Enthalpy of solution and hydrationAQA A-Level Chemistry: Subtopic test

1. 1
   A student is studying what happens when the ionic solid sodium chloride dissolves in water. The process can be considered in two stages: the ionic lattice is first broken up into gaseous ions, and the gaseous ions are then surrounded by water molecules.

   (a)

   Which equation represents the enthalpy of hydration of the sodium ion?

   [1 mark]

   • ANa⁺(g) + aq → Na⁺(aq)
   • BNaCl(s) + aq → Na⁺(aq) + Cl⁻(aq)
   • CNa⁺(aq) → Na⁺(g) + aq
   • DNa⁺(g) + Cl⁻(g) → NaCl(s)

   (b)

   Which statement explains why the enthalpy of hydration of gaseous ions is exothermic?

   [1 mark]

   • ACovalent bonds in water molecules are broken, releasing energy
   • BThe ions must be separated from the lattice, releasing energy
   • CAttractions form between the ions and water molecules, releasing energy
   • DThe ions form an ionic lattice, releasing energy

   (c)

   Define the term enthalpy of hydration.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   The enthalpy of solution of sodium chloride is found using an enthalpy cycle. Data (kJ mol⁻¹): enthalpy of lattice formation of NaCl = −787; enthalpy of hydration of Na⁺(g) = −406; enthalpy of hydration of Cl⁻(g) = −364.

   (a)

   Calculate the enthalpy of solution of sodium chloride.

   [1 mark]

   • A−17 kJ mol⁻¹
   • B−1557 kJ mol⁻¹
   • C+1557 kJ mol⁻¹
   • D+17 kJ mol⁻¹

   (b)

   Which expression gives the enthalpy of solution of an ionic compound?

   [1 mark]

   • AΔH(lattice formation) + ΣΔH(hydration)
   • BΔH(lattice dissociation) + ΣΔH(hydration)
   • CΔH(lattice dissociation) − ΣΔH(hydration)
   • DΔH(lattice formation) − ΣΔH(hydration)

   (c)

   Calculate the enthalpy change when 5.85 g of sodium chloride (Mr = 58.5) dissolves in water. State whether the energy is released or absorbed.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Magnesium chloride dissolves in water. Data (kJ mol⁻¹): enthalpy of lattice dissociation of MgCl₂ = +2526; enthalpy of hydration of Mg²⁺(g) = −1920; enthalpy of hydration of Cl⁻(g) = −364. The enthalpy of hydration of Na⁺(g) is −406 kJ mol⁻¹.

   (a)

   Calculate the enthalpy of solution of magnesium chloride.

   [3 marks]

   (b)

   The enthalpy of hydration of Mg²⁺ is much more exothermic than that of Na⁺. Explain why.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A chemist compares the enthalpies of solution of some Group 1 halides. Enthalpies of lattice formation (kJ mol⁻¹): LiF = −1031; LiCl = −848; KCl = −711. Enthalpies of hydration (kJ mol⁻¹): Li⁺ = −519; F⁻ = −506; Cl⁻ = −364. For potassium chloride the measured enthalpy of solution is +17 kJ mol⁻¹. Relative formula mass of KCl = 74.6.

   (a)

   Calculate the enthalpy of solution of lithium fluoride and of lithium chloride. Explain why the enthalpy of solution of lithium fluoride is less exothermic than that of lithium chloride.

   [6 marks]

   (b)

   Calculate the enthalpy of hydration of K⁺ from the data for potassium chloride, and calculate the energy absorbed when 3.73 g of potassium chloride dissolves. Explain why the enthalpy of hydration of K⁺ is less exothermic than that of Li⁺.

   [6 marks]

   Total for question 4: 12 marks

End of questions