1. Reaction Stoichiometry

2. Objectives Understand the concept of stoichiometry.
Be able to make mass-to-mass stoichiometric calculations.

3. Stoichiometry stoichiometry: the process of calculating quantities
involved in chemical reactions
balanced chemical equations are required
coefficients can represent the number of particles or
the number of moles
N2 (g) H2(g) → 2NH3(g)
1 mole moles moles
mole ratio: ratio of moles involved in a reaction
(using coefficients)

4. Stoichiometry Problem
If 25.7 g of nitrogen gas completely reacts with an
excess of hydrogen gas, what mass of ammonia is
produced?
A B
A = what you know
B = what you want
N2 (g) + 3H2(g) → 2NH3(g)
# g N2 → # mol N2 → # mol NH3 → #g NH3
g A → mol A → mol B → g B
This “pathway” is used in most
stoichiometry problems

5. Stoichiometry Problem
The process of smelting iron ore to iron can be
carried out with the following reaction. How much
ferric oxide is needed to produce 75.8 g of iron
using this method?
Fe2O3(s) CO(g) → 2 Fe(s) + 3 CO2(g)

6. Stoichiometry Problem
A copper penny combines with oxygen in a crucible
to form Cu2O.
Write a balanced chemical equation for the reaction.
What mass of oxygen is needed to completely react
with a 3.12 g copper penny?

7. Objectives
Understand the distinction between actual yield and theoretical yield.
Be able to determine the percentage yield for a chemical reaction.

8. Percentage Yield yield: an amount of product
theoretical yield: the maximum amount of product
that can be produced (calculated using stoichiometry)
actual yield: the actual amount of product produced
in a chemical reaction (what is measured)
The percent
yield cannot
exceed 100%

9. Percentage Yield Problem
What is the percentage yield if 4.65 g of copper is
produced when 1.87 g of aluminum reacts with excess cupric sulfate?
2 Al (s) + 3 CuSO4 (aq) → Al2(SO4)3 (aq) + 3 Cu (s)

10. Objective
You will be able to determine the limiting reagent (or reactant) for a chemical reaction when given the mass of two (or more) reactants.

11. Limiting Reagent
limiting reagent: the reactant that is completely consumed first in a reaction (so it limits the amount of product)
The reactant that yields the least product is the limiting reagent.

12. Limiting Reagent Problem
Which substance is the limiting reagent if 2.50 g of aluminum reacts with 3.50 g of sulfur to produce Al2S3? How much Al2S3 is actually produced?
2 Al + 3 S → Al2S3