Section 9.2 Equation Stoichiometry Chemical Equation – indicates the reactants and products in a rxn; it also tells you the relative amounts of reactants and products involved in the rxn. EX: 2 Mg + O2  2 MgO reactants products Stoichiometry – the study of the mathematical relationships involving chemical formulas and equations.

Chemical equations reflect the law of conservation of mass – matter is neither created nor destroyed in an ordinary chemical rxn. The mass of the products equals the mass of the reactants that reacted. This is represented in an equation by having the same number of each atom on both sides of the equation. Coefficients are used to balance equations.

(Review) Coefficients can stand for: 1) The relative number of atoms, molecules, or formula units 2) The relative number of moles 3) The relative volume of gases EX: 2 H2 + O2  2 H2O 2 molecules H2 + 1 molecule O2  2 molecules H2O 2 mol H2 + 1 mol O2  2 mol H2O 2 volumes H2 + 1 volume O2  2 volumes H2O

***Coefficients tell you the relative number of moles of each reactant and product involved in a rxn.*** mole ratio (coefficient ratio) – a conversion factor that allows you to convert from moles of one substance to moles of another substance in a rxn. EX: CH4 + 2 O2  CO2 + 2 H2O Example mole ratios: What are the possible the mole ratios for: 3 Mg + 2 P  Mg3P2

I. Mole – Mole Problems You are given moles of one substance in a rxn and must find the moles of another substance. Use the mole ratio from the equation to solve Mole-mole problems: Moles of given substance coefficient of substance sought coefficient of given substance = Moles of substance sought

Example mole-mole problems 1. For the rxn N2 + 3 H2  2 NH3 a) How many moles of N2 will react with 8.0 mol of H2? b) How many moles of NH3 can be made from 12.0 mol of H2? 2. For the rxn C5H12O + 6 O2  5 CO2 + 3 H2O a) If 127 mol of CO2 are produced, how many moles of H2O are produced? If 15 mol of O2 react, how many moles of CO2 are produced?

Suppose you have two substances, “A” and “B”, in a rxn. The following flow chart shows the stoichiometric relationship between them: GIVEN GRAMS OF “A” (mass) MOLES OF “A” MOLES OF “B” SOUGHT GRAMS OF “B” SOUGHT (mass) 1 mol A . molar mass of A molar mass of B . 1 mol B coefficient of sought (B) coefficient of given (A)

III. Mass-Mass Problems (3 steps) EX: For the rxn N2 + 3 H2  2 NH3 1) How many grams of H2 will react with 237g of N2? You are given the mass (grams) of one substance in a rxn, and are asked to find the number of grams (mass) of another substance. 2) How many grams of N2 do you need to produce 82.8 g NH3?

The Mole Road

EX2: Decane, C10H22 , is a component of gasoline. 1) How many grams of CO2 will be produced by the complete combustion of 345g of C10H22 ? 2) What mass of oxygen reacted if 3.5 x 104 g of H2O was produced?

EX3: Aluminum metal can be produced by the electrolytic decomposition of aluminum oxide. How many kilograms of Al can be produced by decomposing 1.0 x 103 grams of aluminum oxide? 2) How many grams of oxygen would be consumed in the production of 3.5 x 106 g of Aluminum?

Cake Recipe To make one cake you need: 3 eggs, 4 cups flour, 2 qts of milk, and 1 cup of sugar. You have: 12 eggs, 12 cups flour, 10 qts of milk, and 6 cups of sugar. How many cakes can you make?

Limiting Reactants

In most reactions that occur, you don’t use “exact” amounts of each reactant. Usually one reactant gets used up before the other. limiting reactant – the reactant that gets totally used up in a rxn; it’s the reactant you run out of. (It “limits” the rxn.) excess reactant – any reactant that is not totally used up in a rxn; you still have some of it left after the rxn stops.

