The electro-negativity values come from measurements of the strength of bonds between atoms and from measurements of the amount of electrical energy required to remove an electron from an atom of an element.• Fluorine (F) which has the strongest attraction for electrons of all the reactive elements is assigned an electro-negativity value of 4. All other elements are compared to fluorine.• According to Pauling (1901), if the difference between the electro-negativity values of the two elements is 2.0 or more, the bond is mainly ionic. If the electro-negativity difference is less than 2.0, the bond is mainly covalent.Organic and inorganic compoundsThe distinction between the compounds found in the earth associated with rocks and soil and thoseassociated with living organisms.The two groups of compounds differ in their occurrence and in many of their properties. Thesedifferences provided the basis for one of the earliest efforts to classify compounds.The compounds associated with living organisms called organic compounds, and the earthy compoundsare called inorganic compounds.Organic chemistry is simply the chemistry of the compounds of the clement carbon. Except the carbon-containing compounds such as carbon dioxide, the carbonates, bicarbonates, and carbides are notconsidered to be organic compounds.In other words the properties of organic compounds are largely a result of the covalent bond, and thatinorganic compounds, which have different properties, get their characteristics from the ionic bond.Table- Comparative Properties of Ionic and Covalent Compounds Inorganic (Ionic) Compounds Organic (Covalent) CompoundsMuhammad Fahad Ansari 12IEEM14
  • 3. Environmental Chemistry 3 1 Solids, often crystalline, relatively May be solid, liquid, or gas, depending on the size of the hard and brittle molecules and the degree of intermolecular attraction (polarity and ability to form hydrogen bonds) 2 Relatively high melting and boiling Relatively low melting and boiling points, depending points primarily on the size of the molecules and the amount of intermolecular attraction 3 Relatively soluble in water and Relatively insoluble in water and soluble in organic liquids; insoluble in organic (non polar) polarity and ability to form hydrogen bonds greatly liquids increases water solubility 4 Electrolytes-solutions conduct Most are non electrolytes electricity 5 Relatively stable to heat Relatively unstable to heatBiochemistry: The chemistry that deals with the substances and reaction in living organisms.Electrochemistry: the chemistry dealing with the effects of electricity on chemical reactions and theproduction of electricity through chemical processes. Chemical reactions involve an electron exchangebetween the reactants. These reactions are called oxidation-reduction reactions or electrochemicalreactions. There are two distinctly different classes of electrochemical reactions: those that produceelectrical energy and those that are produced by electrical energy.Chemical ReactionsWhen a chemical reaction occurs, at least one product is formed that is different from thesubstances present before the change occurred. As an example, it is possible to pass an electriccurrent through a sample of water and obtain a mixture of oxygen and hydrogen gases. Thatchange is a chemical reaction because neither oxygen nor hydrogen was present as elementsbefore the change took place.Any chemical change involves two sets of substances: reactants and products. A reactant is anelement or compound present before a chemical change takes place. In the example above,only one reactant was present: water. A product is an element or compound formed as a resultof the chemical reaction. In the preceding example, both hydrogen and oxygen are products ofthe reaction.Chemical reactions are represented by means of chemical equations. A chemical equation is asymbolic statement that represents the changes that occur during a chemical reaction. Thestatement consists of the symbols of the elements and the formulas of the products andreactants, along with other symbols that represent certain conditions present in the reaction.For example, the arrow (or yields) sign, *, separates the reactants from the products in areaction. The chemical equation that represents the electrolysis of water is 2H 2O → 2H2 + O2.Types of Chemical ReactionsMuhammad Fahad Ansari 12IEEM14
  • 4. Environmental Chemistry 4Most chemical reactions can be categorized into one of about five general types: synthesis,decomposition, single replacement, double replacement, and oxidation-reduction. Amiscellaneous category is also needed for reactions that do not fit into one of these fivecategories.Characteristics of each typeSynthesis: Two substances combine to form one new substance:In general: A + B → ABFor example:2 Na + Cl 2 → 2 NaCl or CaO + H 2 O → Ca(OH) 2Decomposition: One substance breaks down to form two new substances:In general: AB → A + BFor example: 2 H 2 O → 2 H 2 + O 2Single Replacement: An element and a compound react such that the element replaces oneother element in the compound:In general: A + BC → AC + BFor example: Mg + 2 HCl → MgCl 2 + H 2Double Replacement: Two compounds react with each other in such a way that they exchangepartners with each other:In general: AB + CD → AD + CBFor example:NaBr + HCl → NaCl + HBrOxidation-reduction: One or more elements in the reaction changes its oxidation state duringthe reaction: In general: A 3+ → A 6+For example: Cr 3+ → Cr 6+Energy changes and chemical kineticsChemical reactions are typically accompanied by energy changes. The equation for thesynthesis of ammonia from its elements is N 2 + 3 H 2 → 2 NH 3 , but that reaction takes placeonly under very special conditions—namely at a high temperature and pressure and in thepresence of a catalyst. Energy changes that occur during chemical reactions are the subject of afield of science known as thermodynamics.In addition, chemical reactions are often a good deal more complex than a chemical equationmight lead one to believe. For example, one can write the equation for the synthesis ofhydrogen iodide from its elements, as follows: H 2 + I 2 → 2 HI. In fact, chemists know that thisreaction does not take place in a single step. Instead, it occurs in a series of reactions in whichhydrogen and iodine atoms react with each other one at a time. The final equation, H 2 + I 2 → 2Muhammad Fahad Ansari 12IEEM14
  • 5. Environmental Chemistry 5HI, is actually no more than a summary of the net result of all those reactions. The field ofchemistry that deals with the details of chemical reactions is known as chemical kinetics.Reaction of sulfuric acid and sugar. The acid dehydrates the sugar forming a pillar of carbon (black) andsteam.Reaction Rates and Chemical EquilibriumThere are few questions related to factors that influence reaction rates • Why must substances such as gasoline be heated before they will start burning? • To what does the term catalytic refer in the catalytic conversion process used in auto emission control systems? • Why do wood shavings burn more rapidly than pieces of wood?The answers to these and other questions involve the rate of chemical reactions.Forward and Reverse Reaction:Ammonia (NH3) manufactured as a fertilizerN2 + 3H2 → 2NH32NH3 → N2 + 3H2N2 + 3H2 ↔ 2NH3Reactants ProductsWhen the rate of the forward reaction equals the rate of the reverse reaction, the system is said to be inchemical equilibrium.Chemical Equilibrium: Exists when two opposing reactions are occurring simultaneously and at the samerate. The maintenance of body weight is an example of a kind of equilibrium. Reaction rates andequilibrium are interrelated.Factors Affecting the Rate of Chemical ReactionsAll the factors involved in the transformation of reactants to products in chemical changes are calledchemical kinetics. The major factors that affect the rate of reactions are the nature of the reactants,temperature, catalysts, and the reactants concentration.Muhammad Fahad Ansari 12IEEM14
  • 6. Environmental Chemistry 6Nature of the reactants: Different substances react at different rates. In general, the readiness withwhich any two substances react together depends largely on the structure of the atoms of the elementsand the nature of the bonds that hold the atoms together in the substance.Temperature: In general, an increase in temperature results in an increase in rate of reaction. Beforereaction can occur between two substances, the bonds already existing between the atoms of eachsubstance must be weakened and broken. An increase in temperature results in greater kinetic energy,and thus motion, of the individual atoms of each substance. The greater energy of motion causes theatoms to tend to pull away from one another. As a result, the existing bonds are weakened or broken.The energy that must be added to weakened bonds so that substances will react is called the energy ofactivation.Many reactions give off energy, the energy liberated by such reactions is usually in the form of heat.Some times radiant energy (for example, light in combustion reactions) and electrical energy may alsobe given off. Reactions that give off (evolve) heat are called exothermic reactions. All combustionreactions are exothermic reactions such as: • C + O2 → CO2 + heat • Mg + F2 → MgF2 + heat • N2 + 3H2 → 2NH3 + heatReactions that absorb heat are called endothermic reactions. The following equations are examples ofendothermic reactions: • 2KClO3 + heat → 2KCl + 3O2 • CaCO3 + heat → CaO + CO2 • 2NH3 + heat → N2 + 3H2One reaction of equilibrium will evolve heat (exothermic) and the other reaction will absorbheat (endothermic). Exothermic N2 + 3H2 -------------------→ 2NH3 + heat ←------------------ EndothermicCatalysts:A catalyst is a substance that changes the rate of a reaction without itself being used up in the process.Catalysts function in several different ways, but their overall effect is to change the reaction rate bylowering the energy of reactants activation.Muhammad Fahad Ansari 12IEEM14
  • 7. Environmental Chemistry 7Reactants Concentration:In general, increasing the concentration of reactants increases the rate of reaction. For example,substances burn more rapidly in pure oxygen than in air, which is only about 20 percent oxygen.The rate of the reaction is proportional to the product of the concentration of the reactants. But the rateof the reaction is equal to the product of the concentration of the reactants times a proportionalityconstant (k). The constant K takes into account the effect of the nature of the reactants, temperature,pressure, and catalyst on the reaction.Rate of Chemical ReactionThe rate or speed with which reactants disappear or a product appears.The rate at which the concentration of one of the reactants decreases, or one of the productsincreases with time.The decomposition of dinitrogen pentoxide (N2O5) in an inert solvent carbon tetrachloride (CCl4)A typical unit for a rate of reaction (mol per L per s)2 N2O5 (in CCl4) → 2N2O4 (in CCl4) + O2 (g)One product, O2, is a gas that is virtually insoluble in the reaction mixture.Decomposition of N2O5 (in CCl4) at 45 0C[N2O5] = 1.40 M Time, s Total volume O2, cm3 (at STP) 0 0 432 1.32 753 2.18 1116 2.89 1582 3.63 1986 4.10 2343 4.46 . . . . . . ∞a 5.93Reaction Kinetics: Reaction MechanismsTermsMuhammad Fahad Ansari 12IEEM14
