Question

Clearly explain (using E values) any difference in the ways in which Fe2+ and Co2+ react with Cr2O7 2- in acidic solution.

Answer 1

Write down the half reactions and the standard reduction potential values.

Cr₂O₇^2- (aq) + 14 H⁺ (aq) + 6 e^- --------> 2 Cr³⁺ (aq) + 7 H₂O; E⁰ = +1.33 V

Co²⁺ (aq) + 2 e^- -------> Co (s); E⁰ = -0.28 V

Fe²⁺ (aq) + 2 e^- -------> Fe (s); E⁰ = -0.44 V

The more positive the reduction potential of an element/compound/substance, the greater is the tendency of the substance to be reduced. Consequently, Cr₂O₇^2- will be reduced to Cr³⁺. The more negative the reduction potential, the greater is the tendency of the element or substance to be oxidized. Fe²⁺ has a higher reduction potential than Co²⁺; consequently, Fe will have a greater tendency to be oxidized while Co will have a (comparatively) lower tendency to be oxidized. However, both Fe and Co will act as reducing agents while Cr₂O₇^2- will be an oxidizing agent.