Question

Why is Li as strong a reducing agent as Cs?

Answer 1

Hint: We know that the alkali metals are very good reducing agents because of their great tendency to lose electrons. The reducing character increases from Na to Cs. However, Lithium (Li) is a stronger reducing agent than Na due to greater hydration energy.

Complete answer:
An element that acts as a reducing agent, must have low ionization energy. Alkali metals act as strong reducing agents as their ionization energy values are low. Since, ionization decreases on moving down from Li to Cs, the reducing property also increases down from Li to Cs.
Lithium ion is small in size and has high ionization enthalpy. Also, more solvent molecules can easily surround around the cationic sphere. On the other hand, 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. Thus, Li has a greater tendency to lose electrons in solution than other alkali metals. The Large amount of hydration energy makes it the strongest reducing agent in spite of its highest ionisation enthalpy.
The tendency of an element to lose electrons in solution is measured by its standard oxidation potential value \[{E^0}_{ext}\]. Since, alkali metals have high \[{E^0}_{ext}\] values, these are strong reducing agents. However, it is observed that Li is the strongest reducing agent amongst alkali metals in solution as \[{E^0}_{ext}\] value of Li is maximum.

Note:
It should be noted that since ionization decreases on moving down from Li to Cs, the reducing property increases in the same order. Thus, Li is the weakest reducing agent while Cs is the strongest reducing agent amongst alkali metals in a free gaseous state.