Electrode potentials and stability

Standard electrode potentials tell us how easily a transition metal or ion changes its oxidation state. Remember the meaning of the sign:

• A more negative E°(M²⁺/M) means the metal is oxidised more easily (it prefers to be an ion).
• A positive value means the change M²⁺ → M happens readily: the ion is easily reduced.

Across the 3d series, E°(M²⁺/M) becomes less negative. Copper is the only one with a positive value (+0.34 V), so copper does not give hydrogen with dilute acids. Its high atomisation enthalpy and ionisation enthalpies are not paid back by its hydration enthalpy.

E° values are decided by three energies together: atomisation (pulling the metal apart), ionisation (removing electrons) and hydration (the ion settling into water).

Read the values with stable configurations in mind:

• E°(Mn³⁺/Mn²⁺) is very positive (+1.5 V): Mn³⁺ is 3d⁴, and gaining one electron gives 3d⁵, the stable half-filled set. So Mn³⁺ is a strong oxidising agent. Co³⁺ is another.
• Mn²⁺ (d⁵) is hard to oxidise; Fe³⁺ (also d⁵) is stable.
• Cr²⁺ is a strong reducing agent: losing one electron turns it into Cr³⁺, 3d³, which in water has a stable half-filled t₂g set. Ti²⁺ and V²⁺ are reducing as well.

Remember: an oxidising agent is itself reduced (it takes electrons), and a reducing agent is itself oxidised (it gives electrons).