Enthalpy of solution, hydration and solubilityEdexcel International A Level Chemistry: Subtopic test

1. 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.

   (a)

   Which equation represents the enthalpy change of hydration of the chloride ion?

   [1 mark]

   • ACl⁻(aq) → Cl⁻(g) + aq
   • B½Cl₂(g) + e⁻ + aq → Cl⁻(aq)
   • CNaCl(s) + aq → Na⁺(aq) + Cl⁻(aq)
   • DCl⁻(g) + aq → Cl⁻(aq)

   (b)

   Which expression gives the enthalpy change of solution of sodium chloride?

   [1 mark]

   • A(lattice energy) + ΔhydH(Na⁺) + ΔhydH(Cl⁻)
   • B−(lattice energy) + ΔhydH(Na⁺) + ΔhydH(Cl⁻)
   • C−(lattice energy) − ΔhydH(Na⁺) − ΔhydH(Cl⁻)
   • D(lattice energy) − ΔhydH(Na⁺) − ΔhydH(Cl⁻)

   (c)

   Calculate the enthalpy change of solution of sodium chloride.

   [2 marks]

   Total for question 1: 4 marks

2. 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.

   (a)

   Why is the enthalpy change of hydration of Mg²⁺ much more exothermic than that of Na⁺?

   [1 mark]

   • AMg²⁺ has a larger radius, so more water molecules fit around it
   • BMg²⁺ has fewer electrons, so it cannot form bonds with water
   • CMg²⁺ has a higher charge and a smaller radius, so a higher charge density that attracts water molecules more strongly
   • DMg²⁺ has a lower charge density than Na⁺

   (b)

   What is the enthalpy change of solution of magnesium chloride?

   [1 mark]

   • A−124 kJ mol⁻¹
   • B+124 kJ mol⁻¹
   • C+240 kJ mol⁻¹
   • D−5172 kJ mol⁻¹

   (c)

   Explain why the lattice energy of magnesium chloride is much more exothermic than that of sodium chloride.

   [2 marks]

   Total for question 2: 4 marks

3. 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.

   (a)

   Calculate the enthalpy change of solution of barium sulfate.

   [3 marks]

   (b)

   Use the data to explain why barium sulfate is much less soluble than magnesium sulfate.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A chemist uses enthalpy and entropy data at 298 K to predict the solubility of ionic compounds in water. Data for sodium chloride: lattice energy −780 kJ mol⁻¹; hydration enthalpies Na⁺ −406 and Cl⁻ −364 kJ mol⁻¹; entropy change of the system on dissolving +43 J K⁻¹ mol⁻¹. Data for silver chloride: lattice energy −916 kJ mol⁻¹; hydration enthalpies Ag⁺ −464 and Cl⁻ −364 kJ mol⁻¹; entropy change of the system on dissolving +33 J K⁻¹ mol⁻¹.

   (a)

   Calculate ΔStotal for dissolving each compound at 298 K, and use the values to predict whether each is soluble.

   [6 marks]

   (b)

   The Group 2 hydroxides become more soluble down the group, with Mg(OH)₂ almost insoluble and Ba(OH)₂ readily soluble. Explain this trend in terms of lattice energy and enthalpy change of hydration.

   [6 marks]

   Total for question 4: 12 marks

End of questions