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: Li+ ion is the most stable in aqueous medium

Standard reduction potential (ERPo) is a measure of tendency of an element to lose electron in aqueous solution. Higher the negative ERPo greater is the ability to lose electrons.
ERPo 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 ERPo 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) Li(s)SublimationLi(g)   Hs=Enthalpy of sublimation
(ii) Li(g)Li+(s)      IE1=ionisation enthalpy
(iii) Li+(g)Li+(aq)    Hn=Enthalpy of hydration
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.