How Can We Describe Chemical Reactions? Chemistry Unit 9
Main IdeasChemical reactions are represented bybalanced chemical equations.There are four main types of chemicalreactions: synthesis, combustion,decomposition, and replacement reactions.Double-replacement reactions occurbetween substances in aqueous solutionsand produce precipitates, water or gases.
ObjectivesRecognize evidence of chemical change.Represent chemical reactions withequationsBalance chemical equations.
Chemical ReactionsThe process by which one or more substances are rearranged to form different substances is called a chemical reaction. Evidence of a chemical reaction is a chemical change: a process of changing one or more substances into a new substance. Evidence: temperature changes, color changes, odor, gas bubbles or appearance of a precipitate
Representing Chemical ReactionsChemical equations – statements that show chemical reactions by the use of chemical formulas and conserved matter with the relative amounts of substances in the reaction.Parts of an equation reaction: Reactants are the starting substances. Products are the substances formed in the reaction.
Representing ReactionsWord Equations: use of words for reactants and products aluminum(s) + bromine(l) → aluminum bromide(s)Skeleton Equations: chemical formulas used for reactants and products but not balanced Al(s) + Br(l) → AlBr3(s) Skeleton equations lack information about how many atoms are involved in the reaction.
Representing ReactionsChemical Equation: Is a statement that uses chemical formulas to show the identities and relative amounts of the substances involved in a chemical reaction. • 2Al(s) +3Br2 2AlBr3(s)
Balancing Chemical EquationsA coefficient in a chemical equation is the number written in front of a reactant or product, describing the lowest whole- number ratio of the amounts of all the reactants and products. The most fundamental law is the law of conservation of mass; a balanced equation shows this law.
Balancing Chemical Equations1. Write the skeleton equation: Make sure chemical formulas are correct. Put in symbols and physical states.liquid sodium carbonate + aqueous calciumchloride yields solid calcium carbonate +aqueous sodium chloride
Balancing Chemical Equations2. Count the atoms of the elements in the reactants Group intact polyatomic ions as a single substance.Na2CO3(l) + CaCl2(aq) CaCO3(s) + NaCl(aq)
Balancing Chemical Equations3. Count the atoms of the elements in the products Group intact polyatomic ions as a single substance.Na2CO3(l) + CaCl2(aq) CaCO3(s) + NaCl(aq)
Balancing Chemical Equations4. Change the coefficients to make the number of atoms of each element equatl on both sides of the equation Never change subscriptsNa2CO3(l) + CaCl2(aq) CaCO3(s) + NaCl(aq)
Balancing Chemical Equations5. Write the coefficients in their lowest possible ratios Na2CO3(l) + CaCl2(aq) CaCO3(s) + NaCl(aq)
Balancing Chemical Equations5. Go back and check math.Na2CO3(l) + CaCl2(aq) CaCO3(s) + 2 NaCl(aq)
ObjectivesClassify chemical reactions.Identify the characteristics of differentclasses of chemical reactions.
Types of Chemical ReactionsChemists classify reactions in order to organize the many types. Synthesis Combustion Decomposition Single Replacement Double Replacement (Metathesis)
SynthesisA synthesis reaction is a reaction in which two or more substances react to produce a single product. When two elements react, the reaction is always a synthesis reaction.
SynthesisA synthesis reaction is a reaction in which two or more substances react to produce a single product. When two compounds react: AB + CD ABCD AB + BC ABC
CombustionIn a combustion reaction, oxygen combines with a substance and releases energy in the form of heat and light. Example: Heated hydrogen reacts with oxygen to produce heat and water in a combustion reaction. This is also a synthesis reaction.
CombustionIn a combustion reaction, oxygen combines with a substance and releases energy in the form of heat and light. Element and oxygen react: A + O2 AO Compound and oxygen react: AB + O2 AO + B
DecompositionA decomposition reaction is one in which a single compound breaks down into two or more elements or new compounds. Decomposition reactions often require an energy source, such as heat, light, or electricity, to occur.
DecompositionA decomposition reaction is one in which a single compound breaks down into two or more elements or new compounds. Compound breaks down into two elements: AB A + B Compound breaks down to form new compounds: ABCD AC + BD
Replacement/Displacem entA reaction in which the atoms of one element replace the atoms of another element in a compound is called a single replacement reaction. A + BX → AX + B
Activity SeriesA metal will notalways replace ametal in a compounddissolved in waterbecause of differingreactivities.An activity series canbe used to predict ifreactions will occur.
