CHEMISTRY CHAPTER 11

Stoichiometry

Main Ideas

The amount of each reactant present at the start of a chemical reaction determines how much product can form.

The solution to every stoichiometric problem requires a balanced chemical equation.

Percent yield is a measure of the efficiency of a chemical reaction.

•Describe the types of relationships indicated by a balanced chemical equation.•State the mole ratios from a balanced chemical equation.

10.1 Defining Stoichiometry: Objectives

Stoichiometry

Stoichiometry is the study of quantitative relationships between the amounts of products formed by a chemical reaction.

Based on the law of conservation of mass.Mass of the reactants must equal the mass of

the products.Molecules are not necessarily balanced on

each side. Atoms, moles and mass do.

Practice Problem #1

C3H8(g) + 5O2(g) 3CO2 + 4H2O(g)

Moles:

Mass:

Molecules are not necessarily balanced on each side. Atoms, moles and mass do.

Mole Ratio

Mole ratio is a ratio between the numbers of moles of any two substances in a balanced chemical equation.

The number of mole ratios that can be written for any equation is (n)x(n-1) where n is the number of substances in the chemical reaction.

Practice Problem #2

4Al(s) + 3O2(g) 2Al2O3(s)

Mole Ratios:

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

Mole Ratios:

Practice Problem #2

2HgO(s) 2Hg(l) + O2(g)

Mole Ratios:

Question

Which of the following is a correct mole ratio for the following equation?

2Al(s) + 3Br2(l) → 2AlBr3(s)

A. 2 mol Al : 3 mol BrB. 3 mol Br2 : 2 mol Al

C. 2 mol AlBr3 : 1 mol Br2

D. 2 mol Br : 2 mol Al

Question

How many mole ratios can be written for the following reaction?

4H2(g) + O2(g) → 2H2O(l)

A. 6 B. 4C. 3D. 2

Practice Problems

Page 371 #1 and 2Page 372 #4,5,6,9

•List the sequence of steps used in solving stoichiometric problems.•Solve stoichiometric problems.

10.2 Stoichiometric CalculationsObjectives

Steps to Solving Stoichiometric Problems

1. Write a balanced equation2. Determine the starting point of the

calculations.1. If units are mass units, convert to moles.

3. Use the correct reaction molar ratio to convert to moles of substance in question.

4. Convert moles to requested units if needed.

Steps to Solving Stoichiometric Problems

Practice Problem #3: Moles to Moles

When burning propane (C3H8), carbon dioxide is produced. How many moles of CO2 are produced when 10.0 mol of C3H8 are burned in excess oxygen?

Practice Problem #3: Moles to Moles

Calculate the moles of CS2 and H2S produced when 1.5 mol S8 is used?

CH4(g) + S8(s) CS2(l) + H2S(g)

Practice Problem #3: Moles to Moles

Sulfuric acid (H2SO4) is formed when sulfur dioxide (SO2) reacts with oxygen and water. How many moles of H2SO4 are produced from 12.5 moles of SO2? How many moles of O2 are needed?

Practice Problem #4: Moles to Mass

Sodium chloride is decomposed into the elements sodium and chlorine. How much chlorine gas, in grams is obtained when 2.50 mol of sodium choride are used?

Practice Problem #5: Mass to Mass

Acid rain (H2SO4) is formed from SO2, O2 and H2O. If 2.5g of SO2 reacts with excess oxygen and water, how much H2SO4 is produced?

Questions

How many moles of CO2 will be produced in the following reaction if the initial amount of reactants was 0.50 moles?

2NaHCO3 → Na2CO + CO2 + H2O

A. 0.25 B. 0.3C. 0.5D. 1.0

Questions

A chemical reaction equation must be ____ in order to make stoichiometric calculations.

A. measured B. controlled C. balanced D. produced

Practice Problems

Page 393 #61-63

•Identify the limiting reactant in a chemical equation.•Identify the excess reactant, and calculate the amount remaining after the reaction is complete.•Calculate the mass of a product when the amounts of more than one reactant are given.

10.3 Limiting ReactantObjectives

Limiting Reactant

The limiting reactant limits the extent of the reaction.

Determine the amount of product formed.A portion of all the other reactants remains

after the reaction stops.

Excess reactants are the reactants leftover when a reaction stops.

Supplies First Aid Kits

500 bandaids85 gauze rolls24 first aid

ointments24 burn sprays65 rolls of tape50 ace bandages

24 bandaids4 gauze rolls1 first aid ointment1 burn spray3 rolls of tape2 ace bandages

Example

Steps to Determining the Limiting Reactant

1. Determine moles of reactants.2. Use mole ratios to determine which reactant

is limiting.3. Analyze the excess reactant

1. Moles reacted2. Mass reacted 3. Excess remaining

Practice Problem #6

If 200.0g of sulfur reacts with 100.0g of chlorine, what mass of disulfur dichloride is produced?

S8(l) + 4Cl2(g) 4S2Cl2(l)

Questions

The mass of the final product in a chemical reaction is based on what?

A. the amount of excess reactant B. the amount of limiting reactant C. the presence of a catalyst D. the amount of O2 present

Questions

What is the excess reactant in the following reaction if you start with 50.0g of each reactant?

P4(s) + 5O2(g) → P4O10(s)

A. O2

B. P4

C. Both are equal. D. unable to determine

Practice Problems

page 383 #23-24

•Calculate the theoretical yield of a chemical reaction from data.•Determine the percent yield for a chemical equation.

10.4 Percent YieldObjectives

Theoretical Yield

Theoretical yield is the maximum amount of product that can be produced from a given amount of reactant.

