Question

Considering the alkali metals, Lithium has the highest hydration enthalpy but the most negative electrode potential, why is it this way?

Answer 1

Dear Student,

Lithium has the most negative electrode potential due to its highest hydration enthalpy which owes to its small size.
Electrode potential represents the overall change
$\mathrm{M}\left(\mathrm{s}\right)\to {\mathrm{M}}^{+}\left(\mathrm{aq}\right)$
It involves three parameters :

1. Sublimation enthalpy for the change $\mathrm{M}\left(\mathrm{s}\right)\to \mathrm{M}\left(\mathrm{g}\right)$
2. Ionisation enthalpy for $\mathrm{M}\left(\mathrm{g}\right)\hspace{0.17em}\to {\mathrm{M}}^{+}\left(\mathrm{g}\right)$
3. Hydration enthalpy for ${\mathrm{M}}^{+}\left(\mathrm{g}\right)\to {\mathrm{M}}^{+}\left(\mathrm{aq}\right)$

Lithium being the smallest ion, has the highest hydration enthalpy which accounts for high negative value of electrode potential.