Exam-style Question 2

Question (4 marks): When copper(I) oxide is added to warm dilute sulfuric acid, a pink-brown solid and a blue solution form:Cu₂O (s) + H₂SO₄ (aq) → Cu (s) + CuSO₄ (aq) + H₂O (l)(a) Give the oxidation number of copper in each copper species. (b) Explain why this is a disproportionation reaction. (c) Give the systematic name of CuSO₄.Answer (a): In Cu₂O, 2x + (−2) = 0, so each Cu is +1. In copper metal it is 0 (an uncombined element). In CuSO₄ the sulfate ion is 2−, so Cu is +2.Answer (b): Copper(I) is both reduced, from +1 to 0, and oxidised, from +1 to +2, in the same reaction. One mark for each change and one for saying that the same element is simultaneously oxidised and reduced.Answer (c): Copper is +2 and sulfur is +6 (x + 4(−2) = −2), so CuSO₄ is copper(II) sulfate(VI).Why the colours? The pink-brown solid is copper metal and the blue solution contains Cu²⁺ (aq) ions, which is evidence for both products.Examiner tip: hydrogen, oxygen and sulfur do not change oxidation number here (+1, −2 and +6 throughout). If only one element changes, and it goes both up and down, the reaction is a disproportionation.