  • 8. Environmental Chemistry 8Activation Energy - The difference in energy between the reactants and the transition state that is theenergy barrier the reactants must overcome to achieve a chemical reaction.Catalyst - A substance that lowers the activation energy for a chemical reaction without beingchemically altered by the reaction.Elementary Step - A reaction that represents a single collision or intramolecular step in a reactionmechanism.Homogeneous Catalyst - A catalyst that is in the same phase as the reactants.Intermediate - A species that is both produced and consumed in a chemical reaction. As such, it doesnot appear in the overall reaction but is proposed to be produced in one elementary step and consumedin another.Kinetics - The study of the rate and mechanism of chemical reactions.Mechanism - The series of elementary steps that combine to produce the path molecules take fromreactant(s) to product(s) in a chemical reaction.Order - In the rate law of a reaction, the power to which the concentration of a reagent is raised. Or,the sum of the powers on the concentration terms in the rate law.Rate - The speed of a reaction measured in amount or reagent consumed or product produced per unittime.Rate Constant - The proportionality constant in the rate law expression. This factor is a measure of theintrinsic reactivity of the reaction but is not constant with respect to temperature.Rate Law - An expression of the dependence of the rate of a reaction on the concentrations ofreactants.Rate Limiting Step - The slowest elementary step in a mechanism. The rate of the reaction must equalthe rate of the slowest step because the reaction can go no faster than its slowest step.Reaction Coordinate Diagram - A plot of free energy versus the reaction coordinate for a reaction thatprovides a pictorial representation of the lowest energy path from reactants to products.Steady-State Approximation - The assumption that the rate of formation and consumption of a highlyreactive intermediate are equal so that the change in intermediate concentration with respect to time isapproximated to be zero.Transition State - The species with the highest energy between reactants and products on a reactioncoordinate diagram, it is a short-lived species that represents a combination of product-like andreactant-like properties.Chemical MechanismsBy describing how atoms and molecules interact to generate products, mechanisms help us tounderstand how the world around us functions at a fundamental level. A mechanism is a series ofelementary steps whose sum is the overall reaction. An elementary step is a reaction that is meant torepresent a single collision or vibration that leads to a chemical change. For a mechanism to beMuhammad Fahad Ansari 12IEEM14
  • 9. Environmental Chemistry 9considered valid, its sum must equal the overall balanced equation, its predicted rate law must agreewith experimental data, and its predictions of intermediates must not be contrary to experimentalobservations. A mechanism may never be proven because we cannot ever see a chemical reaction--boththe time scale of an elementary step and the size of atoms are too small. Furthermore, we must guess atthe identity of many intermediates because they are usually so reactive that they can not be isolated.Instead, a chemist proposes reaction mechanisms and tests their validity against experimental data,ruling out mechanisms that are inconsistent with results. These experiments may be strategicallydesigned to trap an intermediate product to prove its existence as a stepping-point in the total reaction.To aid in our understanding of mechanisms, we will draw reaction coordinate diagrams that trace thefree energy path of a reaction from reactants to products. The activation energy of a reaction representsthe difference in energy between the reactants and the highest point on a reaction coordinate diagram.We will derive the Arrhenius Equation, which relates the rate constant for a reaction to its activationenergy. Local minima on the reaction coordinate diagram are positions occupied by intermediates. Bycomparing the reaction coordinate diagram for a catalyzed and a uncatalyzed process, we can see thatcatalysts function by altering the route the reaction takes from reactants to products without thecatalyst being altered.Properties of MechanismsMechanisms describe in a stepwise manner the exact collisions and events that are required for theconversion of reactants into products. Mechanisms achieve that goal by breaking up the overallbalanced chemical equation into a series of elementary steps. An elementary step is written to mean asingle collision or molecular vibration that results in a chemical reaction. The following picture of anelementary step shows a single collision between water and boron trifluoride:Muhammad Fahad Ansari 12IEEM14
  • 10. Environmental Chemistry 10 Figure %: Schematic representation of an elementary stepThe molecularity of an elementary step describes the number of reactive partners in the elementarystep. For example, the above elementary step is called bimolecular because two molecules collide.Commonly, elementary steps are mono-, bi-, or ter-molecular. The probability of four moleculescolliding at exactly the same place and time is so small that we can safely assume that no reaction willever be tetra-molecular. Because take up a large amount of space, we will represent elementary steps inthis Spark Note as normal reactions with molecular formula line equations. You will know from thecontext (i.e. talking about the steps of a mechanism) whether the reaction is an elementary step or anoverall reaction.To better understand mechanisms, lets consider the following mechanism for the decomposition ofozone, O3:The above mechanism exhibits a property of all mechanisms: it is a series of elementary steps whosesum is the overall balanced reaction. Note the presence of the oxygen atom, O, intermediate in theabove equation. It is an intermediate because it is both created and destroyed in the mechanism anddoes not appear in the net equation.Muhammad Fahad Ansari 12IEEM14
  • 11. Environmental Chemistry 11Another property of mechanisms is that they must predict the experimentally determined rate law. Tocalculate the rate law from a mechanism you need to first know the rate limiting step. The rate limitingstep determines the rate of the reaction because it is the slowest step. You can rationalize that areaction can only go so fast as its slowest step by thinking about what happens when you encounter anaccident on the highway that closes all but one lane. You may have been able to race along at 65 m.p.h.(depending on your states laws) before you reached the lane closure but the slow passage of cars pastthe accident limits your rate. You can only go as fast through that one lane as the slowest car in front ofyou.In the above, the first reaction is labeled as "slow". This reaction is the rate determining step because itis the slowest step. As we have stated, that means that the rate of the overall reaction is equal to therate of the rate determining step. The rate of an elementary step is the rate constant for that stepmultiplied by the concentrations of the reactants raised to their stoichiometric powers. Note that thisrule only applies for elementary steps. The rate of an overall reaction is NOT the product of theconcentrations of the reactants raised to their stoichiometric powers. The rate law for the firstelementary step in the is rate = k [O 3]. Because this step is the rate determining step, the rate law is alsothe rate law for the overall reaction. Using similar techniques we can calculate the rate law predicted byany mechanism. We then check the predicted rate law against the experimentally determined rate lawto test the validity of the proposed mechanism.Reaction Coordinate DiagramsWe can follow the progress of a reaction on its way from reactants to products by graphing the energyof the species versus the reaction coordinate. We will be vague in describing the reaction coordinatebecause its definition is a mess of other variables composed to best make sense of the progress of thereaction. The value of the reaction coordinate is between zero and one. Understanding the meaning ofthe reaction coordinate is not important, just know that small values of reaction coordinate (0-0.2)mean little reaction has taken place and large values (0.8-1.0) mean that the reaction is almost over. It isa kind of scale of the progress of a reaction. A typical reaction coordinate diagram for a mechanism witha single step is shown below:Muhammad Fahad Ansari 12IEEM14
  • 12. Environmental Chemistry 12 Figure %: A reaction coordinate diagram for a single-step reactionNote that the reactants are placed on the left and the products on the right. The choice of the energylevels of the reactants and products is dictated by their energies, those with higher energies are higheron the diagram and those with lower energies are lower on the diagram. The difference is energybetween the reactants and the transition state is called the activation energy. The activation energy isthe height of the energy barrier of the reaction. The transition state is the point of maximum energy onthe diagram which represents a species possessing both reactant-like and product-like properties.Because it is so high in energy, the transition state is very reactive and can never be isolated due to itsextremely short lifetime. The relative energy of the reactants and products, the ΔE on the diagram,determines whether the reaction is exothermic or endothermic. A reaction will be exothermic if theenergy of the products is less than the energy of the reactants. A reaction is endothermic when theenergy of the products is greater than the energy of the reactants. The is for an exothermic reaction.Below is a reaction coordinate diagram for an endothermic reaction. Figure %: Reaction coordinate diagram for an endothermic reactionIf a reaction has n elementary steps in its mechanism, there will be n–1 minima between the productsand reactants representing intermediates. There will also be n maxima representing the n transitionstates. For example, a reaction with three elementary steps could have the following reactioncoordinate diagram.Muhammad Fahad Ansari 12IEEM14
  • 13. Environmental Chemistry 13 Figure %: Reaction coordinate diagram for a three-step reactionOne confusing point about reaction coordinate diagrams is how to determine what the rate determiningstep is. Even experienced chemists consistently get this type of problem wrong. The rate determiningstep is not the one with the highest activation energy for the step. The rate determining step is the stepwhose transition state has the highest energy.Activation Energy and the Arrhenius EquationIntuitively, it makes sense that a reaction with a higher activation barrier will be slower. Think of howmuch harder you must roll a ball up a large hill than a smaller one. Lets consider chemical reactionsmore deeply to derive an equation which describes the relationship between the rate constant of areaction and its activation barrier. To simplify our derivation, we will assume that the reaction has a one-step mechanism. This elementary step represents a collision as shown in . Therefore, the frequency ofthe collisions, f, will be important in our equation. Notice that only a certain orientation of the moleculeswill lead to a reaction. For example, the following collision will not lead to a reaction. The reagentmolecules simply bounce off of one another:Muhammad Fahad Ansari 12IEEM14
  • 14. Environmental Chemistry 14 Figure %: Only specific orientations during a collision will lead to a reaction.Therefore, we will need to include an orientation factor (or steric factor), p, that takes into account thefact that only a certain fraction of collisions will lead to reaction due to the orientation of the molecules.Another factor we must consider is that only a certain fraction of the molecules colliding will haveenough energy to overcome the activation barrier. The Boltzmann distribution is a thermodynamicequation that tells us what fraction of the molecules have a certain amount of energy. As you know, athigher temperatures the average kinetic energy of the molecules increases. Therefore, at highertemperatures more molecules have energy greater than the activation energy--as shown: Figure %: Boltzmann distributions for T1 greater than T2Combining the above considerations, we state the following relationship between the rate constant andthe activation energy, called the Arrhenius equation:The variable k is the rate constant, which is dependent on the frequency of the collisions f, orientationfactor p, activation energy Ea, and temperature T. From the expression for the Arrhenius equation youshould note that a small increase in activation energy leads to a large decrease in rate constant.Furthermore, temperature has a similarly exponential effect on the rate constant. An experimental ruleof thumb is that a 10oC increase in temperature leads to a doubling of the rate constant.One application of the Arrhenius equation that is useful is the determination of the activation energy fora reaction. Taking the natural log of the Arrhenius equation gives a linear equation:Muhammad Fahad Ansari 12IEEM14