Assumes that the reaction goes to completion in the forward direction.

Actual Yield

Actual yield is the amount of product produced when the chemical reaction is carried out in an experiment.

Percent Yield

Percent yield is the ratio of the actual yield expressed as a percent.

Represents how efficient a reaction is in producing the desired product

Percent yield = (actual yield/theoretical yield) x 100

Practice Problem #7

Solid silver chromate (AgCrO4) forms when potassium chromate (K2CrO4) is added to a solution containing .500g of silver nitrate (AgNO3). Determine the theoretical yield of Ag2CrO4. Calculate the percent yield if the reaction yields .455g of Ag2CrO4.

Practice Problem #7

AgNO3 + K2CrO4 Ag2CrO4 + ;

.500g AgNO3, theoretical yield of Ag2CrO4?

percent yield? .455g of Ag2CrO4

Questions

The amount of product that can be produced from a given amount of reactants based on stoichiometric calculations is:

A. actual yield B. percent yield C. theoretical yield D. stoichiometric yield

Questions

You calculate the theoretical yield of a chemical reaction starting with 50.0g of reactant is 25.0g of product. What is the percent yield if the actual yield is 22.0g of product?

A. 88% B. 44% C. 50% D. 97%

Practice Problems

page 387 #28-30

•Apply knowledge and skills from previous units to content learned in this unit.

10.4 Accumulating Content

Practice Problem #8

When an antacid tablet dissolves in water, the fizz is due to a reaction between sodium hydrogen carbonate (baking soda), also called sodium bicarbonate, and citric acid (H3C6H5O7). What are the masses of the products if there is 1.00 gram of each reactant in the tablet. Use the net ionic equation to solve this problem.What is the percent yield if only 1.25g of gas was produced? How does this affect the resulting mass of your other product?

Practice Problem #8

sodium bicarbonate, and citric acid (H3C6H5O7). What are the masses of the products if there is 1.00 gram of each reactant in the tablet. Use the net ionic equation to solve this problem.

Practice Problem #8

What is the percent yield if only 1.25g of gas was produced? How does this affect the resulting mass of your other product?

KEY CONCEPTS

Study Guide

Key Concepts

Balanced chemical equations can be interpreted in terms of moles, mass, and representative particles (atoms, molecules, formula units).

The law of conservation of mass applies to all chemical reactions.

Mole ratios are derived from the coefficients of a balanced chemical equation. Each mole ratio relates the number of moles of one reactant or product to the number of moles of another reactant or product in the chemical reaction.

Key Concepts

Chemists use stoichiometric calculations to predict the amounts of reactants used and products formed in specific reactions.

The first step in solving stoichiometric problems is writing the balanced chemical equation.

Mole ratios derived from the balanced chemical equation are used in stoichiometric calculations.

Stoichiometric problems make use of mole ratios to convert between mass and moles.

Key Concepts

The limiting reactant is the reactant that is completely consumed during a chemical reaction. Reactants that remain after the reaction stops are called excess reactants.

To determine the limiting reactant, the actual mole ratio of the available reactants must be compared with the ratio of the reactants obtained from the coefficients in the balanced chemical equation.

Stoichiometric calculations must be based on the limiting reactant.

Key Concepts

The theoretical yield of a chemical reaction is the maximum amount of product that can be produced from a given amount of reactant. Theoretical yield is calculated from the balanced chemical equation.

The actual yield is the amount of product produced. Actual yield must be obtained through experimentation

Key Concepts

Percent yield is the ratio of actual yield to theoretical yield expressed as a percent. High percent yield is important in reducing the cost of every product produced through chemical processes.

Questions

What law are all stoichiometric calculations based on?

A. law of definite proportions B. law of conservation of mass C. law of conservation of energy D. none of the above

Questions

The mole ratios can be determined only if what?

A. all the reactants are present in equal amounts

B. the reactants do not have coefficients

C. the products do not have coefficients

D. the equation is balanced

Questions

If the following reaction yields 5 mol NaAu(CN)2, how many moles of Au were present as reactants? (Assume all other reactants are in excess).4Au(s) + 8NaCN(aq) + O2 + 2H2O(l) → 4NaAu(CN)2(aq) + 4NaOH(aq)

A. 1 B. 4C. 5D. 20

Questions

In the following reaction, how many moles of NaCN are required to react with 5 mol of Au?4Au(s) + 8NaCN(aq) + O2 + 2H2O(l) → 4NaAu(CN)2(aq) + 4NaOH(aq)

A. 3 B. 5C. 8D. 10

Questions

In the following reaction, what mass of NaOH is produced if 5.0 moles of NaAu are also produced in the reaction? 4Au(s) + 8NaCN(aq) + O2 + 2H2O(l) → 4NaAu(CN)2(aq) + 4NaOH(aq)

A. 20 g B. 50 gC. 200 gD. 400 g

Questions

The SI base unit of amount is ____.A. the gram B. the kilogram C. the mole D. Avogadro’s number

Questions

Zinc reacts with iodine in a synthesis reaction: Zn + I2 Znl2. What is the theoretical yield of Znl2, if 1.912 mol of zinc is used?

A. 6.103 g B. 61.03 gC. 610.3 gD. 0.6103 g

Questions

In a chemical reaction, the statement that matter is neither created nor destroyed is based on what?

A. mole ratio B. law of conservation of mass C. Avogadro’s number D. law of definite proportions

Questions

Which is not a product that must be produced in a double replacement reaction?

A. water B. heat C. precipitates D. gases

Questions

The ____ is the maximum amount of product that can be produced from a given amount of reactant.

A. theoretical yield B. actual yieldC. limiting reactantD. excess reactant