The limiting reactant would be H2. In this example, 10 molecules of H2 react with 7 molecules of O2, according to the equation: 2 H2 + O2  2 H2O The limiting reactant would be H2. The excess reactant would be O2.

Limiting Reactant Problems You are given the initial amounts of the reactants in a rxn, and must determine: a) which reactant is the limiting reactant. You also should be able to determine: b) how much product is produced, and c) how much of the excess reactant is left over. **a) To determine which reactant is the limiting reactant:** Find out how much product can be made from the 1st reactant Find out how much product can be made from the 2nd reactant Which ever reactant produces the LEAST product is the limiting reactant. (In other words: You need to do two mass-mass problems to figure out the limiting reactant.)

EX: 125g of Br2 and 75.0g Li are reacted together according to the equation: 2 Li + Br2  2 LiBr What is the limiting reactant?

**b) The amount of product actually produced is the amount produced by the limiting reactant.** In the example above, 136g of LiBr would actually be produced. EX: 24.0g of CH4 is burned in a closed container that holds 50.0g of O2. a) What is the limiting reactant? b) How many grams of CO2 is produced?

Suppose you have two substances, “A” and “B”, in a rxn. The following flow chart shows the stoichiometric relationship between them: GIVEN GRAMS OF “A” (mass) MOLES OF “A” MOLES OF “B” SOUGHT GRAMS OF “B” SOUGHT (mass) 1 mol A . X molar mass of A molar mass X of B . 1 mol B coefficient of sought (B) X coefficient of given (A)

Practice Limiting Reactant Problems EX: 50.0g of Na is reacted with 50.0g of Cl2 in synthesis reaction. a) what is the limiting reactant? b) how much NaCl is produced?

EX: N2H4 + 2 H2O2  N2 + 4 H2O 65.0g of N2H4 reacts with 75.0g of H2O2 a) what is the limiting reactant? b) how much N2 is produced?

Theoretical and Percent Yield Theoretical Yield – the maximum amount of product that can be produced in a rxn. Theoretical yield is determined by using the rxn’s equation and doing a mass-mass problem. (The amount that “theoretically” could have been produced.) Actual yield – the measured amount of product collected in an experiment. (The amount “actually” produced.)

Percent Yield Actual Yield Percent Yield = x 100 Theoretical Yield A comparison of the amount actually obtained to the amount it was possible to make Actual Yield Theoretical Yield Percent Yield = x 100

Percent Yield Problems 1. In an experiment, 100. g of Na is reacted with excess Cl2. 239 g of NaCl is collected. a) What is the theoretical yield of NaCl? b) What is the percent yield of NaCl? 2. 150. g of O2 is reacted with excess Mg. 300. g of MgO is collected. a) What is the theoretical yield of MgO? b) What is the percent yield of MgO?

Solution Stoichiometry Problems

Review: Molarity moles of solute Molarity (M) = liters of solution Molarity is used to convert between volume of solution and moles of dissolved solute. 1. How many moles of NaCl are in 3.5 L of 2.0 M NaCl? 2. How many moles AgNO3 are in 125 mL 3.00 M AgNO3?

coefficient of sought (B) VOLUME OF “A” GIVEN GIVEN GRAMS OF “A” (mass) MOLES OF “A” 1 mol A . X molar mass of A MOLES OF “B” SOUGHT coefficient of sought (B) X coefficient of given (A) GRAMS OF “B” SOUGHT (mass) molar mass X of B . 1 mol B

Example Solution Stoich Problems 1) Excess Cu is placed in 250 mL of 0.50 M AgNO3 and the following rxn occurs: 2 AgNO3 + Cu  2 Ag + Cu(NO3)2 What mass of Ag can be produced? 2) 125 mL of 2.00 M Pb(NO3)2 is mixed with excess NaCl and the following rxn occurs: 2 NaCl + Pb(NO3)2  PbCl2 + 2 NaNO3 What mass of PbCl2 precipitates?