Enthalpies of Solution & Hydration (CIE A Level Chemistry)

Enthalpy change of solution

• The standard enthalpy change of solution (ΔH_sol^θ) is the enthalpy change when 1 mole of an ionic substance dissolves in sufficient water to form a very dilute solution
• The symbol (aq) is used to show that the solid is dissolved in sufficient water
  • For example, the enthalpy changes of solution for potassium chloride are described by the following equations:

KCl (s) + aq → KCl (aq)

OR

KCl (s) + aq → K⁺ (aq) + Cl^– (aq)

• ΔH_sol^θ can be exothermic (negative) or endothermic (positive)

Enthalpy change of hydration

• The lattice energy (ΔH_latt^θ) of KCl is -711 kJ mol^-1
  • This means that 711 kJ mol^-1 is released when the KCl ionic lattice is formed
  • Therefore, to break the attractive forces between the K⁺ and Cl^- ions, +711 kJ mol^-1 is needed

• However, the ΔH_sol^θ of KCl is +26 kJ mol^-1
• This means that another +685 kJ mol^-1 (711 - 26) is required to break the KCl lattice
• This is compensated for by the standard enthalpy change of hydration (ΔH_hyd^θ)
• For example, the enthalpy change of hydration for magnesium ions is described by the following equation:
  • The standard enthalpy change of hydration (ΔH_hyd^θ) is the enthalpy change when 1 mole of a specified gaseous ion dissolves in sufficient water to form a very dilute solution

Mg²⁺(g) + aq → Mg²⁺(aq)

• • Hydration enthalpies are the measure of the energy that is released when there is an attraction formed between the ions and water molecules
  • Hydration enthalpies are exothermic

• When an ionic solid dissolves in water, positive and negative ions are formed
• Water is a polar molecule with a δ- oxygen (O) atom and δ+ hydrogen (H) atoms which will form ion-dipole attractions with the ions present in the solution
• The oxygen atom in water will be attracted to the positive ions and the hydrogen atoms will be attracted to the negative ions
• Since the ΔH_hyd^θ of KCl is -685 kJ mol^-1, 685 kJ mol^-1 is released in forming these ion-dipole attractions when KCl dissolves in water
  • This compensates for the remaining +685 kJ mol^-1 which was needed to break down the KCl lattice

Interactions of polar water molecules and other ions in solution

The polar water molecules will form ion-dipole bonds with the ions in solution (a) causing the ions to become hydrated (b)