Neutralisation of acids and salt production — AQA GCSE Chemistry
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Neutralisation of acids and salt production explained
When an acid neutralises a base, alkali or carbonate, the salt formed is determined by two things: the acid's anion and the positive ion from the base, alkali or carbonate.
Read the full explanation
Hydrochloric acid supplies Cl⁻ and gives chloride salts; nitric acid supplies NO₃⁻ and gives nitrates; sulfuric acid supplies SO₄²⁻ and gives sulfates. The positive ion comes from the metal or ammonium in the base, alkali or carbonate. For example, sodium hydroxide with hydrochloric acid gives sodium chloride; copper oxide with sulfuric acid gives copper sulfate; calcium carbonate with nitric acid gives calcium nitrate. Hydrogen ions from the acid combine with hydroxide ions to form water, or with carbonate ions to release carbon dioxide and water, while the remaining ions form the salt.
Your focus
- Predict the salt produced when a named acid reacts with a named base, alkali or carbonate.
- Deduce the formula of a salt from the formulae of its positive and negative ions.
- Explain how the acid and the positive ion together determine the identity of the salt.
Neutralisation of acids and salt production exam tips
Marking Points
- Identify the acid used and state the anion it provides: hydrochloric acid gives chloride ions (Cl⁻), nitric acid gives nitrate ions (NO₃⁻), sulfuric acid gives sulfate ions (SO₄²⁻).
- Identify the positive ion from the base, alkali or carbonate, such as Na⁺ from sodium hydroxide, Cu²⁺ from copper oxide, or NH₄⁺ from ammonium carbonate.
- Combine the correct positive and negative ions in the correct ratio so that the charges balance, for example two Na⁺ with one SO₄²⁻ gives Na₂SO₄.
- Name the salt by giving the positive ion name first and the acid-derived anion name second, for example sodium sulfate or ammonium nitrate.
- Write the salt formula using the formulae of common ions, ensuring subscripts show the ratio needed for electrical neutrality.
- Recognise that the reaction may also produce water, and carbon dioxide when a carbonate is used, but the salt is still determined by the acid and the positive ion.
Examiner Tips
- 💡Before naming a salt, write down the acid's anion and the positive ion from the other reactant, then combine them.
- 💡Check the formula by adding up charges: the total positive charge must equal the total negative charge.
- 💡If a carbonate is used, remember that carbon dioxide and water are also produced, but the question may only ask for the salt.
- 💡Practise common ions such as Na⁺, K⁺, Mg²⁺, Ca²⁺, Cu²⁺, Al³⁺, Cl⁻, NO₃⁻, SO₄²⁻ and CO₃²⁻ so you can deduce formulae quickly.
Common Mistakes
- Using the acid name unchanged in the salt name, such as saying hydrochloric acid produces sodium hydrochloric; the correction is to use the anion-derived ending, so sodium chloride.
- Ignoring charge balance when writing formulae, such as writing NaSO₄ for sodium sulfate; the correction is to use two sodium ions with one sulfate ion, giving Na₂SO₄.
- Assuming the positive ion comes from the acid rather than the base, alkali or carbonate; the correction is to take the positive ion from the other reactant, for example Mg²⁺ from magnesium oxide.
- Forgetting that sulfuric acid produces sulfate salts, not sulfide or sulfite salts; the correction is to link sulfuric acid specifically to the sulfate ion SO₄²⁻.