Definition:

Hydration enthalpy is the energy released when 1 mole of gaseous ions gets surrounded by water molecules.

Always negative (exothermic)

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Example:

Na⁺(g) → Na⁺(aq) + energy released

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Factors Affecting Hydration Enthalpy

1. Size of Ion

Smaller ion → higher hydration enthalpy

✔️ Reason:

• Higher charge density
• Strong attraction with water

Order:
Li⁺ > Na⁺ > K⁺ > Rb⁺ > Cs⁺

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2. Charge on Ion

Higher charge → higher hydration enthalpy

✔️ Example:
Mg²⁺ > Na⁺

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Group Trends

Group 1 elements

Decreases down the group
Li⁺ > Na⁺ > K⁺ > Rb⁺ > Cs⁺

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Group 2 elements

Decreases down the group
Be²⁺ > Mg²⁺ > Ca²⁺ > Sr²⁺ > Ba²⁺

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Comparison (Very Important)

Group 2 > Group 1 (hydration enthalpy)

✔️ Example:
Mg²⁺ > Na⁺

👉 Reason:

• Higher charge (+2)
• Stronger attraction with water

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Applications

✔️ 1. Solubility

Higher hydration enthalpy → more soluble

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✔️ 2. Stability of Ions in Solution

Small, highly charged ions are more stable in water

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