Physical and Chemical Change Law of Conservation of Mass Balancing EquationsChemical Reactions
Chemical Reactions Chemical reactions are used in many ways in daily life. A chemical reaction is the process by which a chemical change happens. All chemical reactions are also accompanied by changes in energy
Chemical Reactions Some chemical reactions absorb energy, such as in the chemical reactions that cook food.
Chemical ReactionsOther chemical reactions release energy inthe form of heat, light and/or sound, such asthe burning of wood in a campfire
Chemical ReactionsChemical reactions happen at different rates.Some chemical reactions are fast, such as whenrocket fuel burns.
Chemical ReactionsOther chemicalreactions happenslowly, such asthe formation ofrust on acorroding bicyclechain.
Chemical ReactionsThe chemical reactions in your own body, whichare keeping you alive, are among the fastestchemical reactions known.
Chemical ReactionsScientists are constantly working to find new kindsof chemical reactions in order to produce newsubstances with useful properties.
Chemical ReactionsAll chemical reactions involve the conversion of startingmaterials, called reactants, into new substances, calledproducts. The products have different properties thanthe reactants.
Physical Properties A description of a substance that does not involve forming a new substance.Examples: Colour Texture State Density Solubility Melting point
Chemical Properties A description of what a substance does as it changes into one or more new substances.Examples: Combustibility Corrosion Reaction with acid Bleaching ability
Properties & Change Properties are descriptions similar to an adjective: describes what the substance is like. Change are descriptions similar to a verb: describes what the substance is doing
Physical ChangeA physical change is the change in the state orform of a substance that does not change theoriginal substance.A physical change can result in new physicalproperties but not new chemical properties.
Physical ChangeClasses of physical change:1. Change in state (includes dissolving)2. Change in formExample: Evaporation Cutting paper in half
Chemical changeA chemical change is thetransformation of one ormore substances into newsubstances with newproperties
Visual Clues to a Chemical ChangePresence of a new colourFormation of a precipitate
Visual Clues to a Chemical Change Release of heat or light
Visual Clues to a Chemical Change Production of gas or bubbles Example Reactants: Solid magnesium metal placed into a solution of hydrochloric acid Clue: bubbles / gases Product: hydrogen gas and magnesium chloride
Chemical Equations A chemical reaction is often described by writing a chemical equation
Chemical EquationsA chemical equation uses either words or symbols andformulas to describe the changes that occur during achemical reaction.ExamplesWord equation:Hydrogen gas + oxygen gas waterFormula equation:H2 + O2 H2O
Chemical EquationsEvery chemical equation must have: One or more reactants One or more products An arrow directing reactant to product If there are more than one reactants or products, the chemical names/formulas are separated by a ‘+’ sign
Chemical Equations For example, the chemical reaction between solid magnesium metal and hydrochloric acid is: word equation: magnesium + hydrochloric acid magnesium chloride + hydrogen formula equation: Mg + HCl MgCl2 + H2 Notice that hydrogen is expressed in the formula equation as H2. Recall that pure hydrogen exists as a diatomic molecule. You will need to know which elements exist as molecules when writing formula equations
States of Matter inChemical EquationsThe chemical formulas in a chemical equation willoften include the state of matter of each substance(s) = solid(l) = liquid (e.g. water and oils)(g) = gas(aq) = aqueous (substance is dissolved inwater, e.g. most ionic compounds)Examples:H2 (g) + O2 (g) H2O (l)Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g)
Coefficients inChemical EquationsA coefficient is a whole number that is placed in frontof the symbol of an element to show the ratios of thedifferent substances that are present in the chemicalreactionExample: Mg + HCl MgCl2 + H2 Mg + 2HCl MgCl2 + H2A coefficient of 2 is in front of the formula HClThis means that Mg and HCl combine in a ratio of 1:2
Law of Conservation of MassIn a chemical reaction, the mass of the products alwaysequals the mass of the reactants.In other words, the mass is conserved.
