4.07 Limiting Reactant

Steps for Solving Limited Reactant Problems

1) Balance the equation. 2) Identify the given amounts provided in the word problem, as well as what you are being asked to solve for. 3) Determine if this is a limiting reactant problem. If the word problem provides a given amount for more than one reactant, you know it is a limiting reactant problem. This means you will set up and solve more than two stoichiometry calculations. 4) Set up a stoichiometry calculation for each given amount of reactant, solving for the same unit of the same product in both. 5) The lower answer is the theoretical yield (the amount of product that can be produced by the given amount of reactants), and the reactant that produced the lower answer is the limiting reactant.

Example Three: If 45 grams of nitrogen gas reacts with 32 grams of oxygen gas , how many grams of nitrogen dioxide will be produced? N2 + O2 → NO2

1) Balance the equation: N2 + 2O2 → 2NO2 2) Given: 45 g N2 and 32 g O2. Looking for grams of nitrogen oxide. 3) This is a limiting reactant problem because there are 2 given amounts of reactants. 4) Start with the grams of each of the reactants and solve for grams of the product. 5) O2 is the limiting reactant because there are only 46 grams produced as opposed to 148 g of NO2 produced by N2. N2 is the excess reactant. 45 g x 1 mol N2/ 28 g N2 x 2 mol NO2/ 1 mol N2 x 46 g NO2/ 1 mol N2 = 148 g NO2 32 g x 1 mol O2/ 32 g O2 x 2 mol NO2/ 1 mol O2 x 46 g NO2/ 1 mol N2 = 46 g NO2

Practice Problems

1) How many grams of ZrCl4 can be produced if 123.0 g of ZrSiO4 react with 85.0 g of Cl2? ZrSiO4 + Cl2 → ZrCl4 +SiO2 + O2 Answer: 140 g ZrCl4 2) How many grams of calcium hydroxide will be formed in this reaction when 4.44 g of calcium oxide and 7.77 g of water are available to react? CaO (s) + H2O (l) → Ca(OH)2 (aq) Answer: 5.87 g Ca(OH)2 3) If 11.3 g of sulfur reacts with 14.0 g of oxygen, how many grams of sulfur trioxide could be formed? S + O2 → SO3 Answer: 23.4 g SO3

Example Two: If 35.0 grams of carbon reacts with 58.5 grams of oxygen gas, how many grams of carbon dioxide can be produced? C + O2 → CO2

1) The first thing you need to always make sure you do is balance the chemical equation. C + O2 → CO2 The equation is already balanced. 2) Identify the given amounts provided in the word problem, as well as what you are being asked to solve for. If 35.0 grams of carbon reacts with 58.5 grams of oxygen gas, how many grams of carbon dioxide can be produced? Given amount of reactants are: 35.0 g C and 58.5 g O2 Solving for the theoretical yield of: carbon dioxide. 3) Determine if this is a limiting reactant problem. Because there is a given amount for more than one reactant, this is a limiting reactant problem and you will be setting up two separate stoichiometry calculations. One calculation will start with 35.0 g C and solve for grams of CO2, and the other will start with 58.5 g O2 and solve for grams of CO2. 4) Set up a stoichiometry calculation for each given amount of reactant, solving for the same unit of the same product in both. Start each stoichiometry calculation with the given amount of one of the reactants. Because the word problem asked how many grams of CO2 can be formed, each stoichiometry calculation should solve for grams of CO2. 35.0 g C x 1 mol C/ 12.01 g C x 1 mol CO2/ 1 mol C x 44.01 g CO2/ 1 mol CO2 = 128 g CO2 58.5 g O2 x 1 mol O2/ 32.0 g O2 x 1 mol CO2/ 1 mol O2 x 44.01 g CO2/ 1 mol CO2 = 80.5 g CO2

Example One: If 3.0 moles of HF react with 4.5 moles of SiO2, how many moles of SiF4 can be formed? SiO2 (s) + HF (g) → SiF4 (g) + H2O (l)

1) The first thing you need to always make sure you do is balance the chemical equation. SiO2 (s) + 4 HF (g) → SiF4 (g) + 2 H2O (l) 2) Identify the given amounts provided in the word problem, as well as what you are being asked to solve for. If 3.0 moles of HF react with 4.5 moles of SiO2, how many moles of SiF4 can be formed? Given amount of reactants are: 3.0 moles of HF and 4.5 moles of SiO2 Soving for the theoretical yield of: moles of SiF4 3) Determine if this is a limiting reactant problem. If the word problem provides a given amount for more than one reactant, you know that this is a limiting reactant problem. This means that you will set up and solve more than one stoichiometry calculation. Given information: 3.0 moles of HF and 4.5 moles of SiO2 This is a limiting reactant problem, and you will be setting up two separate stoichiometry calculations (one starting with 3.0 mol HF and the other starting with 4.5 mol SiO2). 4) Set up a stoichiometry calculation for each given amount of reactant, solving for the same unit of the same product in both. Start each stoichiometry calculation with the given amount of one of the reactants. Because the word problem asked how many moles of SiF4 can be formed, each stoichiometry calculation should solve for moles of SiF4. 5) The lower answer is the theoretical yield (the amount of product that can be produced by the given amount of reactants), and the reactant that produced that lower answer is the limiting reactant. 3.0 mol HF x 1 mol SiF4/ 4 mol HF = 0.75 mol SiF4 4.5 mol SiO2 x 1 mol SiF4/ 1 mol SiO2 = 4.5 mol SiF4

Theoretical Yield

The maximum amount of product that can be produced by a given amount of reactant(s). The amount of product able to be made before the limiting reactant runs out.

Excess Reactant

The reactant that is not used up in a reaction. The reactant that is not used completely in a chemical reaction.

Limiting Reactant

The reactant that is used up first in a reaction. The reactant that controls the amount of product able to be produced by a chemical reaction because it is used up completely. Because the reaction stops when one reactant runs out, the limiting reactant determines the amount of product able to be produced in the chemical reaction.