Chapter 11: Stoichiometry
Section 11.1 Defining Stoichiometry
Section 11.2 Stoichiometric Calculations
Section 11.3 Limiting Reactants
Section 11.4 Percent Yield
SECTION11.1 Defining Stoichiometry
Particle and Mole Relationships
Chemical reactions stop when one of the reactants is used up.
Stoichiometry is the study of quantitative relationships between the amounts of reactants used and amounts of
products formed by a chemical reaction.
Stoichiometry is based on the law of conservation of mass.
The mass of reactants equals the mass of the products.
A mole ratio is a ratio between the numbers of moles of any two substances in a balanced equation.
The number of mole ratios that can be written for any equation is (n)(n – 1) where n is the number of species in the
chemical reaction.
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 Br
B.
3 mol Br2 : 2 mol Al
C.
2 mol AlBr3 : 1 mol Br2
D.
2 mol Br : 2 mol Al
SECTION11.2 Stoichiometric Calculations
All stoichiometric calculations begin with a balanced chemical equation.
4Fe(s) + 3O2(g) → 2Fe2O3(s)
Steps to solve mole-to-mole, mole-to-mass, and mass-to-mass stoichiometric problems
1. Write a ___________________________________ for the reaction.
2. Determine where to start your calculations by noting the ______________ of the given substance.
o If mass (in grams) of the given substance is the starting unit, you must ____________________
o If amount (in moles) of the given substance is the starting unit, convert _________ of the given substance
to __________ of the unknown.
3. The end point of the calculation depends on the desired _____________ of the unknown substance.
 If the answer must be in ________________, stop you are finished.
 If the answer must be in ________________, convert moles of unknown to grams of unknown using the
molar mass as the conversion factor.
Ex. The carbon dioxide exhaled by astronauts can be removed from a spacecraft by reacting with lithium hydroxide as
follows:
CO2(g) + LiOH(s) → Li2CO3(s) + H2O
An average person exhales about 20 moles of CO2 per day. How many moles of LiOH would be required to
maintain 2 astronauts in a spacecraft for three days?
Balance equation:
_________CO2 + _____LiOH → ___________Li2CO3 + ___________H2O
Determine moles of given substance:
20 moles per person:
2 people = ___________ moles x _________ days = ____________ moles of CO2
Convert moles of given substance to moles of unknown:
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.
0.3
C.
0.5
D.
1.0
SECTION11.3 Limiting Reactants
Why do reactions stop?
Reactions proceed until one of the reactants is __________________________________
The __________________________ limits the extent of the reaction and, thereby, determines the amount of product formed.
The __________________________ are all the leftover unused reactants.
Determining the limiting reactant is important because the amount of the product formed depends on this reactant.
Ex. S8(l) + 4Cl2(g) → 4S2Cl2(l)
If 200.0g of sulfur reacts with 100.0g of chlorine, what mass of disulfur dichloride is produced?
1. Determine moles of reactants
2. Determine whether the two reactants are in the correct mole ratio, as given in the balanced chemical equation.
Only 1.808 mol of chlorine is available for every 1 mol sulfur, instead of the 4mol of chlorine required by the
balanced chemical equation, thus chlorine is the limiting reactant.
3. Calculate the amount of product formed.
Now that you have determined the limiting reactant and the amount of product formed, what about the excess
reactant, sulfur? How much of it reacted?
1. You need to make a mole-to-mass calculation to determine the mass of sulfur needed to react completely with 1.410
mol of chlorine.
2. Next, obtain the mass of sulfur needed:
3. Knowing that 200.0g of sulfur is available and only 90.42g is needed, you can calculate the amount of sulfur left
unreacted when the reaction ends.
Using an excess reactant can ___________________________________
Using an excess reactant can ___________________________________
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
SECTION11.4 Percent Yield
How much product?
Laboratory reactions do not always produce the calculated amount of products.
Reactants ____________________ to containers.
___________________________________________ form other products.
The ___________________________ is the maximum amount of product that can be produced from a given amount of reactant.
The ___________________________ is the amount of product actually produced when the chemical reaction is carried out in
an experiment.
The percent yield of a product is the ratio of the actual yield expressed as a percent.
________________________ = ________________________ x100
Percent yield is important in the cost effectiveness of many industrial manufacturing processes.
If the theoretical yield of a chemical reaction that began with 50.0 g of reactant is 25.0 g of product. What is the
percent yield if the actual yield is 22.0g of product?
A.
88%
B.
44%
C.
50%
D.
97%