  • 15. Environmental Chemistry 15A graph of ln k versus 1 / T should give a straight line whose slope is - E a / R. By measuring the rateconstant at a range of different temperatures, you can construct a graph to determine the activationenergy of a reaction.CatalysisA catalyst speeds up a reaction without being explicit in the overall balanced equation. It does this byproviding an alternate mechanism for the reaction that has a lower activation barrier than does theuncatalyzed pathway. Compare the catalytic and regular mechanisms for the hydrogenation of ethyleneto ethane and their associated reaction coordinate diagrams in : Figure %: Mechanisms of ethylene hydrogenationAs you can see, the catalyst changes the mechanism of the reaction and lowers the activation energy.The catalyst, because it does not appear in the overall balanced equation has absolutely no effect on thethermodynamics of the reaction.There are two types of catalysts--heterogeneous catalysts and homogeneous catalysts. There is nofundamental difference in how these catalysts work. The difference lies in whether the catalyst is in thesame phase (solid, liquid, or gas) as the reagents. A homogeneous catalyst is in the same phase as thereactants while a heterogeneous catalyst is not. An enzyme is a biological homogeneous catalyst. Acid Base ReactionsMuhammad Fahad Ansari 12IEEM14
  • 16. Environmental Chemistry 16Inorganic CompoundsCenturies ago chemists began to classify substances by grouping them into elements, compounds, andmixtures.Then the elements were classifies further into metals and nonmetals. Likewise, compounds weredivided into organic and inorganic.The organic compounds were eventually grouped into smaller classes such as alcohol, hydrocarbons,ketones, ethers, and so on.The inorganic compounds were further classified as acids, bases, slats and so on.Classes of inorganic Compounds:Compounds were classified on the basis of their properties. However, theories can help you understandwhy the compounds of a given class have similar properties, such as atomic structure and the bonding.Acids-Bases-BuffersImportance of acids, Bases and Buffers (salts)It is fact that, stomach contains acids. Overeating can cause a person to suffer from frequent bouts ofacid indigestion. So that you can always have something on hand to neutralize this inevitable excessstomach acid (Asprin is an acid (acetyl salicylic acid) which may upset the stomach unless properlybuffered. Acids, bases, and buffers are important to the functioning of the human body. Many foods weeat either contain acids or yield acids as they are digested.Yet the acid base balance of your body blood must be maintained with very narrow limits, or you willdie. The concentration of H+ (acidity) of the blood should be approximately 4X10 -8 M. If it increases to approximately 6X10-8 M Or decreases to approximately 2.5X10-8 M death resultsSo that blood acidity is maintained at the proper level.Acids, bases and buffers are also of tremendous importance to industry and commerce.Such as HCl, HNO3, and H2SO4 are manufactured in million-ton quantities annually for a wide variety ofuses.Theories about Acids:• Santé Arrhenius (1859-1927) Swedish chemist explains why water solutions of certain substances conduct electricity. Arrhenius proposed the theory of ionization. • He suggested that these substances, which are called electrolytes, dissociate in water to produce charged particles (ions). The ions are responsible for conducting electricity so thatMuhammad Fahad Ansari 12IEEM14
  • 17. Environmental Chemistry 17 • Acids are electrolytes when they react with metals to yield hydrogen gas (H 2). Arrhenius defined acids as substances that ionize in water to yield hydrogen ions (H +) The thing common to all acids is their ability to yield hydrogen ions (H +) in water. For example: HCl in H2O Yields H+ + Cl- HF in H2O H+ + F- HBr in H2O H+ + Br-• Bronsted-Lowry Concept about Acids: According to this theory, acids are defined simply as proton donors. The hydrogen ion is a proton: the hydrogen atom minus its electron.So acid-base reactions are not limited to reactions between water solutions. Any substance which givesup a proton (hydrogen ion H+) in any reaction is in an acid. This includes all the substances those areacids according to the Arrhenius concept and many others.Acids Composition:In terms of composition, acids consist of the nonmetallic element hydrogen covalently bonded to othernonmetallic element.The system of naming the acids centers around the number of oxygen atoms present.The most common oxygen containing acids of the nonmetal has three oxygen atoms include halogens(Cl, Br, I) nitrogen (N), and carbon (C).These acids are HClO3 chloric acid, HBrO3 bromic acid, HIO3 iodic acid, HNO3 nitric acid and H2CO3carbonic acid.Acids of sulfur (S) and phosphorous (P) contain four oxygen atoms H 2SO4 sulfuric acid and H3PO4phosphoric acid.Only sulfur and phosphorous contain four oxygen atoms the rest contain three oxygen atoms.The number of hydrogen atoms is the same in all the acids of a given nonmetal.Acids of phosphorous contain three hydrogen atoms. The acids of sulfur and carbon contain twohydrogen atoms, all the rest contain only one hydrogen atom.Properties of Acids:The acids occur in all three physical states (gases, liquids, and solids) HCl as a gas, Acetic acid as a liquidand Boric acid as a solid; in the pure liquid form they do not conduct electricity. • Acids have a sour taste. For example, the sour taste of lemons and other citrus fruits is to due to citric acid, one of the organic acids.Muhammad Fahad Ansari 12IEEM14
  • 18. Environmental Chemistry 18 • Acids react with active metals to yield hydrogen. Sodium (Na), potassium (K), calcium (Ca), zinc (Zn), and other active metals react with acids such as hydrochloric and sulfuric to yield hydrogen gas. • Acids react with bases to yield a salt and water. Bases and salts are two other major classes of inorganic compounds. Hydrochloric acid (HCl) reacts with the base sodium hydroxide (NaOH) to yield ordinary table salt, which is sodium chloride (NaCl), and water. • In their pure state some acids are gases, some are liquids, and others are solids. • Acids are electrolytes; they conduct electricity when dissolved in water. However, in the liquid state pure acid compounds such as HCl do not conduct electricity. • Acids turn blue litmus red. Litmus is an organic dye that is used as an indicator. It becomes red when exposed to acids and blue when exposed to bases, so it is a convenient way to distinguish between acids and bases.Properties of Bases:Arrhenius and Bronsted-Lowry concepts: Arrhenius considered bases to be substances that ionize inwater to yield hydroxide ions (OH-)The common bases are metallic hydroxides such as NaOH sodium hydroxide, calcium hydroxideCa(OH-)2. The metallic hydroxide bases are solids that will conduct electricity in molten state-ionicallybonded.NaOH in water Na+ + OH-Ca (OH-)2 in water Ca2+ + 2OH-NH3 + H2O NH4OH NH4+ + OH- (NH3 is a nonmetallic hydroxide)Bronsted-Lowry theory defines bases as protons (H +) acceptors.Water acts as a base in the presence of an acid. It accepts a proton (H +) from HCl and becomes ahydronium ion (H3O+). Hydronium ion can give up the proton (H +) and the chloride ion (Cl -) can accept aproton, they are also an acid and base, respectively.NH3 + H2O NH4+ + OH-Composition of Bases:The bases are named by simply naming the metal and adding the word hydroxide, such as sodiumhydroxide (NaOH), calcium hydroxide Ca (OH -)2, and aluminum hydroxide Al (OH -)3, are the morecommon bases of the non-transition metals.Bronsted-Lowry Concept: the solvent enters into the reaction, and acids and bases occur in solution arecalled conjugate pairs. The hydronium ion is the conjugate acid of the base water, and the chloride ionis the conjugate base of the acid HCl.Muhammad Fahad Ansari 12IEEM14
  • 19. Environmental Chemistry 19Many transition metals may lose varying number of electrons as they react to form compounds. Metalsmay have more then one oxidation number. For example, copper (Cu) may lose one or two electrons,forming two different ions: Cu1+, Cu2+. [Cu OH and Cu (OH-)2] are possible.The ions of metallic element of higher charge (oxidation number) indicate the –ic suffix as cupric, and –ious indicate the lower charge (oxidation number).Salts:Much of the earth’s crust consists of salts. Acids react with bases to form salts and water.HCl + NaOH NaCl + H2OComposition and properties of salts:In general, most inorganic salts consist of a metallic element ionically bonded to the nonmetallicelement or one of the oxygen containing ions (SO 42-, CO32-, NO3- and so on) of the nonmetallic element.The properties of the salts are essentially the same as those of the ionically bonded compounds.The salts are crystalline solids that have high melting and boiling points, and they are electrolytes. Theyare generally more soluble in water and more stable to heat than the organic compounds.In general binary salts are more stable to heat than the ternary salts. The N-to-H bonds in theammonium ions are covalent, as are the bonds between oxygen and other nonmetals. Therefore,ammonium salts and salts containing binary anions (negative ions) are less stable to heat than the binarysalts.Oxides:Oxygen is a special element when combined with hydrogen; it forms the very important compoundwater (hydrogen oxide). If one hydrogen of water is displaced by a metallic element, the hydroxide (OH -)bases result. Oxygen also combines with both the metallic and nonmetallic elements to form otherbinary (two-element) compounds. Many of these compounds are salts (compounds of metal andnonmetal), however, because of oxygen special role they are often classified as oxides.Metallic Oxides:Oxygen readily combines with the metallic elements to form compounds, most of which are ionicallybonded.Properties of Metallic Oxides:The metallic oxides have the same general properties as other ionically bonded compounds. They aresolids with high melting and boiling points and high heat stability.Muhammad Fahad Ansari 12IEEM14
  • 20. Environmental Chemistry 20Na2O + H2O 2NaOHCaO + H2O Ca (OH-)2Nonmetallic Oxides:Oxygen bonds covalently with the other nonmetallic elements to form oxides. A given nonmetallicelement may form more than one oxide, partly because oxygen may bond to the element by bothcovalent and co-ordinate covalent bonds. If there is more than one atom of nonmetal present, this isindicated by prefix (mono, di, tri and so on). System of naming oxides of the nonmetallic elements is asfollowing:CO carbon monoxide CO2 carbon dioxideSiO2 silicon dioxide SO2 sulfur dioxideSO3 sulfur trioxide N2O3 dinitrogen trioxideP2O5 diphosphrous PentoxideProperties of nonmetallic Oxides:The nonmetallic oxides are covalently bonded compounds, related to organic compounds. Several of thenonmetallic oxides dissolve in water and undergo a reaction with it. The nonmetallic oxides may beconsidered to be anhydrides of acids. They form acid in water, he soluble nonmetallic oxides areelectrolytes.CO2 + H2O = H2CO3 in (H2O) = H3O+ + HCO3-SO2 + H2O = H2SO3 in (H2O) = H3O+ + HSO3-Acid-Base ReactionsNa+OH- + H+NO3- Na+NO3- + H2O(aq) (aq) (aq)Base acid salt waterNH3 + HCl NH4+Cl-(gas) (gas) (solid)  base acid saltHCl (in water) H+ + Cl-H2O + HCl H3O+ + Cl-base acid acid baseMuhammad Fahad Ansari 12IEEM14
