Students should be able to: write balanced equations for reactions studied…

The idea

Balancing is a bookkeeping exercise on coefficients only; the formulae themselves are fixed by the chemistry and must never be altered. Work through the elements that appear in fewest species first, leave oxygen and hydrogen until last, and clear any fraction at the end by doubling the whole equation, though examiners will accept fractions and multiples unless the question says otherwise. An unfamiliar reaction is no harder provided the question names the reactants and products: copy their formulae, then balance. For an ionic equation you must also balance charge, which is what makes 4 Mn²⁺ + MnO₄⁻ + 8 H⁺ → 5 Mn³⁺ + 4 H₂O work: the total charge is +15 on each side.

What earns the marks

• Correct formulae for every reactant and product named in the question.
• The numbers of atoms of each element balanced on both sides.
• The total charge balanced in an ionic or half-equation.
• An equation accepted in multiples or with fractions cleared consistently.

Common mistakes and how to put them right

• Altering a formula, such as writing water as H₂O₂, in order to make the atoms balance.

• Balancing the atoms but leaving the charges unequal in an ionic equation.

• Introducing a species that was not named among the reactants or products.

• Forgetting to rebalance hydrogen and oxygen after adding water to one side.

Exam tips

• Tally the atoms of each element under the equation and update the tally after every change.
• Balance charge last in a half-equation by adding electrons to whichever side needs them.
• State symbols are usually ignored unless the question asks for them, so spend your time on the coefficients.