Movement and Interactions: Acids and Alkalis — AQA Combined Science: Synergy
This topic covers reactions of acids, making salts, energy changes in reactions, the pH scale and neutralisation.
Reactions of acids
Acids react in predictable ways:
- acid + metal → salt + hydrogen
- acid + base (metal oxide/hydroxide) → salt + water (neutralisation)
- acid + metal carbonate → salt + water + carbon dioxide
The salt depends on the acid: hydrochloric → chlorides, sulfuric → sulfates, nitric → nitrates. Test hydrogen with a lighted splint (squeaky pop) and carbon dioxide with limewater (turns cloudy).
Making salts
A soluble salt can be made from an acid and an insoluble base (e.g. copper oxide + sulfuric acid → copper sulfate):
- Add excess base to warm acid until no more reacts.
- Filter off the excess solid.
- Crystallise the solution and leave the crystals to dry.
Required practical: preparing a pure, dry sample of a soluble salt.
Energy changes in reactions
- Exothermic reactions transfer energy to the surroundings (temperature rises) — e.g. neutralisation, combustion, many displacement reactions.
- Endothermic reactions take in energy (temperature falls) — e.g. some dissolving and thermal decomposition.
Required practical: measuring temperature changes during reactions such as neutralisation or displacement.
The pH scale and neutralisation
The pH scale (0–14) measures acidity/alkalinity, using universal indicator or a pH probe.
- pH < 7 acidic (produces H⁺ ions); pH 7 neutral; pH > 7 alkaline (produces OH⁻ ions).
- Neutralisation: H⁺ + OH⁻ → H₂O.
Strong and weak acids (Higher Tier)
- Strong acids (hydrochloric, sulfuric, nitric) are fully ionised in water.
- Weak acids (ethanoic, citric, carbonic) are only partially ionised.
- As pH decreases by one unit, the H⁺ concentration increases tenfold.
- Strength (degree of ionisation) is different from concentration (amount dissolved).
Exam tips
- Learn the three general acid reaction patterns and match acids to their salts.
- Know the salt-preparation method steps and the gas tests.
- Define exothermic (temperature rises) and endothermic (temperature falls).
- Distinguish strong/weak (ionisation) from concentrated/dilute (amount dissolved).