22.2 Enthalpy changes in solution

• When a solid ionic compound dissolves in water

  • the ionic lattice breaks up to form gaseous ions
  • the gaseous ions bond to water molecules to form aqueous ions
• Energy is required to break up an ionic lattice into gaseous ions
• Enthalpy change of hydration is the enthalpy change when one mole of gaseous ions dissolves in water to form aqueous ions
• Enthalpy change of hydration is exothermic because bonds are being formed between isolated gaseous ions and water molecules
• Enthalpy change of solution is the enthalpy change when one mole of a solute dissolves in a solvent
• Enthalpy change of solution can be exothermic or endothermic depending on the relative sizes of the lattice enthalpy and the enthalpy changes of hydration
• If lattice enthalpy is less negative than the sum of enthalpy change of hydrations, enthalpy of solution is negative
• If lattice enthalpy is more negative than the sum of enthalpy change of hydrations, enthalpy of solution is positive