Activity SeriesHalogens frequentlyreplace otherhalogens inreplacementreactions.Halogens also havedifferent reactivitiesand do not alwaysreplace each other.
Activity SeriesMetals/Halogens arelisted in order ofreactivity . A lessreactivemetal/halogen willnot replace a morereactivemetal/halogen
Practice ProblemsPage 291 #14-17; page 292 #18-20Page 295 #21-24
Double ReplacementDouble replacement reactions (also called metathesis) occur when ions exchange between two compounds.
Double ReplacementMetathesis reactions often form one ofthree products: The solid product produced during a chemical reaction in a solution is called a precipitate. water – is usually formed with the combination of an acid and a base. A metal salt is also formed. gas – formed when a gas is not a reactant.
ObjectivesIdentify new possible ionic compoundsin a reactionDefine the terms soluble and insolublePredict solids based on solubility rules.
Ionic Compounds in SolutionsIonic compounds in aqueous solutions mix andexchange partners (double replacement). example: Na2SO4(aq) + CaCl2(aq) Some of these products are solids and some of these products remain aqueous.
SolubilitySolubility rules are used to determine the stateof matter of products in an aqueous solution.• Soluble means that the compound dissolves in water.• Insoluble means that the compound remains intact in the solid state in water.
Solubility Rules1. Most nitrate (NO3-) salts are soluble2. Most salts containing the alkali metal ions (Li+, Na+, K+, Cs+, Rb+) and the ammonium ion (NH4+) are soluble.3. Most chloride, bromide, and iodide salts are soluble Exceptions: Ag+, Pb2+, Hg22+
Solubility Rules4. Most sulfate salts are soluble . • Exceptions: Bas+, Pb2+, Hg22+, and Ca2+5. Most hydroxides are only slightly soluble (treat as insoluble). • Exceptions: Na+, K+6. Most sulfide (S2-), carbonate (CO32-), chromate (CrO42-), and phosphate (PO43-) salts are only slightly soluble (treat as insoluble). • Exceptions: any containing Alkali metals and ammonium.
ObjectivesDescribe aqueous solutionsWrite complete ionic and net ionicequations for chemical reactions inaqueous solutions.Predict whether reactions in aqueoussolutions will produce a precipitate,water, or a gas.
Aqueous SolutionsAn aqueous solution contains one or more dissolved substances (called solutes) in water.
Aqueous SolutionsSolution – a homogeneous mixture thatmay contain solids, liquids or gases.Solutes – the substance that is beingdissolved.Solvent – the substance that is mostplentiful and dissolves the othersubstance.
Aqueous SolutionsThere are many possible solutes—sugarand alcohol are molecular compounds thatexist as molecules in aqueous solutions.Molecules that produce hydrogen ions inaqueous solutions are acids.
Aqueous SolutionsIonic compounds can also be solutes inaqueous solutions.When ionic compounds dissolve inwater, their ions separate in a processcalled dissociation.
Types of Aqueous EquationsIonic Equations show ionic detail and dissociation within reactions.Formula equation: 2NaOH(aq) + CuCl2(aq) → 2NaCl(aq) + Cu(OH)2(s)
Types of Aqueous Equations 2NaOH(aq) + CuCl2(aq) → 2NaCl(aq) + Cu(OH)2(s) Ionic equations that show all of the particles in a solution as they actually exist are called complete ionic equations.2Na+(aq) + 2OH–(aq) + Cu2+ (aq)+ 2Cl–(aq) → 2Na+(aq) + 2Cl–(aq) + Cu(OH)2(s) Ions that do not participate in a reaction are called spectator ions and are not usually written in ionic equations.
