Reaction Stoichiometry

Objectives

• Understand the concept of stoichiometry.• Be able to make mass-to-mass stoichiometric

calculations.

Stoichiometry

stoichiometry: the process of calculating quantitiesinvolved in chemical reactions• balanced chemical equations are required• coefficients can represent the number of particles or the number of moles

N2 (g) + 3H2(g) → 2NH3(g)

1 mole 3 moles 2 molesmole ratio: ratio of moles involved in a reaction (using coefficients)

Stoichiometry Problem

N2 (g) + 3H2(g) → 2NH3(g)

If 25.7 g of nitrogen gas completely reacts with an excess of hydrogen gas, what mass of ammonia isproduced?

# g N2 → # mol N2 → # mol NH3 → #g NH3

g A → mol A → mol B → g B

A B A = what you knowB = what you want

This “pathway” is used in most stoichiometry problems

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

Stoichiometry Problem

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

Objectives

• Understand the distinction between actual yield and theoretical yield.

• Be able to determine the percentage yield for a chemical reaction.

Percentage Yield

theoretical yield: the maximum amount of productthat can be produced (calculated using stoichiometry)actual yield: the actual amount of product producedin a chemical reaction

percentage yie ld actua l yie ldtheore tica l yie ld 100

The percent yield cannot exceed 100%

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)

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.

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.

Which substance is the limiting reagent if 3.50 g of sulfur reacts with 2.50 g of aluminum to produce Al2S3? How much Al2S3 is actually produced?