Lithium is a highly reactive alkali metal that can react with water to produce hydrogen gas and an alkaline solution. However, lithium's high hydration enthalpy results in its slow reaction with water, despite being highly reactive. This phenomenon can be explained by the following factors:

Hydration Enthalpy:

The hydration enthalpy is the energy released when a cation attracts water molecules around it. The hydration enthalpy of lithium is exceptionally high because of its small size and high charge density. This high hydration energy makes it difficult for water molecules to penetrate the lithium cation's hydration shell and initiate the reaction.

Surface Tension:

Water has a high surface tension, which means that it forms a layer on the surface of a metal, reducing its contact with the metal's surface. This layer of water molecules acts as a barrier to prevent further reaction, making it difficult for lithium to react with water.

Passivation:

When lithium reacts with water, a thin layer of lithium hydroxide is formed on the metal's surface, which acts as a passivating layer. This passivating layer prevents further reaction between lithium and water by slowing down the reaction rate.

Conclusion:

Thus, the high hydration enthalpy of lithium, surface tension of water, and the passivating layer of lithium hydroxide are the reasons why lithium reacts slowly with water despite being highly reactive. However, once the passivating layer is broken, the reaction speeds up due to the formation of more lithium hydroxide.

Lithium has a very small size so there is very less space for water molecules to attach to it therefore only a few water molecules hydrate it . and it is therefore difficult to hydrate lithium so high hydration enthalpy is seen in lithium .