1. Limiting Reactants and Percent Yield
Reactions are rarely carried out with exactly the required amount of each reactants
In most reactions, one or more reactants is in excess.

2. Limiting Reactants
Limiting reactant: The substance that runs out first.
Excess reactant: The substance that is left over after the reaction
So…If you have 50 bicycle frames and 80 bicycle wheels, how many bicycles can be made. What limits the number of bicycles?

3. Why Use an Excess Reactant?
To ensure that the other reactant is completely used up
Cheaper and easier to recycle

4. Steps for determining Limiting Reactants
Write a balanced equation
For each reactant, calculate the amount of product formed
Smallest answer indicates:
Limiting reactant
Amount of product

5. excess limiting amount produced
Ex 1) g Zn reacts with 82.0 g HCl. Identify the limiting and excess reactants. How much hydrogen will be formed?
Zn + 2HCl → ZnCl2 + H2
excess
79.1 g Zn
1 mol Zn
1 mol H2
2 g H2 =
2.42
g H2
65.39 g Zn
1 mol Zn
1 mol H2
82. g HCl
1 mol HCl
1 mol H2
2 g H2 =
1.13
g H2
36.46 g HCl
2 mol HCl
1 mol H2
limiting
amount produced

6. amount produced limiting excess
Ex 2) g Zn reacts with 65.3 g HCl. Identify the limiting and excess reactants. How much hydrogen will be formed?
Zn + 2HCl → ZnCl2 + H2
amount produced
limiting
54.7 g Zn
1 mol Zn
1 mol H2
2 g H2 =
1.67
g H2
65.39 g Zn
1 mol Zn
1 mol H2
65.3 g HCl
1 mol HCl
1 mol H2
2 g H2 =
1.79
g H2
36.46 g HCl
2 mol HCl
1 mol H2
excess

7. Percent Yield
Batting average in baseball is a measurement of the percent of times a player successfully completes a time at bat.

8. Percent Yield
Similar calculations show success of a chemical reaction. Why?
most reactions do not produce the predicted amount of product.
Reactions fail to go to completion
Liquid adheres to containers or evaporates
Solids get stuck in filter paper or lost in the purification process.

9. Percent Yield Theoretical yield- the amount that should be produced.
(found by stoichiometry)
Actual yield- the amount that is actually produced.
(found by experimentation)
Percent yield- the percent you actually got

10. Percent Yield

11. Percent Yield
Ex3) If the theoretical yield is 57.3 g P4O10, the reaction experimentally produced 52.5 g. What is the percent yield?
Sometimes you must first calculate the theoretical yield (stoichiometry)