  • 21. Environmental Chemistry 21The hydronium ion (H3O+) is capable of giving up the proton (to the base) and becoming a watermolecule again, it is an acid. Thus the hydronium ion is the conjugate acid of the base water.Na+OH- (in water) Na+ + OH-Substances other than metallic hydroxides that are bases when dissolved in waterNH3 + H2O NH4+ + OH-Base acid acid baseAcids of halogensThe acids of chlorineHClO4 Perchloric acidHClO3 Chloric acidHClO2 Chlorous acidHClO hypochlorous acidHCl Hydrochloric acidHI Hydroiodic acidHF Hydrofluoric acidHBr Hydrobromic acidAcids of NitrogenHNO3 Nitric acidHNO2 Nitrous acidAcids of CarbonH2CO3 Carbonic acidAcids of SulfurH2SO4 Sulfuric acidMuhammad Fahad Ansari 12IEEM14
  • 22. Environmental Chemistry 22H2SO3 Sulfurous acidAcids of PhosphorousH3PO4 Phosphoric acidH3PO3 Phosphorous acidOrganic acid (weak acid)HC2H3O2 Acetic acidBases of non-transition elementsLi, Na, K (OH-)Ba, Ca, Mg (OH-)2Al (OH-)3Bases of transition elementsCu1+OH-Cu2+(OH-)2Fe2+ (OH-)2Fe3+ (OH-)3 Breaking Bonds: Oxidation-Reduction ReactionsCharge transfer reactions - Oxidation/Reduction (Redox) ReactionsMuhammad Fahad Ansari 12IEEM14
  • 23. Environmental Chemistry 23Chemical reactions involve making and breaking bonds. So far we have considered making andbreaking ionic bonds between ions in aqueous salt solutions. Now we want to discuss makingand breaking covalent bonds. When we break covalent bonds, there are three ways we canseparate the two atoms involved in the bond. Consider for example a C-X bond. If when thebond is broken, the electrons are left with C, C takes on a negative charge and is called a carbonanion. If the electrons leave with X, then C is left with a positive charge and is a carbon cation.If one electron stays with each atom the C has no charge, but does not have an octet. Thecarbon is called a free radical.Oxidation-Reduction Reactions:Chemical reactions involves an electron exchange between the reactants, in which one substance loseselectrons and another substance gains electrons or gains a greater share of them these reactions arecalled oxidation-reduction reactions.Oxidizing agent: Substance that causes another substance to lose electronsReducing agent: Substance that loses electrons in a chemical reaction; it reduces another substance bylosing electrons to it.Chemical Reactions:Example: Reactants Product2Na + Cl2 → 2 Na+ Cl-Loses GainsOxidized ReducedMuhammad Fahad Ansari 12IEEM14
  • 24. Environmental Chemistry 24Reducing agent Oxidizing agentOxidation Number or Oxidation State:Reflects the number of electrons lost or gained in relation to the elemental state.Simple and arbitrary rules to assign an oxidation number to a substance: • The oxidation number of an element in its free or uncombined form is zero. For example: Na0, Mg0, S0, O20, and so on. • The oxidation number of a mono-atomic (one-atom) cation (positive ion) or anion (negative ion) is equal to its charge. In other words, the oxidation number of mono-atomic ions equals the number of electrons it has lost or gained. For example: Na1+, Mg2+, Al3+, Cl1-, S2-, and so on. • The oxidation number of oxygen in compounds is usually -2. The exceptions are the peroxides, such as H2O2, and compounds of oxygen and fluorine, such as OF2. • The oxidation number of hydrogen in compounds is usually +1. The hydrides, such as NaH, are exceptions. • In the formula for a compound, the sum of the positive oxidation numbers must equal the sum of the negative oxidation numbers. For example: Mg2+S2-, Na1+Cl1-, K1+Mn7+O48-, H22+S6+O48-, and so on. • In complex ions such as SO42-, PO43-, ClO31-, the algebraic sum of the oxidation numbers of the individual atoms in the ion equals the charge on the ion. For example: in SO42-, since oxygen is always -2 there will be a total of -8 oxidation number due to oxygen, therefore, S must have an oxidation number of +6 in order for the ion to have oxidation number of -2. SO42- x +4(-2) = -2 (net charge on ion) x -8 = -2 x = 8 -2 = 6Muhammad Fahad Ansari 12IEEM14
  • 25. Environmental Chemistry 25 So the oxidation number of S in SO42- , Cl in ClO31- and P in PO43- are ClO31- x + 3(-2) = -1 (net charge on ion) X-6=-1 X=6–1=5 PO43- x + 4(-2) = -3 (net charge on ion) X-8=-3 X= 8 – 3 = 5Oxidation/Reduction (Redox) ReactionsWe have just discussed acid/base reactions, which involve proton transfer. Now we will discussanother kind of charge transfer, electron transfer or oxidation/reduction reactions. Inoxidation/reduction reactions, there is a transfer of charge - an electron - from one species toanother. Oxidation is the loss of electrons and reduction is a gain in electrons. Use theseacronyms to help you remember: Leo Ger - Loss of electrons oxidation; Gain of electronsreduction or Oil Rig - Oxidation involves loss, Reduction involves gain - of electrons.) Thesereactions always occur in pair. That is, an oxidation is always coupled to a reduction. Whensomething gets oxidized, another agent gains those electrons, acting as the oxidizing agent, andgets reduced in the process. When a substance gets reduced, it gains electrons from somethingthat gave them up, the reducing agent, which in the process gets oxidized. Its just like an acidbase reaction. An acid reacts with a base to form a new acid and base.Reactions in which a pure metal reacts with a substance to form a salt are clearly oxidationreactions. Consider for example the reaction of sodium metal and chlorine gas.2Na(s) + Cl2 2NaCl(s)Muhammad Fahad Ansari 12IEEM14
  • 26. Environmental Chemistry 26Na is a pure metal. (Although it really exists as sodium ions surrounded by a sea of electrons),consider it for our purposes to exist as elemental Na, which has a formal charge of 0. Likewise,Cl2 is a pure element. To determine the charge on each Cl atoms, we divide the two bondedelectrons equally between the two Cl atoms, hence assigning 7 electrons to each Cl. Hence theformal charge on each Cl is 0.In a similar fashion we can determine the oxidation number of an atom bonded to anotheratom. We can assign electrons to a bonded atoms, compare that number to the number in theouter shell of the unbonded atoms, and see if there is an excess or lack. In these cases, thesame number of electrons get assigned to each atoms as when we are calculating formalcharge. Hence the oxidation numbers are equal to the formal charge in these examples. Clearly,Na went from an oxidation number and formal charge of 0 to 1+ and Cl from 0 to 1-. Therefore,Na was oxidized by the oxidizing agent Cl2, and Cl2 was reduced by the reducing agent Na.Lets consider other similar redox reactions: • 2Mg(s) + O2(g) 2MgO(s) • Fe(s) + O2(g) Fe oxides(s) • C(s) + O2(g) CO2(g)In the first two reactions, a pure metal (with formal charges and oxidation numbers of 0) loseelectrons to form metal oxides, with positive metal ions. The oxygen goes form a formal chargeand oxidation number of 0 to 2- and hence is reduced.What about the last case? Each atom in both reactants and products has a formal charge of 0.This reaction, a combustion reaction with molecular oxygen, is also a redox reaction. Where arethe electrons that are lost or gained?Muhammad Fahad Ansari 12IEEM14
  • 27. Environmental Chemistry 27This can be determined by assigning the electrons in the different molecules in a way slightlydifferent than we did with formal charge. For shared (bonded) electrons, we give bothelectrons in the bond to the atom in the shared pair that has a higher electronegativity. Nextwe calculate the apparent charge on the atom by comparing the number of assigned electronsto the usual number of outer shell electrons in an atom (i.e. the group number). This apparentcharge is called the oxidation number. When we use this method for the reaction of C to CO 2,the C in carbon dioxide has an oxidation number of 4+ while the two oxygens have an oxidationnumber of 2- . Clearly, the C has "lost electrons" and has become oxidized by interacting withthe oxidizing agent O2. as it went from C to CO 2. If the atoms connected by a bond are identical,we split the electrons and assign one to each atom. In water, the O has an oxidation number of2- while each H atom has an oxidation number of 1+. Notice that the sum of the oxidationnumbers of the atoms in a species is equal to the net charge on that species.What we have done is devise another way to count the electrons around an atom and theresulting charges on the atoms. See the animation below to review electron counting, and the 3"types of charges" - partial charges, formal charges, and now oxidation numbers.Consider an O-X bond, where X is any element other than F or O. Since O is the second mostelectronegative atom, the two electrons in the O-X bond will be assigned to O. In fact all theelectrons around O (8) will be assigned to O, giving it always an oxidation number of 2-. This willbe true for every molecule we will study this year except O 2 and H2O2 (hydrogen peroxide). Nowconsider a C-H bond. Since C is nearer to F, O, and N than is H, we could expect C (en 2.5) to bemore electronegative than H (en 2.1). Therefore, both electrons in the C-H bond are assigned toC, and H has an oxidation number of 1+. This will always be true for the molecules we study,except H-H. A quick summary of oxidation numbers shows that for the molecules we will study:Muhammad Fahad Ansari 12IEEM14
  • 28. Environmental Chemistry 28 • O always has an oxidation # of 2- (except when it is bonded to itself or F) • H always has an oxidation # of 1+ (except when it is bonded to itself) • The sum of the oxidation numbers on a compound must equal the charge on the compound (just like the case of formal charges)Notice in each of the reactions above, oxygen is an oxidizing agent. Also notice that in each ofthese reactions, a pure element is chemically changed into a compound with other elements.All pure, uncharged elements have formal charges and oxidation numbers of 0. When theyappear as compounds in the products, they must have a different oxidation number. Thedisappearance or appearance of a pure element in a chemical reaction makes that reaction aredox reaction.Now lets consider a more complicated case - the reaction of methane and oxygen to producecarbon dioxide and water:CH4 + O2 H2O + CO2Since H has an oxidation # of 1+, the oxidation # of C in CH 4 is 4-, while in CO2 it is 4+. Clearly Chas been oxidized by the oxidizing agent O2. O2 has been reduced to form both products.Now consider a series of step-wise reactions of CH4 ultimately leading to CO2.You should be able to determine that the oxidation numbers for the central C in each moleculeare 4-, 2-, 0, 2+, and 4+ as you proceed from left to right, and hence represent step-wiseoxidations of the carbon. Stepwise oxidations of carbon by oxidizing agents different than O 2Muhammad Fahad Ansari 12IEEM14
  • 29. Environmental Chemistry 29are the hallmark of biological oxidation reactions. Each step-wise step releases smaller amountsof energy, which can be handled by the body more readily that if it occurred in "one step", asindicated in the combustion of methane by O2 above.You may have learned in a previous course that in oxidation reactions, there is an increase inthe number of X-O bonds, where X is some atom. Alternatively, it also involves the decrease ofX-H bonds. Reduction would be the opposite case - decreasing the number of X-O bonds and/orincreasing the number of X-H bonds. This rule applies well to the above step-wise example.Redox reactions are common in nature. Some common redox reactions are those that occur inbatteries, when metals rust, when metals are plated from solutions, and of course combustionof organic molecules such as hydrocarbons (like methane and gasoline) and carbohydrates (likewood). A simple redox reaction that leads to the plating or deposition of a pure metal from asolution of that metal is shown below.Cu (s) Cu (aq) + 2e- (half equation/reaction)Ag+ (aq) + e- Ag (s) (half equation/reaction)Cu(s) + 2Ag+(aq) Cu2+(aq) + 2Ag(s)In this reaction, pure silver metal - Ag(s) is plated on the surface of Cu(s) In this reaction: • Cu is oxidized and is the reducing agent • Ag+ is reduced and is the oxidizing agentIf you look at the products, you could imagine they could also react in a reverse of the originalreaction to produce the original reactants.Cu2+(aq) + 2Ag(s) Cu(s) + 2Ag+(aq)Muhammad Fahad Ansari 12IEEM14
