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Equations Law of Conservation of Matter Balancing Chemical Equations 3-1 Stoichiometry Stoichiometry The study of quantitative relationships between substances undergoing chemical changes. Law of Conservation of Matter In chemical reactions, the quantity of matter does not change. The total mass of the products must equal that of the reactants. 3-2 Chemical equations Chemist’s shorthand to describe a reaction. It shows: • All reactants and products • The physical state of all substances • Any conditions used in the reaction CaCO3 (s) Reactant CaO (s) + CO2 (g) Products A balanced equation shows the relationship between the quantities of all reactants and products. 3-3 Balancing chemical equations Each side of a chemical equation must have the same number of each type of atom. CaCO3 (s) Reactants 1 Ca 1C 3O CaO (s) + CO2 (g) Products 1 Ca 1C 3O 3-4 Balancing chemical equations Step 1 Count the number of atoms of each element on each side of the equation. Step 2 Determine which atom numbers are not balanced. Step 3 Balance one atom at a time by using coefficients in front of one or more substances. Step 4 Repeat steps 1-3 until everything is balanced. 3-5 Balance the following HCl + Ca _____> CaCl2 + H2 Step 1 & 2 1 H 2 H - not balanced 1 Cl 2 Cl - not balanced 1 Ca 1 Ca Step 3 2 HCl + Ca _____> CaCl2 + H2 Step 4 2H 2 Cl 1 Ca 2H 2 Cl 1 Ca The balanced equation 2 HCl + Ca _____> CaCl2 + H2 3-6 Balance the following C2H6 + O2 _____> CO2 + H2O Step 1 & 2 2C 6H 2O 1 C - not balanced 2 H - not balanced 3 O - not balanced Step 3 Balance biggest molecule first, C2H6 C2H6 + O2 _____> 2 CO2 + 3 H2O Step 4 2C 2C 6H 6H 2O 7 O - not balanced 3-7 Balance the following C2H6 + O2 _____ > 2 CO2 + 3 H2O Step 3a Now let’s balance O2 C2H6 + 3.5 O2 _____> 2 CO2 + 3 H2O Step 4a 2C 2C 6H 6H 7O 7O You can’t have 3.5 O2 so double the equation! Balanced Equation 2 C2H6 + 7 O2 _____> 4 CO2 + 6 H2O 3-8 Balance the following Sometimes equation are too difficult to balance simply by inspection. Try this one: Cu + HNO3 Cu(NO3)2 + H2O + NO Step 1 & 2 1 Cu 1H 1N 3O 1 Cu - balanced 2 H - not balanced 3 N - not balanced 8 O - not balanced Step 3 The N and O elements appear in more than just one react and one product, which means they must be balanced simultaneously. 3-9 Balancing chemical equations with Algebra Step 1 Write correct chemical formulas for all reactants and products. Step 2 Assign a variable (a, b, c, etc.) for the coefficient of each reactant and product. Step 3 Write an expression for each element by counting the number of atoms present in each reactant and multiplying by the coefficient, then repeat for the elements on the product side. 3 - 10 Balancing chemical equations with Algebra Step 4 The sum of the reactant values must equal the sum of the product values. Step 5 Let one variable have a value of 1 and solve for the other variables. Step 6 Express all of the coefficients as a whole-number ratio. Step 7 Recheck by counting the total number of atoms of each element in the reactants as compared to those in the products. The must be equal. 3 - 11 Balance an Equation Using Algebra a Cu + b HNO3 c Cu(NO3)2 + d H2O + e NO For Cu a = c For H b = 2d For N b = 2c + e For O 3b = 6c + d + e Let d = 1 b = 2(1) = 2 2 = 2c + e e = 2 - 2c 3(2) = 6c + 1 + (2 - 2c) c = 3/4 = a e = 2 - 2(3/4) e = 2/4 or 1/2 3/4Cu + 2HNO3 3/4Cu(NO3)2 + 1H2O + 1/2NO