Balanced chemical equations

Word equations

A chemical reaction can be summarised using a word equation.

sulfur + oxygen → sulfur dioxide

Chemical equations

In a chemical equation the names of substances are replaced by symbols and formulae.

S + O2 → SO2

Conservation of atoms

Atoms cannot be created or destroyed. This means the number of atoms in the reactants at the start of the reaction must equal the number of atoms in the products the end.

The equation above (S + O2 → SO2) is correct.

Atoms in the reactantsAtoms in the products
1 × S1 × S
2 × O2 × O

This means that the mass of the reactants must equal the mass of the products. This is known as conservation of mass.

Question

How many of each type of atom are there in the following?

C atomsO atoms
CO
CO2
2CO
2CO2
C atomsO atoms
CO11
CO212
2CO22
2CO224

Balancing chemical equations

In a chemical equation, if the names of substances are replaced by their symbols and formulae, most reactions are not correct - there is a different number of atoms in the reactants than in the products.

To make the chemical equation correct it must be balanced:

StepAction
1Write a word equation for the reaction.
2Write a simple equation by writing the name of each substance with its symbol or formula.
3Check whether the equation is balanced by counting the atoms of each element.
4If necessary, change the number of one of the molecules to make the atoms of at least one element balance. You may need to multiply both sides if the number of an atom on the right is not a multiple of the atoms on the left.
5Repeat steps 3 and 4 until the chemical equation is balanced.

Example

Carbon monoxide reacts with oxygen to produce carbon dioxide. Write the balanced chemical equation for this reaction.

StepResultEquation balanced?
1carbon monoxide + oxygen → carbon dioxide
2CO + O2 → CO2
3Reactants: 1 × C, (1 × O) + (2 × O) = 3 × O. Products: 1 × C, 2 × ONot balanced.
42CO + O2 → CO2
Step 3 (repeat)Reactants: 2 × C, (2 × O) + (2 × O) = 4 × O. Products: 1 × C, 2 × O.Not balanced.
Step 4 (repeat)2CO + O2 → 2CO2
Step 3 (repeat)Reactants: 2 × C, 4 × O. Products: 2 × C, (2 × 2 × O) = 4 × O.This chemical equation is now balanced.
Question

Write a balanced chemical equation for the complete combustion of methane (CH4) with oxygen (O2).

Word equation: methane + oxygen → carbon dioxide + water

Simple chemical equation: CH4 + O2 → CO2 + H2O

Reactants: 1 × C, 4 × H, 2 × O

Products: 1 × C, 3 × O, 2 × H not balanced

Try: CH4 + O2 → CO2 + 2H2O

Reactants: 1 × C, 4 × H, 2 × O

Products: 1 × C, 4 × H, 4 × O not balanced

Try: CH4 + 2O2 → CO2 + 2 H2O

Reactants: 1 × C, 4 × H, 4 × O

Products: 1 × C, 4 × H, 4 × O balanced

Balanced chemical equation: CH4 + 2O2 → CO2 + 2H2O

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