Types of Aqueous Reactions2Na+(aq) + 2OH–(aq) + Cu2+ (aq)+ 2Cl–(aq) → 2Na+(aq) + 2Cl–(aq) + Cu(OH)2(s) Formulas that include only the particles that participate in reactions are called net ionic equations. 2OH–(aq) + Cu2+(aq) → Cu(OH)2(s)
Types of Aqueous ReactionsSome reactions produce more water molecules. HBr(aq) + NaOH(aq) → H2O(l) + NaBr(aq) Without spectator ions H+(aq) + OH–(aq) → H2O(l)
Types of Aqueous ReactionsGases that are commonly produced are carbon dioxide, hydrogen cyanide (aq-hydrocyanic acid) , and hydrogen sulfide (aq-hydrosulfuric acid).2HI(aq) + Li2S(aq) → H2S(g) + 2LiI(aq)HCl(aq) + NaHCO3(aq) → H2CO3(aq) + NaCl(aq) H2CO3(aq) decomposes immediately. H2CO3(aq) → H2O(l) + CO2(g)
Types of Aqueous ReactionsTwo reactions can be combined and represented by a single chemical reaction
Types of Reactions in Aqueous Solutions1. Precipitate is formed- when a compound forms from ions, an exothermic reaction takes place. The ions by themselves are less stable and therefore of higher energy than when combined in a compound. This change in energy (increase in stability) ‘drives’ the reaction. 2NaOH(aq) + CuCl2(aq) 2NaCl(aq) + Cu(OH)2(s) Net:
Types of Reactions in Aqueous Solutions2. Water is formed- even though a double reaction takes place, solutions may look the same since water will still be the dominant substance. HBr(aq) + NaOH(aq) H2O(l) + NaBr(aq) Net:
Types of Reactions in Aqueous Solutions3 Gas is formed- for the same reasons that a precipitate is formed, a gas will also form. Common gases in double replacement reactions are: carbon dioxide, hydrogen cyanide, and hydrogen sulfide. 2HI(aq) + Li2S(aq) H2S(g) + 2LiI(aq) Net:
Question 1What is the solvent in an aqueous solution?A. hydrogenB. sodium ionsC. waterD.alcohol
Question 2An equation that includes only the particles that participate in a reaction is called:A. net ionic equationB. spectator ionsC. complete ionic equationD.reduced ionic equation
Accumulating ContentObjective: Apply knowledge and skills from previous unitsto content learned in this unit.• Why do polyatomic ions stay intact in an aqueous solution?
Accumulating ContentObjective: Apply knowledge and skills from previous unitsto content learned in this unit.• How do naming rules change when working with gases vs. aqueous acids?
Accumulating ContentObjective: Apply knowledge and skills from previous unitsto content learned in this unit. How does lattice energy relate to solubility rules?
Key ConceptsSome physical changes are evidence that indicate achemical reaction has occurred.Word equations and skeleton equations provideimportant information about a chemical reaction.A chemical equation gives the identities andrelative amounts of the reactants and products thatare involved in a chemical reaction.
Key ConceptsBalancing an equation involves adjusting thecoefficients until the number of atoms of eachelement is equal on both sides of the equation.Classifying chemical reactions makes them easierto understand, remember, and recognize.Activity series of metals and halogens can be usedto predict if single-replacement reactions willoccur.
Key ConceptsIn aqueous solutions, the solvent is always water.There are many possible solutes.Many molecular compounds form ions when theydissolve in water. When some ionic compoundsdissolve in water, their ions separate.When two aqueous solutions that contain ions assolutes are combined, the ions might react withone another. The solvent molecules do not usuallyreact.
Key ConceptsReactions that occur in aqueous solutionsare double-replacement reactions.
Question 1The law of conservation of mass requires what in a chemical reaction equation?A. both sides of the equation to contain the same substancesB. the reactants to have the same amount of molecules as the productsC. both sides to have the same amount of atoms of each elementD. the products to have fewer molecules than the reactant
Question 1A reaction that gives off heat is what type of reaction?A. single replacement reactionB. double replacement reactionC. synthesis reactionD. combustion reaction
Question 1Ions that are present in a solution and do not participate in a chemical reaction when another substance is added are called ____.A. spectator ionsB. reactantsC. productsD. net ions
Question 1A double replacement reaction produces all of the following except ____.A. gasesB. solidsC. lightD. water
Question 1What type of reaction is the following?2H2O(l) + energy → H2(g) + O2(g)A. synthesis reactionB. decomposition reactionC. combustion reactionD. replacement reaction
Question 1What type of reaction is the following?2H2(g) + O2(g) → 2H2O(l)A. replacement reactionB. synthesisC. combustion reactionD. double replacement reaction
Question 1A precipitate forms in a double replacement reaction only if:A. the reactivities of the compounds differB. the new compound is denser than waterC. the new compound is soluble in waterD. the new compound is not soluble in water
Question 1A ____ is a statement that uses chemical formulas to show the identities and relative amounts of the substances involved in a chemical reaction.A. word equationB. skeleton equationC. chemical equationD. balanced equation
Question 1Predict the type of reaction.LiBr2 (aq) + 2NaOH (aq) → ____A. synthesis reactionB. combustion reactionC. single replacement reactionD. double replacement reaction
Question 1Which reactions are essentially the opposite of synthesis reactions?A. single-replacementB. decompositionC. combustionD. double-replacement