Enthalpy of solution and hydrationEdexcel A-Level Chemistry: Mind map

Exam questions on Enthalpy of solution and hydration

1. A student investigates an instant cold pack in which potassium chloride dissolves in water and the temperature of the water falls. In energy cycles, lattice energy is taken as the enthalpy change when one mole of the solid is formed from its gaseous ions. For potassium chloride the lattice energy is −717 kJ mol⁻¹, and the enthalpy changes of hydration are −322 kJ mol⁻¹ for K⁺ and −364 kJ mol⁻¹ for Cl⁻.
   Calculate the enthalpy change of solution of potassium chloride and use your answer to explain why the temperature of the water falls.2 marks
2. A chemist compares how strongly different ions are attracted to water molecules when they dissolve. She is looking at the enthalpy change of hydration of a series of gaseous ions, and wants to explain the differences in terms of ionic charge and radius.
   Explain why the enthalpy change of hydration of Mg²⁺ is much more exothermic than that of Na⁺.2 marks
3. Energy data (in kJ mol⁻¹) are given for sodium chloride and magnesium chloride. Lattice energy (formation of the solid from gaseous ions): NaCl −787; MgCl₂ −2526. Enthalpy change of hydration: Na⁺ −406; Mg²⁺ −1920; Cl⁻ −364. The enthalpy change of solution of NaCl is +17 kJ mol⁻¹.
   Calculate the enthalpy change of solution of magnesium chloride.3 marks