Conservation of Mass No atoms are destroyed and no new atoms are produced during a chemical reaction. Instead, the atoms in the reactants are simply rearranged to form the products Chemical bonds between atoms are broken and new ones are formed, and the atom simply reconnect in new ways
Conservation of MassThe rearrangement of atoms that occurs during a chemicalreaction can be illustrated using models or diagrams.In this equation, there are equal numbers of hydrogen atoms (4)and equal numbers of oxygen atoms (2) on both the reactantsside and the products side.word equation: hydrogen + oxygen waterformula equation: H2 + O2 H2O
Conservation of MassWhen the number of each kind of atom is the same inthe reactants and products, the equation is said to bebalanced.balanced equation: 2H2 + O2 2H2O
Conservation of Mass
Skeleton Chemical EquationsA chemical equation that iscomplete except for coefficientsis called an unbalanced equationor skeleton equation.Example:Skeleton equation: H2 + O2 H2OBalanced equation: 2H2 + O2 2H2O
Balancing Chemical Equations To balance a chemical equation, begin by counting the number of atoms of each element in the skeleton equation. Balance by placing coefficients in front of the chemical formulas until the number of atoms in the reactants equals to the products.
Rules for using coefficients Use only whole numbers. Check that the coefficients in the equation are the lowest common factor. Never change a subscript in a formula to help make atoms balance!
Hints to help balance equationsBalance atoms of elements in any complicatedlooking formulas first and balance atoms ofpure elements last. H2
Hints to help balance equations Hydrogen atoms and/or oxygen atoms will often appear in many or all of the formulas of the reactants and products. When this is the case, balance other elements first, balance hydrogen second last and oxygen last.
Hints to help balance equationsYou may be able to treat polyatomic ions as aunit.Example: If NO3- appears in the reactants and productsof a skeleton equation, count the number of NO3-groups rather than the number of N and O atomsseparately.
Balancing Chemical EquationsExample 1:Balance the following chemical equation: AlBr3(s) + Cl2(g) AlCl3(s) + Br2(g)1.) Count the number of atoms in the reactants and products:
Balancing Chemical EquationsExample 1: 2AlBr3(s) + Cl2(g) AlCl3(s) + 3Br2(g)Balance the number of bromine atoms by adding a coefficientof 2 in front of AlBr3 and a coefficient of 3 in front of Br2. Countthe atoms again:
Balancing Chemical EquationsExample 1: 2AlBr3(s) + Cl2(g) AlCl3(s) + 3Br2(g)The number of aluminum atoms is no longer equal.
Balancing Chemical EquationsExample 1: 2AlBr3(s) + Cl2(g) 2AlCl3(s) + 3Br2(g)Balance the number of aluminum atoms by adding a coefficientof 2 in front of AlCl3. Count the atoms again:
Balancing Chemical EquationsExample 1: 2AlBr3(s) + Cl2(g) 2AlCl3(s) + 3Br2(g)The number of chlorine atoms is no longer balanced.
Balancing Chemical EquationsExample 1: 2AlBr3(s) + 3Cl2(g) 2AlCl3(s) + 3Br2(g)Balance the number of chlorine atoms by adding a coefficient of3 in front of Cl2. Count the atoms again:
Balancing Chemical EquationsExample 1: 2AlBr3(s) + 3Cl2(g) 2AlCl3(s) + 3Br2(g) The equation is balanced!
Balancing Chemical EquationsTry it!Balance the following chemical equations:1. Al + F2 AlF32. Ca + H2O Ca(OH)2 + H23. CaCl2 + Na3PO4 Ca3(PO4)2 + NaCl
Balancing Chemical EquationsTry it!Balance the following chemical equations:1. 2Al + 3F2 2AlF32. Ca + 2H2O Ca(OH)2 + H23. 3CaCl2 + 2Na3PO4 Ca3(PO4)2 + 6NaCl