Explain trends in $M^{2+}/M$ standard electrode potentials.

The stability of $\mathrm{M}^{2+}$ ions in aqueous medium depends on three factors :

$(i)$ Enthalpy of atomisation

$(ii)$ Summation of first and second ionization enthalpies.

$(iii)$ Hydration enthalpy

An element in $\mathrm{M}^{2+}$ state in aqueous medium is more stable if the electrode potential $\left(\mathrm{M}^{2+} / \mathrm{M}\right)$ value is more negative. Across the period, the tendency to form $\mathrm{M}^{2+}$ ion decreases.

Except copper, all elements of first transition series show negative values of electrode potentials. The exceptional behaviour of the copper due to low enthalpy of atomisation and very high summation of first and second ionization enthalpies which is not compensated by its hydration enthalpy $\left(\mathrm{Cu}^{2+}\right)$.

Because of positive electrode potential, copper doesnot liberate hydrogen gas from dilute acids and reacts only with oxidizing acids such as nitric acid and hot concentrated sulphuric acid.

The electrode potentials of $\mathrm{Mn}, \mathrm{Ni}$ and $\mathrm{Zn}$ are more negative than expected. However, the electrode potential values of $\mathrm{Mn}$ and $\mathrm{Zn}$ are lowered because of low second ionization enthalpies while $Ni$ has exceptionally more negative electrode potential due to its high hydration enthalpy.