  • 30. Environmental Chemistry 30In this reaction, pure copper metal - Cu(s) would be plated on the surface of the Ag(s). In thisreaction: • Ag(s) is oxidized and is the reducing agent • Cu2+ is reduced and is the oxidizing agentWhy doesnt this reverse reaction also occur? It actually does to a small extent. You couldactually envision the original reaction as reversible:Cu(s)RA + 2Ag+(aq)OA Cu2+(aq)OA + 2Ag(s)RAWhere RA indicates which reactants/products are potential reducing agents in the forward andreverse reactions, and OA indicates potential oxidizing agents. Which way does this reactiongo? Suffice it to say, the reaction goes in the direction from the strongest oxidizing and reducingagents to the weakest. In the above example, Cu(s) is the stronger RA and Ag+ is the strongerOA. In redox, reactions, a pure element is either consumed or formed in the chemical reaction. This would include all oxidation reactions involving O 2. A reaction which increases the number of bonds from an atom to oxygen represents an oxidation of that atom and a reduction of oxygen. A decrease in the number of bonds from an atom to oxygen represents a reduction of the atom. Both an oxidizing and reducing agent must be present.Microbial Redox ProcessImportant redox reactions that are carried out by microorganisms are summarized here:The notation [CH2O] is used to denote a fragment of an arbitrary carbohydrate.Photosynthetic Production of BiomassMuhammad Fahad Ansari 12IEEM14
  • 31. Environmental Chemistry 31Photosynthetic microorganisms (algae and some bacteria) carryout photosynthesis reactions, in thesereactions, energy-rich carbohydrate molecules are produced by combining carbon dioxide and water,using energy derived from sunlight.These reactions can be written:From a Redox Perspective:CO2 + H2O [CH2O] + O2Carbon is reduced from oxidation state +4 to 0, and oxygen is oxidized from -2 to 0Aerobic Respiration:In the presence of oxygen, microorganisms degrade biomass to form carbon dioxide and water.Chemical energy that is released can be used by the organisms.[CH2O] + O2 CO2 + H2OThis process is the reverse of photosynthesis, carbon is oxidized and oxygen is reduced.Nitrogen Fixation• In the atmosphere, nitrogen is almost entirely in the form of N 2 and is in oxidation state 0.• The nitrogen in biological system is mostly in the form of an amine –NH 2, Which is very closely related to ammonia (NH3) and ammonium ion (NH4+) here nitrogen is in oxidation state -3.• Nitrogen in water and soil is in form of nitrate (NO 3-) in which nitrogen is in oxidation state +5.Microorganisms play an essential role in the movement of nitrogen among these oxidation states.Compounds such as ammonia and nitrate contain a single nitrogen atom as fixed nitrogen species.Certain groups of bacteria are capable of converting gaseous nitrogen to fixed nitrogen, in the form ofthe ammonium ion.Energy from the oxidation of biomass to CO 2 is used to reduce the nitrogen in N 2 to ammonium.3[CH2O] + 2N2 + 3H2O + 4H+ 3CO2 + 4NH4+NitrificationIn the nitrification, nitrogen in the ammonium ion is oxidized from -3 to +5, with oxygen as oxidizer.NH4+ + 2O2 NO3- + 2H+ + H2OMuhammad Fahad Ansari 12IEEM14
  • 32. Environmental Chemistry 32Plants absorb nitrogen more efficiently in the form of nitrate than an ammonium, so redox reaction canenhance the effectiveness of ammonia-based agricultural fertilizers.Nitrate Reduction or DenitrificationWhen oxygen is not available as the oxidizer to degrade biomass, microorganisms can use nitrate as theoxidizer (electron acceptor).Nitrate Reduction: is used in some wastewater treatment systems to convert fixed nitrogen to N 2 gas,which can then be safely released to the atmosphere. This process is called denitrification, sincenitrogen is removed from the aqueous system.Nitrogen in municipal wastewater begins in a reduced state (-3), the overall process involved two steps:Nitrification in an aerobic reactor, followed by denitrification in an anaerobic reactor, four nitrogenatoms, being reduced from +5 to 0, can fully oxidize five carbon atoms from 0 to +4NH4+ + 2O2 NO3- + 2H+ + H2O (aerobic reaction)5[CH2O] + 4NO3- + 4H+ 5CO2 + 7H2O + 2N2 (anaerobic reaction)Sulfate Reduction:Some environments that contain biodegradable materials lake both oxygen and nitrate to serve as theoxidizing agent, in such cases, sulfate may serve that role.The conversion of one sulfur atom from +6 in sulfate to -2, in hydrogen sulfide oxidizes two carbonatoms from 0 to +4 oxidation states.2[CH2O] + 2H+ + SO42- 2CO2 + 2H2O + H2SThis reaction can occur in stagnant anaerobic marine sediments that are supplied with decayingbiomass, algae or seaweed accumulation.If the rate of biomass accumulation is high, oxygen can be rapidly depleted from the sediments.Methane Formation (Methanogenesis):In the absence of oxygen, nitrate, and sulfate, biomass can still be converted to carbon dioxide as:2[CH2O] CO2 + CH4This is an interesting redox reaction, since the two carbon atoms begins in oxidation state zero (0). Onecarbon atom is oxidized to +4, and the other is reduced to -4.Muhammad Fahad Ansari 12IEEM14
  • 33. Environmental Chemistry 33Methane generation process is exploited in seawater treatment to convert excess microbiologicalmaterial to gases, which are more easily handled for disposal.Table-Oxidation States of Some Chemical Elements Oxidation Element Species Formula state -2Hydrogen sulfide H2S 0Elemental sulfur S Sulfur +4Sulfur dioxide SO2 +6Sulfate ion SO42- -4Methane CH4 0Soot, graphite C Carbon +2Carbon monoxide CO +4Carbon dioxide CO2 -3Ammonia NH3 0Nitrogen gas N2 +2Nitric oxide NO Nitrogen +3Nitrite ion NO2- +4Nitrogen dioxide NO2 +5Nitrate ion NO3- -2Almost all compounds - Oxygen -1Hydrogen peroxide H2O2 0Oxygen gas O2 0Hydrogen gas H2 Hydrogen +1Hydrogen ion H+ -1Chlorine ion Cl- 0Chlorine gas Cl2 Chlorine +1Hypochlorous acid HOCl +7Perchloric acid HClO4Hints when trying to predict products: • First look for obvious precipitation reactions - two aqueous salt solutions mixed together with one of the salts containing an ion that quite often is insoluble (such as Fe, Ba, Ag, Hg, Pb) • Then looks for other possible precipitation reactions in which one of the above ions is present in an aqueous salt but the other solution is an acid which might provide an anion (like sulfate or chloride) that could precipitate with the above ion.Muhammad Fahad Ansari 12IEEM14
  • 34. Environmental Chemistry 34 • Next identify easy acid/base reactions - in which you have an easily identifiable acid (nitric, sulfuric, hydrochloric) and a hydroxide salt as the base • Now look for easily identifiable redox reactions in which O2 is a reactant and probably an oxidizing agent and the other reagent is a hydrocarbon, a carbohydrate, a metal, etc. The hydrocarbon and carbohydrate will react to form carbon dioxide and water, and the metal will react to form oxides. • Finally look for other redox reactions, such as when you have a pure metal interacting with a ion of another metal or an acid which could dissolve it to form a salt of the metal. Precipitation ReactionAn aqueous solution is one that is occurring in water. What makes water significant is that itcan allow for substances to dissolve and/or be dissociated into ions within it.Reaction TypesSeveral schemes have been developed to categorize chemical reactions. The one we will use most oftenis charge transfer: These reactions involve making and breaking bonds and the transfer of chargebetween substances. If a proton is transferred from one substance to another, the reaction is called anacid/base reaction. An electron transfer occurs in an oxidation/reduction reactions. Finally soluble ionsof salts can be "transferred" towards each other in solution and form an insoluble solid. This reaction iscalled a precipitation reaction.Properties of PrecipitatesPrecipitates are the products of a precipitation reaction, in which certain cations and anionscombine to produce and insoluble solid. The determining factors of the formation of a precipitatecan vary: some depend on temperature, such as solutions used for buffers, while others aredependent only on solution concentration. The solids produced in precipitate reactions arecrystalline. The solid can then be suspended throughout the liquid or it can fall to the bottom ofthe solution. The liquid that remains above the precipitate is called the supernatant liquid.Here is a diagram of the formation of a precipitates in the solution.Muhammad Fahad Ansari 12IEEM14
  • 35. Environmental Chemistry 35Precipitation and Double Replacement ReactionsMost precipitation reactions that occur are almost always either single replacement reactions ordouble replacement reaction. Double replacement reactions make up the majority of precipitationreactions, and we will be using a double replacement reaction for this particular explanation. Theequation for a double-replacement reaction is as follows: AB + CD → AD + CBAs illustrated above, a double replacement reaction occurs when two ionic reactants dissociateand bond with the respective anion or cation from the other reactant. This can be thought of as"switching partners," that is, the two pairs pictured above "lose" their partner and form a bondwith a different partner:A double replacement reaction is specifically classified as a precipitation reaction when thechemical equation in question occurs in aqueous solution and one of the products formed isinsoluble. An example of a double replacement reaction is as follows: CdSO4(aq) + K2S(aq) → CdS(s) + K2SO4(aq)As you can see, both of the reactants are aqueous and the one of the products is solid. Becausethe reactants are ionic, and the fact that they are aqueous, i.e. in water, means that these reactantswill dissociate and thus are soluble. However, there are certain solubility rules that dictate thatsome ionic molecules are not insoluble in water. These molecules will form a solid thatprecipitates throughout the solution.Muhammad Fahad Ansari 12IEEM14
  • 36. Environmental Chemistry 36Solubility Rules Whether an aqueous double replacement reaction forms a precipitate is dictated by the solubilityrules. They are as follows: • Salts formed with group 1 cations and NH 4+ cations are soluble. There are some exceptions for certain Li+ salts. • Acetates (C2H3O2-), nitrates (NO3-), and perchlorates (ClO4-) are soluble. • Bromides, chlorides, and iodides are soluble • Sulfates (SO42-) are soluble. Exceptions to this rule are sulfates formed with Ca 2+, Sr2+, and Ba2+. • Salts containing silver, lead, and mercury (I) are insoluble. • Carbonates (CO22-), phosphates (PO43-), sulfides, oxides, and hydroxides (OH -) are insoluble. Sulfides formed with group 2 cations and hydroxides formed with calcium, strontium, and barium are exceptions to the rule.Make sure to follow the lower-numbered guidelines in case of conflict between two rules.Do you understand these concepts so far? Here are some examples to help you find out.Complete the double replacement reaction and indicate the states of matter that the products arein each chemical equation.Example 1:1. NaOH(aq) + MgCl2(aq) →Because double replacement reactions "switch partners," we can predict the products of thisreaction regardless of their states of matter. That means that the Na + cation bonds with Cl- anion,and the Mg2+ cation bonds with the (OH)- to create:2NaOH + MgCl2 → 2NaCl + Mg(OH)2Now we must use consult the solubility rules to determine whether or not these products aresoluble. If we consult the rules, we find that both group 1 cations (Na +) and chlorides are soluble,so we can conclude that NaCl will be soluble in water. However, hydroxides are insoluble, andthus Mg(OH)2 is insoluble in water; that is, Mg(OH) 2 is a precipitate. So the resulting equationis:2NaOH(aq) + MgCl2(aq) → 2NaCl(aq) + Mg(OH)2(s)Example 2:2.CoCl2(aq) + Na2SO4(aq) →Based on our knowledge of double replacement reactions, we can predict that the products of thisreaction are CoSO4 and NaCl. Upon looking at the solubility rules, we can determine that CoSO 4is soluble because sulfates are soluble. Similarly, we find that NaCl is soluble, because chloridesare soluble. Thus the resulting equation looks as follows (after balancing, of course):CoCl2(aq) + Na2SO4(aq) → CoSO4(aq) + NaCl(aq)This particular example is important because both the reactants and the products are aqueous.This means that no precipitation is formed. As we will see in the next section, the presence of aMuhammad Fahad Ansari 12IEEM14
