(a)(i) Distinguish between a strong acid and a concentrated acid. (ii) What is meant by amphoteric oxide? Give one example. (b)(i) Describe the manufacture ...

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(a)(i) Strong acid vs concentrated acid: a strong acid is one that ionises (dissociates) completely into ions in aqueous solution, whereas a concentrated acid is one containing a large amount of acid (solute) in a small amount of water (little water). Strength refers to degree of ionisation; concentration refers to amount of solute per unit volume.

(a)(ii) Amphoteric oxide: an oxide that can react with both acids and bases to form a salt and water, e.g. aluminium oxide, Al₂O₃ (or ZnO, PbO).

(b)(i) Contact process for H₂SO₄:

1. Sulphur (or a sulphide ore) is burnt in air to give sulphur(IV) oxide: \(\text{S} + \text{O}_2 \to \text{SO}_2\).
2. The purified, dry SO₂ and excess air are passed over a catalyst of vanadium(V) oxide (V₂O₅) at about 450 °C and 1-2 atm, oxidising it to sulphur(VI) oxide: \(2\text{SO}_2 + \text{O}_2 \rightleftharpoons 2\text{SO}_3\).
3. The SO₃ is absorbed in concentrated H₂SO₄ to form oleum (fuming sulphuric acid): \(\text{SO}_3 + \text{H}_2\text{SO}_4 \to \text{H}_2\text{S}_2\text{O}_7\).
4. The oleum is diluted carefully with water to give concentrated H₂SO₄: \(\text{H}_2\text{S}_2\text{O}_7 + \text{H}_2\text{O} \to 2\text{H}_2\text{SO}_4\).

(b)(ii) Equations:

• As a dehydrating agent (removing the elements of water from sugar): \(\text{C}_{12}\text{H}_{22}\text{O}_{11} \xrightarrow{\text{conc. H}_2\text{SO}_4} 12\text{C} + 11\text{H}_2\text{O}\).
• As an oxidizing agent (hot concentrated acid with copper): \(\text{Cu} + 2\text{H}_2\text{SO}_4 \to \text{CuSO}_4 + \text{SO}_2 + 2\text{H}_2\text{O}\).

(b)(iii) Tetraoxosulphate(VI) acid is classified as a heavy chemical because it is manufactured and used in very large (industrial) quantities and is relatively cheap and impure.