Heat Of Hydration Formula

The heat of hydration is described as the quantity of energy produced when one mole of ions undergo a hydration process. It is a specific form of dissolution energy and the solvent used is water.

The enthalpy of a hydrated salt is the change in heat when 1 mole of an anhydrous substance combines with a requisite number of water molecules to form the hydrate. The heat of hydration can be determined if the heat of the solutions of anhydrous salt and the hydrated forms are known.

Anhydrous salts readily combine with water to form hydrates and dissolve with the evolution of heat. The only difference between hydrate and anhydrous salt is the heat is evolved as the heat of hydration in the formation of hydrates.  

The heat of hydration formula is given by:

Heat of hydration = (ΔH _solution – ΔH _{lattice energy})

Where

ΔH _solution  = Heat of the solution

ΔH _{lattice energy} = Lattice energy of the solution

Example 1

The sodium chloride lattice enthalpy is ΔH for NaCl →→ Na⁺ + Cl^– is 700 kJ/mol. To make 1M NaCl the solution heat is +5.0kJ/mol. Determine the heat of hydration of Na+ and Cl-, where the heat of hydration of Cl- is -300kJ/mol.

Solution:

Given data

Lattice energy = 700 kJ/mol

Heat of solution = 5.0kJ/mol

Heat of hydration of Cl^– = -300kJ/mol

Substitute the values in the given formula

Heat of hydration = (ΔH_solution – ΔH_{lattice energy})

                                       = 5 – 700

Therefore, Heat of hydration = -695

Heat of hydration of Na⁺ + Cl^– = -695

Heat of hydration of Na⁺ = -695 – (-300)

Therefore, Heat of hydration of Na+ = -395