  • 37. Environmental Chemistry 37precipitate dictates whether a double replacement reaction in aqueous reaction goes intocompletion.Net Ionic EquationsThe formation of a precipitate is a very important component of a double replacement reactionthat occurs in aqueous solution: it allows us to write what is called a net ionic equation. Tobetter understand the definition of a net ionic equation, lets look back on the equation for thedouble replacement reaction: AB + CD → AD + CBBecause this particular reaction is a precipitation reaction, we can assign states of matter to eachvariable pair. AB(aq) + CD(aq) → AD(aq) + CB(s)The first step to writing a net ionic equation is to separate the soluble (aqueous) reactants andproducts into their respective cations and anions. Precipitates, as we know, do not dissociate inwater, so we leave the solid in the reaction alone. The resulting equation would look somethinglike: A+(aq) + B-(aq) + C+(aq) + D-(aq) → A+(aq) + D-(aq) + CB(s) If we look at this equation, we notice that the cation A +(aq) and the anion D-(aq) are present onboth sides. These ions are called spectator ions, because they do not contribute to the formationof the precipitate. In the formation of the net ionic equation, we cancel out these spectator ions: A+(aq) + B-(aq) + C+(aq) + D-(aq)→ A+(aq) + D-(aq) + CB(s)The net ionic equation would then be: C+(aq) + B-(aq) → CB(s)It is important to note that the net ionic equation is reliant on the formation of a precipitate. Ifboth products are aqueous, a net ionic equation cannot be written because all ions are consideredto be spectator ions. Therefore, no reaction occurs.Dissolution and Precipitation of SolidsTable-Solubility Products for Some Ionic Solids (at T=25oC) Compound Equilibrium relationship Ksp Aluminum hydroxide Al(OH)3 ↔ Al3+ + 3OH- 1 X 10-32 M4 Cadmium hydroxide Cd(OH)2 ↔ Cd2+ + 2OH- 2 X 10-14 M3 Calcium carbonate CaCO3 ↔ Ca2+ + CO32- 5 X 10-9 M2 Calcium fluoride CaF2 ↔ Ca2+ + 2F- 3 X 10-11 M3 Calcium hydroxide Ca(OH)2 ↔ Ca2+ + 2OH- 8 X 10-6 M1 Calcium phosphate Ca3(PO4)2 ↔ 3Ca2+ + 2PO43- 1 X 10-27 M5 Calcium sulfate CaSO4 ↔ Ca2+ + SO4 2- 2 X 10-5 M2 Chromium(III) hydroxide Cr(OH)3 ↔ Cr3+ + 3OH- 6 X 10-31 M4 Iron(II) hydroxide Fe(OH)2 ↔ Fe2+ + 2OH- 5 X 10-15 M3 Iron(III) hydroxide Fe(OH)3 ↔ Fe3+ + 3OH- 6 X 10-38 M4 Magnesium carbonate MgCO3 ↔ Mg2+ + CO32- 4 X 10-5 M2Muhammad Fahad Ansari 12IEEM14
  • 38. Environmental Chemistry 38 Magnesium hydroxide Mg(OH)2 ↔ Mg2+ + 2OH- 9 X 10-12 M3 Nickel hydroxide Ni(OH)2 ↔ Ni2+ + 2OH- 2 X 10-16 M3Practice Problems:Here are a few practice problems to help you determine whether you understand the material.From the given information, write the net ionic equation for the reaction, including states ofmatter. Highlight the text after the number under the solutions section for the solutions. • Fe(NO3)3(aq) + NaOH(aq) → • AlSO4(aq) + BaCl2(aq) → • HI(aq) + Zn(NO3)2(aq) → • CaCl2(aq) + Na3PO4(aq) → • Pb(NO3)2(aq) + K2SO4 (aq) → Solutions and ColloidsAll chemical substances may be classified as elements, compounds, or mixtures.Mixtures:Mixtures are similar to compounds in that both can be broken down into simpler substances. Butmixtures and compounds are different:The ratio of weight of the simpler substances may vary in mixtures, but the ratio is always the same incompounds.For example, the weight ratio of hydrogen to oxygen in water is always the same; however the ratio ofalcohol to water varies in mixtures.Solutions are mixtures because their composition may vary.A solution is homogeneous mixture on an ionic or molecular scale of two or more different substances.Homogeneous: means made up of similar parts. In homogeneous mixtures the substances are uniformlymixed in one another.Heterogeneous: made up of dissimilar parts. In heterogeneous mixtures the distribution of substances isnot uniform.For example, if fresh raw milk is allowed to stand for a period of time, the cream rises to the surface in aclearly detectable layer. Raw milk is heterogeneous mixture. Including tea, coffee, coke, seawater andsweets are the examples.ColloidsThe primary difference between colloids and solutions is the size of their particles.Muhammad Fahad Ansari 12IEEM14
  • 39. Environmental Chemistry 39In colloids the dispersed particles are large. Particles reflect light.Colloids are quite common, such different substances as milk, shaving cream, smoke, and fog are allcolloids.Colloid such as milk is like a sugar solution in that the dispersed (dissolved) substances cannot beseparated out by filtration. A suspension such as sand in water can be separated by filtration.Factors Affecting SolutionThe amount of the solute that will dissolve in a specific amount of a given solvent is called the solubilityof the solute.The primary factors affecting on the solubility of one substance in another are the nature of the solute,and the solvent, the temperature, and the pressure of the gas (in solution of gases in liquids).The nature of the solute and solventA rule of thumb about solubility is “likes dissolve in likes”.That is, polar solutes dissolve in polar solvents and non polar solutes dissolve in non polar solvents.Example: NaCl in H2O = solution of sodium chloride in water (polar) Oil dissolve in gasoline (non polar)Temperature:For most common solution of solids in liquids, the solubility of the solute in the solvent generallyincreases as the temperature increases.For example: the solubility of table sugar (sucrose) in water is:179 grams per 100 ml of H2O at 0 oC and487 grams per 100 ml of H2O at 100 oCException: the solubility of gases in liquids decrease with increased temperature.Pressure:Pressure has a significant effect on the solubility of gases in liquids.Muhammad Fahad Ansari 12IEEM14
  • 40. Environmental Chemistry 40Henry’s Law: states that the solubility of a gas in a liquid is directly proportional to the pressure of thegas above the liquid.Thus, if the pressure of gas above a liquid is doubled, its solubility doubled.The factors affecting the Rate of DissociationIn general, the rate with which a given solute will dissolve in a given solvent is determined by the size ofthe solute particles, the rate of stirring, and the temperature.Size of the solute particles:The more finely divided solute, the more rapidly it will dissolve, the larger the surface area of the soluteexposed to the solvent, the more rapidly it dissolves. The surface area of a substance increases veryrapidly as it is subdivided into smaller particles.Stirring:The more vigorous the stirring, the more quickly the solvent in contact with the solute is changed, thusthe more rapid the rate of dissociation.Temperature:At higher temperature both the solute and solvent particles have, greater energy of motion. Because oftheir increased kinetic energy, the solute particles break away from one another more rapidly. • Solution ConcentrationQualitative and Percent Methods:• In using a solution you often need to know the amount (weight or volume) of solute in the solutionThe reaction between many substances (acids and bases, for example) takes place in solution.• When you are dealing with the chemical properties of the solution, you need to know the amount of solute in a given amount of the solution.Also, when solutes are dissolved in liquids, the presence of the solute affects the physical properties ofthe liquid.Muhammad Fahad Ansari 12IEEM14
  • 41. Environmental Chemistry 41For example, when sugar is dissolved in water, the boiling point of the solution is higher and the freezingpoint of the solution is lower than for pure water.The change in the properties of the liquids depends on the ratio of solute to solvent molecules.• So when you are considering some of the physical properties of solutions, you need to know the amount of solute per amount of solvent.• Dilute Solution: contains relatively small amount of solute.• Concentrated Solution: contains relatively large amount of solute.• Saturated Solution:-That contains the amount of dissolved solute necessary for there to be equilibrium between dissolvedand un-dissolved solute.-Its concentration depends on the solubility of the solute, the temperature, and the pressure for the gas.-A saturated solution is not necessarily a concentrated solution. Saturated solutions of relativelyinsoluble substances contain a very small amount of solute. They are therefore, relatively dilute. Onother hand, saturated solutions of soluble substances are quite concentrated.• Supersaturated Solution:For most substances a hot saturated solution contains more solute than a cold saturated solution of thesubstance. The solution will contain more solute than a saturated solution at that cooler temperature.• Percent concentration:-There are three ways in which percent is used to express concentration.To avoid confusion, specify ‘percent by weight”, “percent by volume”, and so on.-One of the three general methods used to express the concentration of solutions in terms of thequantity of solute per amount of solution.-Percent concentration expresses the number of parts of solute per 100 parts of solution.• Percent by Weight-Solution concentration in a percent is to indicate the weight of solute per 100 grams of solution.Thus, a 5 percent solution contains 5 grams of solute per 100 grams of solution.Muhammad Fahad Ansari 12IEEM14
  • 42. Environmental Chemistry 42-Each milliliter (cubic centimeter) of water weights 1 gram. Percent by weight = weight of solute/weight of solution X 100• Percent by Volume: The concentration solution, of one liquid in another is often expressed as percent by volume. Percent by Volume = volume of solute / volume of solution X 100• Percent by Weight-Volume: In medicine, concentration is often expressed in terms of mass of solute per volume of solution. mg % = mg of solute / ml of solution X 100For example,The concentration of glucose (sugar) in the blood, expressed in terms of milligrams of glucose per 100ml of blood.-There are many substances dissolved in the blood, and the concentrations of these substances normallyvary a lot from time to time. So the weight of a given volume of blood fluctuates almost continuously.For this reason the weight of a given substance such as glucose per reference VOLUME of blood ratherthan per reference WEIGHT of blood is used.Percent by Weight Calculation:Problem: Calculate the percent by weight concentration for a solution that contains 9.00 g of sucrose(table sugar) in 30.0 g of solution.Solution: Percent sucrose = 9.00 g sucrose / 30.0 g solution X 100 = 0.300 g sucrose / 1.00 g solution X 100 = 30.0 percentProblem: How would you prepare 500 g of 12.0 percent NaCl solution?(Note: A 12.0 percent solution contains 12.0 g of NaCl per 100 g of solution)Solution: g NaCl = 12.0 g NaCl / 100 g solution = 0.120 g NaCl / 1.00 g solution X 500 g solution = 60.0 NaCl Wt. H2O = 500 g – 60.0 = 440 g.Therefore, to prepare 500 g of 12.0 percent NaCl solution, you would weight out 60.0 g NaCl anddissolve it in 440 g (440 ml) of water.Problem: How many grams of sugar must be added to 500 g (500 ml) of water to prepare a 15.0 percentsolution? (100 g of solution will contain 15.0 g of sugar and 85.0 g of water).Muhammad Fahad Ansari 12IEEM14
