Enthalpy of Solution - Key Terms (Edexcel International A Level Chemistry)

Enthalpy of solution

• The standard enthalpy change of solution (Δ_solH) is the enthalpy change when 1 mole of an ionic substance dissolves in sufficient water to form an infinitely 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)

• Δ_solH^ꝋcan be exothermic (negative) or endothermic (positive)

Enthalpy of hydration

• The standard enthalpy change of hydration (Δ_hydH) is the enthalpy change when 1 mole of a specified gaseous ion dissolves in sufficient water to form an infinitely dilute solution
  • For example, the enthalpy change of hydration for magnesium ions is described by the following equation:

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

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