Question

From the given data of E° values, answer the following questions:
E°_M²⁺/M (V): Cr = –1.18, Mn = –1.18, Fe = –0.44, Co = –0.28, Ni = –0.25, Cu = +0.34
(I) Why does E°_M²⁺/M show an irregular trend in the above values?
(II) Why is the E°_Cu²⁺/Cu value exceptionally positive?
(III) Why is the E°_Mn²⁺/Mn value highly negative?

Hint:

(I) Irregular trend: The standard electrode potential E° M 2+ /M depends on three energy terms taken together: the enthalpy of atomisation (sublimation), the sum of the first two ionisation enthalpies, and the hydration

Answer 1

(I) Irregular trend: The standard electrode potential E°_M²⁺/M depends on three energy terms taken together: the enthalpy of atomisation (sublimation), the sum of the first two ionisation enthalpies, and the hydration enthalpy of the M²⁺ ion. Across the transition series these three quantities do not change smoothly, so their net balance is irregular. Hence E°_M²⁺/M does not follow a regular trend.

(II) Cu exceptionally positive: For copper the sum of its first two ionisation enthalpies is very high and its enthalpy of atomisation is also high, but these are not compensated by its hydration enthalpy. In other words, the energy required to convert Cu(s) to Cu²⁺(aq) is not balanced by the energy released, so the change is energetically unfavourable. This makes Cu a poor reducing agent and gives it a positive (exceptional) electrode potential.

(III) Mn highly negative: For manganese the M²⁺ ion (Mn²⁺) has a stable half-filled 3d⁵ configuration. This extra stability of Mn²⁺ makes it easy to form, so Mn metal is readily oxidised to Mn²⁺. Easy formation of the +2 ion corresponds to a highly negative E°_Mn²⁺/Mn value.