Reaction Stoichiometry

I. Introduction to Reaction Stoichiometry

A. Definitions

1. Stoichiometry - deals with the amount

of reactants and products in chemical

reactions

2. Mole Ratio - conversion factor that

relates the number of moles of any two

substances involved in a chemical

reaction

B. Types of Stoichiometry Problems

(A is the given, B is the unknown)

1. mole-mole (mole A mole B)

2. mole-mass (mole A mass B)

3. mass-mole (mass A mole B)

4. mass-mass (mass A mass B)

C. Diagram A is the given

B is what you need to find - the unknown

*All problems will include a mole B

conversion factor mole A

1 mole A # mole B molar mass B

MASS A -----> MOLE A ------> MOLE B ------> MASS B

molar mass A # mole A 1 mole B

III. Mole Ratios and Coefficients in Equations

A. 2H2 + O2 ----------> 2H2O

1) H2, O2 2) H2, H2O 3) O2, H2O

2 mol H2 1 mol O2 2 mol H2 2 mol H2O 1mol O2 2 mol H2O

1 mol O2 2 mol H2 2 mol H2O 2 mol H2 2 mol H2O 1 mol O2

B. 2KClO3 ----------> 2KCl + 3O2

1) KClO3, KCl 2)KClO3 , O2 3) KCl, O2

II. Solving Stoichiometry Problems (All Problems Include Mole Ratios)A. Mole-Mole electricity

1. 2 Na + Cl2 ----------> 2NaCl A – Given B - find

a) 4 moles of Na will react with (?) moles of Cl2? 4.0 mol Na X 1.0 mol Cl2 = 2 mol Cl2

1 2.0 mol Na

A. Mole –Mole Problem

1. 2 Na + Cl2 ----------> 2NaCl

A - Given B - Find

b)6 moles of Na will form (?) moles of NaCl?

6 mol Na X 2 mol NaCl = 6 mol NaCl

1 2 mol Na

B. Mole-Mass

1. CaO + H2O ----------> Ca(OH)2

A B

a)2 moles of CaO will produce(?)grams of Ca(OH)2

2 mol CaO x 1 mol Ca(OH)2 x 74.1g Ca(OH)2 =

1 1 mol CaO 1 mol Ca(OH)2

148 g Ca(OH)2

1. CaO + H2O ----------> Ca(OH)2

b) How many grams of CaO are needed to

form 2 moles of Ca(OH)2?

2 mol Ca(OH)2 x 1 mol CaO x 56.1g CaO =

1 1 mol Ca(OH)2 1 mol CaO

112g CaO

C. Mass-Mole

1. Zn + 2HCl ----------> ZnCl2 + 2H2

a) How many moles of zinc chloride are

formed when 196.2 grams of zinc react with hydrochloric acid?

196.2g Zn x 1 mol ZnCl2 = 3.001 mol ZnCl2

1 65.38g Zn

1. Zn + 2HCl ----------> ZnCl2 + 2H2

b) How many moles of zinc are necessary to

form 67.70 grams of zinc chloride?

67.70g ZnCl x 1 mol ZnCl2 x 1 mol Zn =

1 136.29 g ZnCl2 1 mol ZnCl2

.4967 mol Zn

D. Mass-Mass

1. CH4 + 2O2 ----------> CO2 + 2H2O

a) How many grams of carbon dioxide are formed

when 64.0 grams of oxygen react with methane?

64.0g O2 x 1 mol O2 x 1 mol CO2 x 44.0g CO2 =

1 32.0g O2 2 mol O2 1 mol CO2

44.0g CO2

1. CH4 + 2O2 ----------> CO2 + 2H2O

b) How many grams of methane are needed to

produce 48.0 grams of water?

48.0g H2O x 1 mol H2O x 1 mol CH4 x 16.0g CH4 =

1 18.0g H2O 2 mol H2O 1 mol CH4

21.3g CH4

III. Limiting Reactant

A. Definitions

1. limiting reactant – limits the extent of

reaction and determine the amount of

product (reactant that is used up first)

2. excess reactant – portion of reactant

that remains after a reaction is

complete

B. What are some examples that model

limiting reactant and excess reactant?

1. hamburger + bun

2. cake recipe requires 2 eggs and

3 cups of flour, and 1 cup sugar

C. How is the Limiting Reactant

Determined?

1. Divide number of moles of each

reactant by its coefficient in the

balanced equation

2. The substance with the smaller

number is the limiting reactant.

3. The substance with the larger number

is the excess reactant.

D. Example Determine the limiting reactant,the excess reactant reactant and the number of

moles of each product? (6 moles of H2 combined

with 3 moles of N2)

1. 3 H2(g) + N2(g) 2NH3(g) 6 mol 3 mol

a) 6/3 = 2 3/1 = 1

H2 is excess N2 is limiting

b) 3 mol N2 x 2 mol NH3 = 6 mol NH3

1 1 mol N2

Determine the limiting reactant, the excessreactant and the number of moles of each product

2. Zn(s) + 2HCl(aq) ZnCl2(aq) + H2(g) 2.5 mol Zn +3.5 mol HCl are combined

a) 2.5/1 = 2.5 3.5/2 = 1.75 Zn is excess HCl is limiting

b) 3.5 mol HCl x 1 mol ZnCl2 = 1.8 mol ZnCl2 1 2 mol HCl

c) 3.5 mol HCl x 1 mol H2 = 1.8 mol H2

1 2 mol HCl

IV. Percent Yield

A. Definitions

1. theoretical yield – maximum amount of

product that can be produced from a given

amount of reactant

2. actual yield – amount of product actually

produced during a reaction carried out in

the lab

3. percent yield – ratio of actual yield to the

theoretical yield expressed as a percent

% Yield = Actual yield (experiment results) X 100%

Theoretical yield (stoichiometric calculations)

B. Procedure

1. Determine theoretical yield ( mass-

mass stoichiometric calculation)

2. Divide actual yield (experiment results

in grams) by theoretical yield

3. Multiply by 100% to get percent yield

1. Zinc reacts with iodine in a synthesis reaction.

a. Determine the theoretical yield if 125.0g of zinc is used.

b. Determine the % yield if the mass of the product zinc

iodide recovered is 556g. Zn(s) + I2(s) ZnI2(s)

a)125.0g Zn x 1 mol Zn x 1 mol ZnI2 x 329.2g ZnI2 = 707.4g

1 65.4g Zn 1 mol Zn 1 mol Zn

Theoretical Yield is 707.4 g ZnI2

b) Actual yield………….. 556 = 78.6% (percent yield)

Theoretical yield……. 707.4

2. Water decomposes when subject to an electric current

to form hydrogen gas and oxygen gas. In the lab, 60.0 g of

oxygen is recovered when 72.0g of water is decomposed.

What is the theoretical yield? What is the percent yield?

electricity

2 H2O 2H2 + O2

72.0g H2O x 1 mol H2O x 1 mol O2 x 32.0g O2 = 64.0g O2

1 18.0g H2O 2 mol H2O 1 mol O2

Actual Yield…………60.0g = 93.8% (percent yield)

Theoretical Yield…...64.0g