  • 43. Environmental Chemistry 43Solution: g sugar = 15.0 g sugar / 85.0 g H2O X 500 g H2O = 88.2 g sugar.Problem: How many grams of water must be added to 16.0 g of sugar to give a 4.00 percent solution?(100 g of solution will contain 4.00 g of sugar and 96.0 g of H2O)Solution: g H2O = 96.0 g H2O / 4.00 g sugar X 16.0 g sugar = 384 g H2O • Solution ConcentrationMoles of solute per Volumes of solution:-The concentration of solute in a solution in terms of percent tells simply the solution is dilute orconcentrated. Molarity tells how many molecules of solute are involved.-However, chemical reaction occurs between molecules of the solutes.-The weight of a mole (Avogadro’s number of molecules) of one substance may be quite different fromthe weight of a mole of another substance.For example,-A mole of sodium hydroxide (NaOH – molecular wt. = 40.0) weights 40.0 grams and a mole hydrobromicacid (HBr – molecular wt. = 80.9) weights 80.9 grams.-A 10 percent solution of NaOH and a 10 percent of HBr contain the same weight of solute per 100grams of solution. But the number of molecules of solute in each solution is very different.-Expressing concentration in terms of the moles of solute per volume of solution is usually more usefulin chemistry than concentration expressed as percent.Molarity (M) M = no. of moles of solute / 1 liter of solution-The molarity expresses the number of moles of solute per unit volume of solution-not volume ofsolvent or weight of solution.-The volume of solvent depends on both the molarity of the solution and the density (weight per unitvolume) of the solute.The following relationships are used in solving problems dealing with molar concentration: • 1 mole = 1 gram-molecular weight (g-molecular wt.) • Molarity = M = no. of moles of solute / no. of liters of solution = no. of moles of solute / 1 liter of solutionMuhammad Fahad Ansari 12IEEM14
  • 44. Environmental Chemistry 44Problem: What is the molarity of a solution if 5 liter of it contains 4 moles of solute?Solution: M = 4 moles of solute / 5 liter of solution = 0.8 mole of solute / 1 liter of solution = 0.8 M • No. of moles of solute = M X no. of liters solution = moles of solute / 1 liter of solution X no. of liters of solutionM (moles of solute in 1 liter) times the number of liters of a solution gives the number of moles of solutein that volume of the solution.Problem: How many moles of solute are in 0.30 liter of 1.5 M solution?Solution: moles solute = 1.5 moles / 1 liter X 0.30 liter = 0.45 mole solute • No. of moles of solute = no. of grams of solute X 1 mole of solute / 1 g-molecular wt. of soluteProblem: How many moles of NaOH (molecular wt. = 40.0) are in 100 g of the compound?Solution: moles NaOH = 100 g NaOH X 1 mole NaOH / 40.0 g NaOH = 2.5 moles NaOH • No. of grams of solute = no. of moles of solute X g-molecular wt. of solute / 1 mole of soluteProblem: What is the weight in grams of 0.25 moles of NaOH (molecular wt. = 40.0)?Solution: g NaOH = 0.25 mole of NaOH X 40.0 g NaOH / mole NaOH = 10 g NaOH. Chemical EquivalencyMolar concentration has its limitations because all molecules do not react on a one-to-one basis.For example,Muhammad Fahad Ansari 12IEEM14
  • 45. Environmental Chemistry 45The equations for the reaction of NaOH solution with hydrochloric acid, sulfuric acid, and phosphoricacid:NaOH + HCl NaCl + H2O2NaOH + H2SO4 Na2SO4 + 2H2O3NaOH + H3PO4 Na3PO4 + 3H2O1 mole of NaOH equal to 1 mole of HCl2 moles of NaOH equals to 1 mole of H 2SO43 moles of NaOH equals to 1 mole of H 3PO4Normality (N)The number of equivalents (gram-equivalent weight) of solute per liter of solutionThus, one normal (1N) solution contains 1 gram-equivalent weight of solute per liter of solution Normality (N) = no. of equivalent of solute / 1 liter of solutionEquivalent WeightThe equivalent weight of a substance is the molecular weight of the substance divided by its equivalencyper mole in the particular reaction. Substance Molecular weight Equivalency Equivalent weight Per mole NaOH 40.0 1 40.0 / 1 = 40.0 HCl 36.5 1 36.5 / 1 = 36.5 H2SO4 98.0 2 98.0 / 2 = 49.0 H3PO4 98.0 3 98.0 / 3 = 32.7Calculations related to NormalityThe following relationships are important when making calculations related to normal concentration: • N = no. of equivalents of solute / no. of liters of solutionMuhammad Fahad Ansari 12IEEM14
  • 46. Environmental Chemistry 46 = no. of eq. of solute / 1 liter of solution • 1 eq. = 1 g-eq. wt. • 1 g-eq. wt. = g-molecular wt. / equivalency per mole of solute = g-molecular wt. X 1 mole / no. of eq. per mole of solute(Remember, equivalency per mole equals the number of protons an acid molecule yields or a basemolecule accepts in the reaction under consideration.) • No. of eq. (g-eq. wt.) = no. of g of solute X 1 eq. of solute / g-eq. wt. of solute • No. of eq. (no. of g-eq. wt.) of solute = N X no. of liters of solution = eq. of solute / liter X no. of liters. • No. of g of solute = no. of eq. of solute X g-eq. wt. of solute / eq. of solute • N = M X no. of eq. of solute / 1 mole of solute M = N X 1 mole of solute / no. of eq. of soluteProblem: Calculate the normality of a solution that contains 2.8 equivalents of solute in 1.4 liter ofsolution.Solution: N = 2.8 eq. solute / 1.4 liter solution = 2.0 eq. solute / 1 liter solution = 2.0 NProblem: Calculate the number of equivalents of solute in 500 ml of 6.0 N solutionSolution: eq. solute = 6.0 eq. / liter solution X {500 ml X 1 liter / 1000 ml} = 3.0 eq.Problem: A solution was prepared by dissolving 101 g of Ca (OH) 2 (molecular wt. = 74.1) in enoughwater to yield 5.00 liters of solution. What is the normality of the solution? [Assume that each Ca(OH) 2molecules accept two protons in reaction].Solution:First, calculate the gram-equivalent weight of Ca (OH) 2.g-eq. wt. Ca(OH)2 = 74.1 g Ca(OH)2 / mole Ca(OH)2 X 1 mole Ca(OH)2 / 2 eq. Ca(OH)2 = 37.0 g Ca(OH)2 per eq.ThanN = 101 g ca(OH)2 / 5.0 liter solution X 1 eq. Ca(OH)2 / 37.0 g Ca(OH)2 = 0.55 eq. Ca(OH)2 / 1 liter solution = 0.55 NMuhammad Fahad Ansari 12IEEM14
  • 47. Environmental Chemistry 47Problem: A solution was prepared by dissolving 6.0 g of NaOH (molecular wt. = 40) in 200 ml of solution.Calculate the normality of the solution.Solution: g-eq. wt. NaOH = 40 g NaOH / mole NaOH X 1 mole NaOH / 1 eq. NaOH = 40 g NaOH / eq.(NaOH has only one OH- per molecule, so it can accept one proton per molecule-or 1 mole of NaOH canaccept 1 mole of protons).N= 6.0 g NaOH / 200 ml solution X 1 eq. wt. NaOH / 40 g NaOH X 1000 ml solution / 1 liter of solution = 0.75 eq. wt. / 1 liter = 0.75 NProblem:How would you prepare 1.2 liter of 2.4 N H 2SO4 solution? (Assume that H2SO4 donates both protonswhen it reacts)Solution:g-eq. wt. H2SO4 = 98 g H2SO4 / mole H2SO4 X 1 mole H2SO4 / 2 g-eq. wt = 49 g H2SO4 / eq. wt. H2SO4g H2SO4 = [2.4 eq. H2SO4 / 1 liter solution X 1.2 liter solution] X 49 g H 2SO4 / eq. wt. H2SO4Therefore, you would dissolve 140 g of H2SO4 in enough H2O to make 1.2 liter solution.Problem:What volume of 0.900 N H3PO4 solution can be prepared from 147 g of H 3PO4 (molecular wt. = 98.0)?(Assume that all three hydrogen ions of H 3PO4 are replaced).Solution:g-eq. wt. H3PO4 = 98.0 g H3PO4 / mole H3PO4 X 1mole H3PO4 / 3 g-eq. wt. = 32.7 g H3PO4 / g-eq. wt. N = no. of g-eq. wt. / liter of solutionTherefore:Liter solution = no. of g-eq. wt. / N= no. of g-eq. wt. / eq. per 1 liter = no. of g-eq. wt. X 1 liter / eq.1 H3PO4 solution = [147 g H3PO4 X 1 g-eq. wt. H3PO4 / 32.7 g H3PO4] X 1 liter / 0.900 eq.Muhammad Fahad Ansari 12IEEM14
  • 48. Environmental Chemistry 48 = 5.00 liter solution Solvents and SorbentsDevelopment of more efficient solvents is crucial in reducing the cost of carbon dioxideabsorption from combustion exhaust gases. Several solvents have been proposed for carbondioxide absorption, but the question is how the best candidate could be selected. Alkanolaminessuch as monothanolamine (MEA), diethanolamine (DEA), di-isopropanolamine (DIPA) andmethyldiethanolamine (MDEA) have been the most common solvents used over the years forabsorption process in removal of acid gases, CO2 and H2S from industrial and combustionexhaust gas streams. However, these alkanolamines are still deficient for carbon dioxideabsorption due to inherent problems associated with their use in CO 2 capture process. Differentfactors affect the efficacy of a solvent for carbon dioxide absorption, these include solventsolubility, vapor pressure, molecular weight and foaming tendency, degradation and corrosionproperties; others are reaction kinetics, heat of reaction and regeneration energy requirement aswell as the cyclic capacity. Environmental and cost factors are also to be considered.Chemical SolventsThe most commonly used technology today for low concentration CO 2capture is absorption withchemical solvents. This chemical absorption process is adapted from the gas processing industrywhere amine-based processes have been used commercially for the removal of acid gasimpurities from process gas streams. However, problems of scale, efficiency, and stabilitybecome barriers when chemical solvents are used for high volume gas flows with a relativelysmaller fraction of valuable product. The processes require large amounts of material undergoingsignificant changes in conditions, leading to high investment costs and energy consumption. Inaddition, degradation and oxidation of the solvents over time produces products that arecorrosive and may require hazardous material handling procedures.Amine SolventsThe currently preferred chemical solvent technology for carbon capture is amine-based chemicalabsorbent. CO2 in the gas phase dissolves into a solution of water and amine compounds. Theamines react with CO2 in solution to form protonated amine (AH+), bicarbonate (HCO3-), andcarbamate (A CO2 -) [9]. As these reactions occur, more CO 2 is driven from the gas phase intothe solution due to the lower chemical potential of the liquid phase compounds at thistemperature. When the solution has reached the intended CO2 loading, it is removed from contactwith the gas stream and heated to reverse the chemical reaction and release high-purity CO 2. TheCO2-lean amine solvent is then recycled to contact additional gas. The flue gas must first becooled and treated to remove reactive impurities such as sulfur, nitrogen oxides, and particulatematter.Otherwise, these impurities may react preferentially with the amines, reducing the capacity forCO2, or irreversibly poisoning the solvent. The resulting pure CO 2 stream is recovered atpressures near atmospheric pressure. Compression, and the associated energy costs, would berequired for geologic storage.Alkanolamines, simple combinations of alcohols and ammonia, are the most commonly usedcategory of amine chemical solvents for CO2 capture. Reaction rates with specific acid gasesMuhammad Fahad Ansari 12IEEM14
