Which has higher enthalpy of hydration? Justify your answer.
Answer: LiF has higher (more exothermic) enthalpy of hydration than NaF.
Reason: Li⁺ ion is smaller than Na⁺ ion. Smaller ion → higher charge density → stronger attraction to water molecules → more energy released → higher enthalpy of hydration.
Answer: NaNO₃ has higher (more exothermic) enthalpy of hydration than KNO₃.
Reason: Na⁺ ion is smaller than K⁺ ion. Smaller ion → higher charge density → stronger attraction to water molecules → more energy released → higher enthalpy of hydration.
Intermediate Level
Factors Affecting Hydration Enthalpy
Factor
Effect
Reason
Example
Ionic Charge
Increases with charge
Stronger ion-dipole attraction
MgO > NaCl
Ionic Radius
Increases with decreasing radius
Higher charge density
MgSO₄ > CaSO₄
Charge Density
Charge density = ionic chargeionic radius
- Higher charge density → stronger attraction to water molecules → higher hydration enthalpy.
Concept Assessment Exercise 6.8
a. LiF or NaF - LiF has higher enthalpy of hydration because Li⁺ is smaller than Na⁺.
b. KNO₃ or NaNO₃ - NaNO₃ has higher enthalpy of hydration because Na⁺ is smaller than K⁺.
Important Points
1. Hydration enthalpy is influenced by charge density.
