Question

Why is lithium the strongest reducing agent?

Answer 1

First we know that A reducing agent is an element or compound that loses an electron to an electron recipient (oxidizing agent) in a redox chemical reaction. Examples of reducing agents are the earth metals, formic acid, oxalic acid, and sulfite compounds. We also know that the alkali metals are strong reducing agents because of their great tendency to lose electrons.

Complete answer:
Any element loses its electrons if it has low ionization energy. Hence an element must have low ionization energy to act as a reducing agent. Since ionization decreases on moving down from Li to Cs, the reducing character also increases down from Li to Cs.
Lithium ions are small in size and have high ionization enthalpy. Because of its small size, it is extensively hydrated and has very high hydration enthalpy. This high hydration enthalpy compensates for the high energy needed to remove electrons. The large amount of hydration energy makes it the strongest reducing agent in spite of its highest ionization enthalpy. Hence lithium is the strongest reducing agent due to greater hydrogen energy.

Note:
The tendency of an element to lose electrons in solution is measured by its standard oxidation potential value ${E^o}_{ext}$. Since alkali metals have high ${E^o}_{ext}$ values, they are strong reducing agents. Since, lithium is the strongest reducing agent among alkali metals in solution as ${E^o}_{ext}$ value of lithium is maximum.