AQA GCSE Chemistry 4.4.2 Reactions of Acids Exam Questions

About AQA GCSE Chemistry 4.4.2 Reactions of Acids

AQA GCSE Chemistry 4.4.2 covers the main reactions of acids and how to make salts from them. You learn reactions of acids with metals, metal oxides, hydroxides and carbonates, the required practical for preparing a pure, dry sample of a soluble salt, and the pH scale. Separate Chemistry students also study titrations, and Higher tier students explain strong and weak acids and how pH relates to hydrogen ion concentration. It is part of section 4.4, assessed on Paper 1, and links to concentration in 4.3.4 and electrolysis in 4.4.3. Method descriptions and naming salts are tested in almost every series.

What you need to know

• Acids and metals Acid + metal → salt + hydrogen, for example Mg + 2HCl → MgCl2 + H2. Higher tier only: explain this as a redox reaction in terms of electrons.
• Neutralisation Acid + base (metal oxide or hydroxide) → salt + water. Acid + metal carbonate → salt + water + carbon dioxide. Hydrochloric acid gives chlorides, sulfuric acid gives sulfates and nitric acid gives nitrates.
• Soluble salts required practical Add an excess of insoluble metal oxide or carbonate to warm acid until no more reacts, filter off the excess solid, then heat the filtrate to evaporate some water and leave it to crystallise. For example, copper(II) oxide and sulfuric acid make copper(II) sulfate crystals.
• pH and neutralisation Acids produce H+ ions in aqueous solution and alkalis produce OH- ions. Neutralisation: H+ + OH- → H2O. pH can be measured with universal indicator or a pH probe; pH 7 is neutral.
• Titrations Separate Chemistry only: the volumes of acid and alkali that react exactly can be found by titration with an indicator. See 4.3.4 for calculations.
• Strong and weak acids (Higher tier only) Strong acids (hydrochloric, nitric, sulfuric) are completely ionised in water; weak acids (ethanoic, citric, carbonic) are only partially ionised. As pH decreases by one unit, H+ concentration increases by a factor of 10. Strong and weak are not the same as concentrated and dilute.

Common mistakes in 4.4.2 Reactions of Acids

• Confusing strong with concentrated. Strength is about how fully the acid ionises; concentration is about how much acid is in a given volume.
• Using excess acid when making a salt from an insoluble base. Add excess of the solid base so all the acid reacts, then filter off the excess.
• Heating the solution to dryness. Evaporate some of the water then leave to crystallise, so the crystals are not damaged.
• Forgetting carbon dioxide as a product when acids react with carbonates.
• Stating that a decrease of 1 pH unit doubles the H+ concentration. It increases it by a factor of 10.

Try three 4.4.2 Reactions of Acids exam-style questions