Understanding Hydration Enthalpy

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Let us understand how hydration enthalpy works. Read on to learn how it leads to its solubility and lattice energy..

What is the Hydration Enthalpy?

Hydration enthalpy is basically when one mole of gaseous ions is surrounded by molecules of water and they end up forming an aqueous solution; it is the enthalpy change which occurs.

→ When these ions and molecules interact, the energy is released due to this interaction, making it an exothermic process (−ΔH).

Breakdown of Hydration Enthalpy

When we want to learn about solubility, ionic strengths and more, it is hydration enthalpy we study, since it :

High charged/ Small ions → high hydration enthalpy seen
Low charged/ Large ions → low hydration enthalpy seen

Charge density formula:

Hydration enthalpy ∝ z² / r

(where z = ionic charge, r = ionic radius)

Here is a complete understanding of hydration enthalpy.

Electronic Configuration of Water Molecule (for Hydration)

Since there is sp³ hybridisation of oxygen in Water (H₂O), it has a bent geometry.

2 H (σ bond), O (2 Lone pairs)
O–H bonds → polar solvent
Ion- dipole interaction → enabled due to polarity, it stabilises the ions

Formation of Hydrated Ions

The gaseous ions, when introduced into water, result in :
−ve end of O to surround→ cations
+ve end of H to surround→ anions

O and H surrounding cations and anions in NaCl

And there is stabilisation of energy, which is in the form of hydration enthalpy.

Factors which affect Hydration Enthalpy

Charge in ion (z): Higher charge → stronger ion-dipole interaction → higher hydration enthalpy
Size of ion (r): Smaller ion → higher charge density → stronger hydration → higher hydration enthalpy
Polarising power (z/r): Hydration enthalpy ∝ (charge/size)

Bonding and Energetics in Hydration Enthalpy

Being part of the dissolution process in ionic compounds, the following properties will:

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ΔHsolution = ΔHlattice + ΔHhydration

ΔHlattice is endothermic (+ve) (energy required to break the lattice).

ΔHhydration is exothermic (−ve).

If ΔHsolution is negative, the salt dissolves easily.

Details At A Glance

Property	Details
Process	Hydration of gaseous ions
Type of enthalpy	It will always be exothermic
Governing factor	Charge density (z/r)
Strongest hydration	Small, highly charged ions (e.g., Li⁺, Mg²⁺, Al³⁺)
Weakest hydration	Large, low-charged ions (e.g., Cs⁺, I⁻)
Relation to solubility	Higher hydration enthalpy will increase solubility (if it is greater than lattice enthalpy)

Formal Explanation of Stability of Hydration Enthalpy

In the aqueous solution, the hydration enthalpy lowers the potential energy of solution which in turn ends up stabilizing the ions.

For e.g., compounds like NaCl will dissolve easily since lattice enthalpy will be fulfilled by hydration enthalpy.

Summing Up

Hydration enthalpy is the energy released when in water, gaseous ions dissolve. The hydration enthalpy is dependent on the charge density of ions. Also, in ionic compounds in water, it plays a quite important role in factors like stability, solubility, etc.