Lithium is the strongest reducing agent though it has the highest ionization energy in its group. Which of the following factors is responsible for making Li the strongest reducing agent.
A ) Large heat of atomisation.
B ) Smaller size.
C ) Large sublimation energy.
D ) Large amount of hydration enthalpy.

Hint: Consider the factors that affect the reducing ability of metals. Ionization energy and hydration energy are two important factors. Hydration energy depends on the size of metal cation.

Complete answer:
> Higher is the ionization energy, lower is the reducing tendency. Thus, lithium, having higher ionization energy is expected to have lower reducing tendency. But in reality lithium is the strongest reducing agent. This anomaly can be explained on the basis of hydration energy.
> Lithium ions have a small size. Due to this, it can attract a large number of water molecules. Several water molecules surround lithium cation. In this process a large amount of hydration energy is released. Thus, lithium has high hydration energy. Higher is the hydration energy, higher is the reducing tendency.
> In the group of alkali metals, lithium has the smallest ionic size. Due to this lithium has the highest hydration energy and lithium is the strongest reducing agent.

Hence, the correct option is D ) Large amount of hydration enthalpy.

Note: In the redox reaction, a reducing agent itself undergoes oxidation. A reducing agent supplies electrons to the substance being reduced. Hence, reducing agent is expected to have low ionization enthalpy. Metals have low ionization energy so that they can readily donate an electron and complete octet. However, the effect of hydration energy outweighs the effect of ionization energy.