Stoichiometry

Limiting Reactions and Percentage Yield

Stoichiometry Objectives

Describe a method for determining which of two reactants is a limiting reactant

Calculate the amount in moles or mass in grams of a product, given the amounts in moles or masses in grams of two reactants, one of which is in excess

Distinguish between theoretical yield, actual yield, and percentage yield

Calculate percentage yield, given the actual yield and quantity of a reactant

Stoichiometry Limiting Reactions

The limiting reactant is the reactant that limits the amount of the other reactant that can combine and the amount of product that can form in a chemical reaction

The excess reactant is the substance that is not used up completely in a reaction

Stoichiometry Limiting Reactants

Sample Problem

Silicon dioxide (quartz) is usually quite unreactive but reacts readily with hydrogen fluoride according to the following equation

If 6.0 mol HF is added to 4.5 mol SiO2, which is the limiting reactant?

SiO2(s) + 4HF(g) → SiF4(g) + 2H2O(l)

Limiting Reactants

Sample Problem Solution

Given: amount of HF = 6.0 mol

amount of SiO2 = 4.5 mol Unknown: limiting reactant

4

4

mol SiFmol HF mol SiF produced

mol HF

4

2 42

mol SiFmol SiO mol SiF produced

mol SiO

Stoichiometry

Limiting Reactants

Sample Problem Solution

Stoichiometry

4

2 42

1 mol SiF4.5 mol SiO 4.5 mol SiF produced

1 mol SiO

4

4

1 mol SiF6.0 mol HF 1.5 mol SiF produced

4 mol HF

HF is the limiting reactant.

Stoichiometry Percentage Yield

The theoretical yield is the maximum amount of product that can be produced from a given amount of reactant

The actual yield of a product is the measured amount of that product obtained from a reaction

The percentage yield is the ratio of the actual yield to the theoretical yield, multiplied by 100

actual yieldpercentage yield 100

theorectical yield

Stoichiometry Percentage Yield

Sample Problem

Chlorobenzene, C6H5Cl, is used in the production of many important chemicals, such as aspirin, dyes, and disinfectants. One industrial method of preparing chlorobenzene is to react benzene, C6H6, with chlorine, as represented by the following equation

When 36.8 g C6H6 react with an excess of Cl2, the actual yield of C6H5Cl is 38.8 g.

What is the percentage yield of C6H5Cl?

C6H6 (l) + Cl2(g) → C6H5Cl(l) + HCl(g)

6 6 6 5 6 56 6

6 6 6 6 6 5

6 5

1 mol C H 1 mol C H Cl 112.56 g C H Cl36.8 g C H

78.12 g C H 1 mol C H 1 mol C H Cl

53.0 g C H Cl

Percentage Yield

Sample Problem Solution

Given: mass of C6H6 = 36.8 g

mass of Cl2 = excess

actual yield of C6H5Cl = 38.8 g

Unknown: percentage yield of C6H5Cl

Theoretical Yield

Stoichiometry

6 6 6 5 6 56 6 6 5

6 6 6 6 6 5

mol C H mol C H Cl g C H Clg C H g C H Cl

g C H mol C H mol C H Cl

Stoichiometry Percentage Yield

Sample Problem Solution

Percentage Yield

6 5

actual yieldpercentage yield C H Cl 100

theorectical yield

38.8 gpercentage yield 100

53.0 g73.2%

Stoichiometry

Sodium carbonate reacts with nitric acid according to the following equation.

Na2CO3(s) + 2HNO3 2NaNO3 + CO2 + H2O

How many moles of Na2CO3 are required to produce 100.0 g of NaNO3?

Stoichiometry

Phosphorus burns in air to produce a phosphorus oxide in the following reaction:

4P(s) + 5O2(g) P4O10(s)

What mass of phosphorus will be needed to produce 3.25 mol of P4O10?

Stoichiometry

Aluminum will react with sulfuric acid in the following reaction.

2Al(s) + 3H2SO4(l) Al2(SO4)3(aq) + 3H2(g)

How many moles of H2SO4 will react with 18 mol Al?

Stoichiometry

In the commercial production of the element arsenic, arsenic(III) oxide is heated with carbon, which reduces the oxide to the metal according to the following equation:

2As2O3 + 3C 3CO2 +4As

If 8.87 g of As2O3 is used in the reaction and 5.33 g of As is produced, what is the percentage yield?

Stoichiometry

A process by which zirconium metal can be produced from the mineral zirconium(IV) orthosilicate, ZrSiO4, starts by reacting it with chlorine gas to form zirconium(IV) chloride.

ZrSiO4 + 2Cl2 ZrCl4 + SiO2 + O2

What mass of ZrCl4 can be produced if 862 g of ZrSiO4 and 950. g of Cl2 are available?

Stoichiometry

Calculate the mass of silver bromide produced from 22.5 g of silver nitrate in the following reaction:

2AgNO3(aq) + MgBr2(aq) 2AgBr(s) + Mg(NO3)2(aq)

Stoichiometry

Use the following equation for the oxidation of aluminum in the following problems.

4Al + 3O2 2Al2O3

Which reactant is limiting if 0.32 mol Al and 0.26 mol O2 are available?

Stoichiometry

Hydrogen peroxide breaks down, releasing oxygen, in the following reaction:

2H2O2(aq) 2H2O(l) + O2(g)

What mass of oxygen is produced when 1.840 mol of H2O2 decomposes?

Stoichiometry

Hydrogen is generated by passing hot steam over iron, which oxidizes to form Fe3O4, in the following equation.

3Fe(s) + 4H2O(g) 4H2(g) + Fe3O4(s)

If 625 g of Fe3O4 is produced in the reaction, how many moles of hydrogen are produced at the same time?

Stoichiometry

Aluminum oxidizes according to the following equation:

4Al + 3O2 2Al2O3

Powdered Al (0.048 mol) is placed into a container containing 0.030 mol O2. What is the limiting reactant?

Stoichiometry