What is hydration enthalpy? And explain

What is hydration enthalpy?? And explain , ’ small size of Li has high hydration enthalpy which accounts for high negative E value and high reducing power.’’ How hydration enthalpy is related to reducing power???

Hydration enthalpy is the amount of energy released when one mole of gaseous ions combine with water to form hydrated ions.
Lithium which is an alkali metal has high negative value of E-. It indicates high reducing power.
Reducing power is sum of three energies such as ionisation energy, sublimation energy and hydration energy as follow:

First step:
M(s) → M(g) (sublimation energy)

Second step:
M(g) → M+( g) (ionisation energy)

Third step:
M+(g) + H2O → M+(aq) (hydration enthalpy)

Reducing power = sublimation energy + ionisation energy + hydration energy

Lithium being small in size has high ionization enthalpy. On the other hand because of small size it is extensively hydrated and has very high hydration enthalpy. This high hydration enthalpy compensates the high energy needed to remove electron (in second step). Thus Li has greater tendency to lose electrons in solution than other alkali metals. Hence, Li is the strongest reducing agent.

Thus, the abnormally large reducing power of lithium is due to its large hydration energy.

Large hydration energy of Lithium is related to its smallest size.