a. State which pairs of substances i to iv below might catalyse the reaction:

S₂O₈^2–(aq) + 2I^–(aq) → 2SO₄^2–(aq) + I₂(aq)

Explain your answer.

i. Ni²⁺(aq) / Ni(s)                               E^θ = –0.25 V

ii. Mn³⁺(aq) / Mn²⁺(aq)               E^θ  = +1.49 V

iii. Ce⁴⁺(aq) / Ce³⁺(aq)                 E^θ = +1.70 V

iv. Cu²⁺(aq) / Cu⁺(aq)                   E^θ= +0.15 V

b. Describe in terms of oxidation number change, which species are being oxidised and which are being reduced in these equations:

i. SO₂(g) + NO₂(g) → SO₃(g) + NO(g)

ii. NO + 1/2O₂ → NO₂(g)