Heat Of Hydration Formula


Heat of Hydration Formula

Heat of hydration is defined as the amount of energy released when one mole of ions undergo hydration. It is a special type of dissolution energy where in the solvent is water.

The enthalpy of a hydrated salt is the change in heat, when 1 mole of an anhydrous substance combines with 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 heat of hydration in the formation of hydrates.  

The heat of hydration formula is given by:

Heat of hydration = ΔH solutionΔH lattice energy


ΔH solution  = Heat of the solution

ΔH lattice energy = Lattice energy of the solution

Example 1

The heat of solution of anhydrous and hydrated copper sulphate are – 65 and lattice energy is 11 KJ respectively. Determine the heat of hydration.


Given parameters are

Heat of solution = – 65

Lattice energy = 11 KJ

Substitute the values in the given formula,

Heat of hydration = ΔHsolution – ΔHlattice energy

                            = – 65 – 11

Heat of hydration = -56

Example 2

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


Given data,

Lattice energy = 700 kJ/mol

Heat of solution = 5.0kJ/mol

Substitute the values in the given formula,

Heat of hydration = ΔHsolution – ΔHlattice 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



Practise This Question

Two satellites S1 and S2 revolve round a planet in coplanar circular orbits in the same sense. Their periods of revolution are 1 hour and 8 hours respectively. The radius of the orbit of S1=104km When S2 is closest to S1 find  speed of S2 relative to S2 and  the angular speed of S1 actually observed by an astronaut at S1.

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