23.2 Enthalpies of Solution and Hydration

2026 Syllabus Objectives

By the end of these notes, you should be able to:

  1. Define and use the term enthalpy change with reference to hydration (ΔH꜀hyd) and solution (ΔH꜀sol)
  2. Construct and use an energy cycle involving enthalpy change of solution, lattice energy, and enthalpy change of hydration
  3. Carry out calculations using these energy cycles
  4. Explain, in qualitative terms, how ionic charge and ionic radius affect the size of the enthalpy change of hydration

1. Enthalpy Change of Solution (ΔH꜀sol)

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What is it?

When an ionic solid (like salt) is placed in water, it dissolves — meaning the crystal structure breaks apart and the individual ions spread out into the water. The energy change that happens during this process is called the enthalpy change of solution.

Definition: The standard enthalpy change of solution (ΔH꜀sol) is the energy absorbed or released when 1 mole of an ionic solid dissolves in enough water to form a very dilute solution, measured under standard conditions.

The symbol "aq" in equations represents a very large amount of water (essentially an infinite amount, so the solution is very dilute).

The General Equation

XY(s) + aq→X+(aq) + Y−(aq)\text{XY(s) + aq} \rightarrow \text{X}^{+}\text{(aq) + Y}^{-}\text{(aq)}

The ionic solid on the left breaks apart into its aqueous ions (ions surrounded by water) on the right.

Examples

CompoundEquationΔH꜀sol
Sodium chloride (NaCl)NaCl(s) + aq → Na⁺(aq) + Cl⁻(aq)+3.9 kJ mol⁻¹
Magnesium chloride (MgCl₂)MgCl₂(s) + aq → Mg²⁺(aq) + 2Cl⁻(aq)−55 kJ mol⁻¹

Can ΔH꜀sol be positive or negative?

Yes — it can be either:

  • Negative (exothermic): Energy is released when the solid dissolves. The solution gets warmer.
  • Positive (endothermic): Energy is absorbed when the solid dissolves. The solution gets cooler.

What does this tell us about solubility?

The value of ΔH꜀sol gives us a clue about whether a substance will dissolve easily:

  • Negative or small positive value → The substance is likely to be soluble in water.
  • Large positive value → The substance is likely to be relatively insoluble in water.

For example, NaCl has ΔH꜀sol = +3.9 kJ mol⁻¹ (small positive → soluble), while AgCl has ΔH꜀sol = +65.7 kJ mol⁻¹ (large positive → relatively insoluble).

⚠️ Important note: No ionic salt is completely insoluble — the terms "soluble" and "insoluble" are relative. Even "insoluble" salts dissolve a tiny amount.

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