Stoichiometry involves balancing equations

Stoichiometry is the quantitative study of the amounts of reactants and products in chemical reactions.

The key steps are:

1. Write the unbalanced chemical equation.
2. Balance the equation by adding stoichiometric coefficients.

Stoichiometric coefficients are numbers placed before each species to ensure the number of atoms of each element is the same on both sides of the equation.

Worked example 1 - Writing the balanced equation for the combustion of propane

Propane gas (C₃H₈) undergoes complete combustion in oxygen gas (O₂) to produce carbon dioxide gas (CO₂) and liquid water (H₂O). Write a balanced equation for this reaction.

Step 1: Write the unbalanced equation with reactants on the left and products on the right:

C₃H₈ + O₂ ➔ CO₂ + H₂O

Step 2: Count the number of each type of atom on both sides, adding stoichiometric coefficients to balance them:

Balance the carbon atoms by adding the coefficient "3" to CO₂:

• C₃H₈ + O₂ ➔ 3CO₂ + H₂O

Balance the hydrogen atoms by adding the coefficient "4" to H₂O:

• C₃H₈ + O₂ ➔ 3CO₂ + 4H₂O

Balance the oxygen atoms by adding the coefficient "5" to O₂:

• C₃H₈ + 5O₂ ➔ 3CO₂ + 4H₂O

Step 3: Include state symbols for each species (see table below):

C₃H_8(g) + 5O_2(g) ➔ 3CO_2(g) + 4H₂O_(l)

Mole ratios relate reactant and product amounts

The stoichiometric coefficients in a balanced chemical equation indicate the relative amounts of the reacting species.

The ratio of these coefficients is called the mole ratio. Reactants will always react in amounts proportional to their mole ratio to produce amounts of products proportional to their mole ratio.

For example, the balanced equation for respiration is:

C₆H₁₂O_6(aq) + 6O_2(g) ➔ 6CO_2(g) + 6H₂O_(l)

• The mole ratio of reactants C₆H₁₂O_{6 :} O₂ is 1:6.
• The mole ratio of products CO₂ : H₂O is 6:6, which simplifies to 1:1.

This means:

• C₆H₁₂O₆ and O₂ react in a 1:6 mole ratio.
• For each mole of glucose (C₆H₁₂O₆) consumed, 6 moles of CO₂ and 6 moles of H₂O are produced.