The reducing power of a metal depends on various factors. Suggest the factor which makes Li, the strongest reducing agent in aqueous solution.

1. Sublimation enthalpy

2. Ionisation enthalpy

3. Hydration enthalpy

4.  Electron-gain enthalpy

Hint: ${\mathrm{Li}}^{+}$ ion is the most stable in aqueous medium

Standard reduction potential $\left({\mathrm{E}}_{\mathrm{RP}}^{\mathrm{o}}\right)$ is a measure of tendency of an element to lose electron in aqueous solution. Higher the negative ${\mathrm{E}}_{\mathrm{RP}}^{\mathrm{o}}$ greater is the ability to lose electrons.
${\mathrm{E}}_{\mathrm{RP}}^{\mathrm{o}}$ depends on
(i) enthalpy of sublimation
(ii) ionisation enthalpy
(iii) enthalpy of hydration
Thus, in aqueous medium, order of reactivity of alkali metals is Na < K < Rb < Cs ${\mathrm{E}}_{\mathrm{RP}}^{\mathrm{o}}$ value of Li is least (-3.04V) among all alkali metals.
The formation of Li+(aq) from Li involves following steps are as follows:
(i)
(ii)
(iii)
For alkali metals, enthalpies of sublimation are almost same. IE1 value of Li is endothermic and highest and hydration is exothermic and maximum for Li+.
The highly exothermic step (iii) for smallest Li+ makes it strongest reducing agent.