Question

What is hydration enthalpy of alkali metals?

Answer 1

For answering we should learn about alkali metals and Hydration enthalpy first, its factors and how it is calculated. We will discuss Hydration enthalpy of each element belonging to alkali metals.

Complete answer:
Alkali metals belong to S-block elements and the valence shell configuration of these elements is $\mathop {ns}\nolimits^1$ . These are soft metals which can be cut through knives due to their low boiling point and low melting point, they are highly reactive metals with very low ionisation enthalpy due to high electropositivity of this metal. These metals are highly sensitive to hydrolysis, they quickly react with water to form metal hydroxides and Hydrogen gas.
Hydration is defined as the process of addition of water molecules to an atom, ion or molecule. It is also known as solvation.
Hydration enthalpy is the amount of heat energy released due to formation of new bonds between water molecules and ions; it can also be defined as the amount of energy released by one mole of ions during formation of hydrated species. It mainly depends upon the charge-to- size ratio.
Since size of atoms increases down the group in alkali metals and charge remains the same Hydration enthalpy decreases down the group which means Lithium have highest hydration enthalpy and Caesium have lowest hydration enthalpy among alkali metals.
Hydration enthalpy of Alkali metals:
Lithium: $- 506kJ/mol$
Sodium: $- 406kJ/mol$
Potassium: $- 330kJ/mol$
Rubidium: $- 310kJ/mol$
Caesium: $- 276kJ/mol$

Note:
The hydration enthalpy has a negative value because it is the energy released during the formation of water and ion bonds. So, pay attention when you are comparing Hydration enthalpies of different elements you can ignore the negative sign.