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In aqueous solution, the standard reduction potential for the NO3- ion (as oxidising agent) is +0.96 V. The standard reduction potentials of some metal ions are: V2+ (aq) + 2 e- -> V E0 = -1.19 V Fe3+ (aq) + 3 e- -> Fe E0 = -0.04 V Au3+ (aq) + 3 e- -> Au E0 = +1.40 V Hg2+ (aq) + 2 e- -> Hg E0 = +0.86 V Which pair of metals can be oxidised by NO3- in aqueous solution?

1. V and Hg
2. Hg and Fe
3. Fe and Au
4. Fe and V

Correct answer: Fe and V

Solution

NO3- (E0 = +0.96 V) oxidises a metal when the metal's reduction potential is below +0.96 V (so the metal is the more easily oxidised species). V (-1.19) and Fe (-0.04) both qualify; Hg (+0.86) is borderline but Au (+1.40) cannot be oxidised. The cleanly correct pair is Fe and V.

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