  • 49. Environmental Chemistry 49differ among the various amines. In addition, amines vary in their equilibrium absorptioncharacteristics and have different sensitivities with respect to solvent stability and corrosion.Alkanolamines can be divided into three groups :• Primary amines, including monoethanol amine (MEA) and diglycolamine (DGA)• Secondary amines, including diethanol amine (DEA) and diisopropyl amine(DIPA)• Tertiary amines, including triethanol amine (TEA) and methyldiethanol amine(MDEA)MEA, relatively inexpensive and the lowest molecular weight, is the amine that has been usedextensively for the purpose of removing CO2 from natural gas streams. MEA has a high enthalpyof solution with CO2, which tends to drive the dissolution process at high rates. However, thisalso means that a significant amount of energy must be used for regeneration. In addition, a highvapor pressure and irreversible reactions with minor impurities such as CO2 and CS2 result insolvent loss.Research on improved chemical solvents seeks a high absorption capacity for CO 2 without acorresponding large energy requirement for regeneration. Other desirable properties include highchemical stability, low vapor pressure, and low corrosiveness. It has been shown that solventsbased on piperazine-promoted K2CO3 can have reaction rates approaching that of MEA, butcurrently have lower capacity. Sterically hindered amines have been developed with similarcapacity and possibly less regeneration energy requirement than conventional MEA absorbents.These modified amines attempt to balance good absorption and regeneration characteristicsunder some conditions due to the reduced chemical stability of the amine- CO 2 anion [9].Controlled species selectivity is also possible with these compounds.Physical AbsorptionAbsorbents allow a gas to permeate a solid or liquid under one set of conditions, and desorbunder others. The rate of absorption or desorption is temperature and pressure dependent.Smaller differences in conditions require less energy, but require more absorbent to capture CO 2at an equivalent rate.Physical SolventsAbsorption in most current physical solvent systems occurs at high partial pressure of CO 2 andlow temperatures. The solvents are then regenerated by either heating, pressure reduction, or acombination of both. The interaction between CO 2 and the absorbent is weak relative to chemicalsolvents, decreasing the energy requirement for regeneration. Capacity can be higher thanchemical solvents, since it is not limited by the stoichiometry of the chemical system.Physical solvent scrubbing of CO2 is well established. Selexol, a liquid glycol-based solvent, hasbeen used for decades to process natural gas, both for bulk CO 2 removal and H2S removal.Glycol is effective for capturing both CO 2 and H2S at higher concentration. However, the CO 2 isreleased at near atmospheric pressure, requiring recompression for transportation and geologicstorage.Muhammad Fahad Ansari 12IEEM14
  • 50. Environmental Chemistry 50The Rectisol process, based on low-temperature methanol, is another physical solvent processthat has been used for removing CO 2. Glycerol carbonate is interesting because of its highselectivity for CO2, but it has a relatively low capacity.Mixed Chemical-Physical SolventsSome CO2 capture applications benefit from a mixture of physical and chemical solvents.The most commonly used examples are Sulfinol, a mixture of the physical solvent sulfolane andthe amines DIPA or MDEA, and Amisol, a mixture of methanol and secondary amines. Thesehybrid solvents attempt to exploit the positive qualities of each constituent under specialconditions.Physical AdsorptionPhysical adsorption relies on the affinity of CO 2 to the surface of a material under certainconditions without forming a chemical bond. Adsorbents can separate CO 2 from a stream bypreferentially attracting it to the material surface at high pressures through weak interactionssuch as van der Waals forces. During capture, the chemical potential of the adsorbed CO 2 islower than the chemical potential of CO2 in the gas mixture.Regenerable Physical AdsorbentsRegenerable adsorbents must have the ability to reverse the chemical potential of the adsorbedphase upon changing the conditions to remove the CO 2. This is done primarily through changesin pressure or stripping with an easily separable gas such as steam.Limited temperature changes can improve efficiency, but take time cycle due to the heat capacityof the adsorbent material. Since adsorption is a surface phenomenon, a successful adsorbent willhave a high surface area to volume ratio. The central advantage of physical adsorption methodsis the possibility for low energy requirement to regenerate the sorbent material and the quickregeneration time associated with changing the pressure.Proposed absorbents include activated carbon, zeolites (molecular sieves), and promotedhydrotalcites. Current zeolite systems can produce nearly pure streams of CO 2, but have highenergy penalties due to vacuum pumps and dehumidification equipment. Hydrotalcites are mosteffective at high temperatures (450-600 K), enabling capture inside or near combustion orgasification chambers . Research is required to decrease the pressure difference requirement andincrease the capacity of current adsorbents.Membrane Separation ProcessesMembrane systems include thin barriers that allow selective permeation of certain gases,allowing one component in a gas stream to pass through faster than the others. Membraneseparation can be considered a steady-state combination of adsorption and absorption. Asuccessful membrane allows the desired gas molecule to adsorb to the surface on one side, oftenat higher pressure. The molecule then absorbs into the membrane interior, eventually reachingthe other side of the membrane where it can desorb under different conditions, such as lowpressure.Membrane gas separation processes have been widely used for hydrogen recovery in ammoniasynthesis, removal of CO2 from natural gas, and nitrogen separation from air. Each of themembranes used in these capacities could be applied to carbon capture. Commonly usedMuhammad Fahad Ansari 12IEEM14
  • 51. Environmental Chemistry 51membrane types for CO2 and H2 separation include polymeric membranes, inorganicmicroporous membranes, and palladium membranes.Polymeric membranes, including cellulose acetate, polysulfone, and polyimide are the mostcommonly used for separation of CO 2 from nitrogen, but have relatively low selectivity to otherseparation methods. Inorganic membranes, able to withstand high temperatures, are capable ofoperating inside combustion or gasification chambers. Membrane reactors based on inorganicmembranes with palladium catalyst can reform hydrocarbon fuels to mixture of H 2 and CO2 andat the same time separating the high-value H 2. Combining membranes with chemical solventshas also been proposed. Despite an extra energy requirement, this arrangement may eliminateproblems associated with direct contact between the liquid solvent and gas mixture.Most membranes have inherent difficulty achieving high degrees of gas separation due tovarying rates of gas transport. Stream recycling or multiple stages of membranes may benecessary to achieve CO2 streams amenable to geologic storage, increasing energy consumption.However, the potential for high surface area could reduce the chemical potential differencerequired to drive gas separation.ChemisorptionGas molecules can chemically bond to the surface of some materials. The process is calledassociative if the molecule bonds in whole to the surface and dissociative if the gas moleculebreaks up in order to form a bond. Chemisorbents are often composed of an active surface layersupported by an inert substrate. Proposed systems use small particles as substrates in order toprovide large surface area. Regeneration drives the chemical reaction in reverse, often at elevatedtemperature.Metal Oxide Air SeparationAir separation allows a pure stream of oxygen to react with the fuel, creating an effluent of onlyCO2 and water or other useful products such as hydrogen. It is often easier with currenttechnology to separate CO2 from water or hydrogen than from nitrogen. Reactive metal exposedto air will oxidize rapidly. The metal oxidation reaction is highly exothermic (ΔHOX ~ -950kJ/mol). The oxides can then be endothermically (ΔHRED ~ -150 kJ/mol) regenerated byexposing them to a high temperature reducing environment. The oxygen combines with the fossilfuel to form carbon oxides and varying amounts of hydrogen-containing species depending onthe type of fuel. The chemistry and geometry of this separation has allowed recent small-scalestudies to obtain a nearly 100% pure stream of oxygen to react with the fuel. Phase separation ofwater from the resulting effluent could produce a pure stream of CO 2. This complete process iscommonly called chemical looping separation.Research in metal oxide air separation is focused on cost and the physical and chemical stabilityof the oxygen carriers over many cycles. The particles usually consist of a reactive oxide and asupporting inert oxide. While various oxygen carrier particles are under consideration, copper,iron, manganese, and nickel are the most promising reactive metals.No large-scale demonstration has been performed, but models predict that a power systemutilizing metal oxide air separation has significant advantages. The lower irreversibilityassociated with the regeneration step relative to conventional combustion add to the already lowenergy requirement of the inherent separation of CO 2 from nitrogen. Energy analyses show theMuhammad Fahad Ansari 12IEEM14
  • 52. Environmental Chemistry 52resulting overall energy penalty could be as low as 400 kJ/kg CO 2 for a natural gas combinedcycle plant, assuming idealized chemical stability of the oxygen carrier.Dry Chemical AbsorbentsUnder some conditions, CO2 can undergo a reversible chemical reaction with a dry absorbentmaterial. The chemical reaction can be reversed by changing the conditions, resulting in therelease of pure CO2. Sodium carbonate supported on an inert particle has been proposed as suchan absorbent. An exothermic reaction of sodium carbonate with CO 2 and water held at 60 to70ºC forms primarily sodium bicarbonate. The products must be heated to 120 to 200ºC toreverse the reaction. Lithium zirconate is also being investigated for its high capacitychemisorbtion separation of CO2 at high temperatures.Chemical BondingSome gas separation technologies use materials that create stable, thermodynamically favoredchemical bonds with a gas in a mixture or a gas in solution. These materials can either beendothermically regenerated and used in a loop much like the sorbent technologies or form stablewaste materials to be stored.CO2 MineralizationSome minerals will undergo thermodynamically favorable reactions with CO 2, separating it froma gas stream and forming a stable, chemically bonded product. Although most proposed cycleshave problems with kinetics due to the relatively low enthalpy of reaction, separation andconversion to a stable storage medium is accomplished in one step. Open-ended cycles such asthis have the advantage of not requiring regeneration. The reactant minerals can be considered aseparate resource that provides the energy requirement for separation.One proposed method involves a reaction of CO 2 with CaCO3, or limestone, and water to formcalcium and bicarbonate ions. These ions can be deposited into the ocean, short circuiting theresidence of carbon in the atmosphere. Another method proposes enhancing the otherwise slowmechanism of silicate weathering. A gas stream containing CO 2 could react with magnesiumsilicate to form magnesium carbonate and pure silicate. The large volumes of material involvedpresent significant challenges for transportation and handling.Phase SeparationBelow certain temperatures, gas molecules are moving slow enough to succumb to weakintermolecular forces. Depending on the partial pressure of other gases in a mixture, condensinggases will form a distinct phase with a composition different from that of the vapor that is easilyseparated.CO2 ClathrateClathrates are a phase of water in which the hydrogen-bonded structures encapsulate “guest”molecules of gas. The preferential formation of CO 2 clathrate over other fossil fuel conversioneffluent gases could be used as a method to capture CO2. The CO2 clathrates could later bedissociated, producing a pure stream of CO 2. Formation of CO2 hydrates occurs at about 140 atmand at temperatures near the freezing point of water.Muhammad Fahad Ansari 12IEEM14
  • 53. Environmental Chemistry 53Muhammad Fahad Ansari 12IEEM14