Enthalpies of Solution & Hydration (Cambridge (CIE) A Level Chemistry): Revision Note

Enthalpy Change of Hydration & Solution

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 under standard conditions

• 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)

  • 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 under standard conditions

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

Ion–Dipole interactions during dissolution

• When an ionic solid dissolves in water, it breaks into positive and negative ions

• Water is a polar molecule, with a δ⁻ oxygen atom and δ⁺ hydrogen atoms

• The oxygen atom is attracted to positive ions (cations)

• The hydrogen atoms are attracted to negative ions (anions)

• These attractions form ion–dipole bonds between the water molecules and the ions

Interactions of polar water molecules and other ions in solution