$\mathrm{E}^{\circ}$ of $\mathrm{Cu}$ is $+0.34 \mathrm{~V}$ while that of $\mathrm{Zn}$ is $-0.76 \mathrm{~V}$. Explain.

In case of copper, the hydration enthalpy of $\mathrm{Cu}^{2+}$ does not balance the summation of sublimation enthalpy and ionization enthalpies and so it has positive $\mathrm{E}^{\circ}$ values.

However, in case of $\mathrm{Zn}$ the second ionization enthalpy is low because electrons is to be removed from $4 s$-orbital to attain stable $3 d^{10}$ configuration. Hence $\mathrm{Zn}$ has negative $\mathrm{E}^{\circ}$ value.