Enthalpy of solution, hydration and solubilityEdexcel International A Level Chemistry: Mind map

Exam questions on Enthalpy of solution, hydration and solubility

1. Sodium chloride dissolves in water: NaCl(s) + aq → Na⁺(aq) + Cl⁻(aq). Data (kJ mol⁻¹): lattice energy of NaCl −780 (formation of the solid from gaseous ions); enthalpy change of hydration of Na⁺ −406; enthalpy change of hydration of Cl⁻ −364.
   Calculate the enthalpy change of solution of sodium chloride.2 marks
2. Magnesium chloride is used as a de-icing salt and dissolves in water: MgCl₂(s) + aq → Mg²⁺(aq) + 2Cl⁻(aq). Data (kJ mol⁻¹): lattice energy of MgCl₂ −2524; enthalpy change of hydration of Mg²⁺ −1920 and of Cl⁻ −364. For comparison, the lattice energy of NaCl is −780 and the enthalpy change of hydration of Na⁺ is −406. The ionic radius of Na⁺ is 0.102 nm and that of Mg²⁺ is 0.072 nm.
   Explain why the lattice energy of magnesium chloride is much more exothermic than that of sodium chloride.2 marks
3. Magnesium sulfate (Epsom salts) dissolves readily in water, but barium sulfate is so insoluble that it can be swallowed as a 'barium meal' for X-ray imaging. Data (kJ mol⁻¹): lattice energy of MgSO₄ −2928 and of BaSO₄ −2423; enthalpy change of hydration of Mg²⁺ −1920, of Ba²⁺ −1305 and of SO₄²⁻ −1099; enthalpy change of solution of MgSO₄ −91.
   Calculate the enthalpy change of solution of barium sulfate.3 marks