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

1. 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⁻.

   (a)

   Which statement defines the enthalpy change of hydration of an ion?

   [1 mark]

   • AThe enthalpy change when one mole of gaseous ions dissolves in water to form one mole of aqueous ions at infinite dilution
   • BThe enthalpy change when one mole of an ionic solid dissolves in water to form a solution at infinite dilution
   • CThe enthalpy change when one mole of an ionic solid is formed from its gaseous ions
   • DThe enthalpy change when one mole of water is formed from its elements in their standard states

   (b)

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

   [1 mark]

   • AΔH hydration(K⁺) + ΔH hydration(Cl⁻)
   • BΔH lattice energy + ΔH hydration(K⁺) + ΔH hydration(Cl⁻)
   • CΔH lattice energy − ΔH hydration(K⁺) − ΔH hydration(Cl⁻)
   • D−ΔH lattice energy + ΔH hydration(K⁺) + ΔH hydration(Cl⁻)

   (c)

   Calculate the enthalpy change of solution of potassium chloride and use your answer to explain why the temperature of the water falls.

   [2 marks]

   Total for question 1: 4 marks

2. 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.

   (a)

   Which of these ions has the most exothermic enthalpy change of hydration?

   [1 mark]

   • ANa⁺
   • BMg²⁺
   • CK⁺
   • DCl⁻

   (b)

   Which statement about the enthalpy change of hydration of ions is correct?

   [1 mark]

   • AIt is always endothermic because bonds in water must be broken
   • BIt becomes more exothermic as the ionic radius increases
   • CIt is always exothermic because ions attract the polar water molecules
   • DIt depends only on the charge on the ion and not on its radius

   (c)

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

   [2 marks]

   Total for question 2: 4 marks

3. 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.

   (a)

   Calculate the enthalpy change of solution of magnesium chloride.

   [3 marks]

   (b)

   Use the data to explain why the enthalpy change of solution of magnesium chloride is more exothermic than that of sodium chloride.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student studies how the charges and sizes of ions affect the energy changes when ionic solids form and dissolve. For potassium bromide, the lattice energy is −679 kJ mol⁻¹, the enthalpy change of solution is +20 kJ mol⁻¹ and the enthalpy change of hydration of K⁺ is −322 kJ mol⁻¹. Ionic radii in nm: Na⁺ 0.102, Mg²⁺ 0.072, Cl⁻ 0.181, O²⁻ 0.140.

   (a)

   Calculate the enthalpy change of hydration of the bromide ion. Then suggest, with a reason, how the enthalpy change of hydration of the iodide ion compares with that of the bromide ion.

   [6 marks]

   (b)

   A student states that magnesium oxide has a more exothermic lattice energy than sodium chloride and that Mg²⁺ has a more exothermic enthalpy change of hydration than Na⁺, and that both facts have a similar cause. Evaluate the student's statement, referring to ionic charge and radius.

   [6 marks]

   Total for question 4: 12 marks

End of questions