Potassium chloride, KCl, and magnesium chloride, MgCl2, are both ionic solids. [Table 1.1] energy change value/kJ mol-1 standard enthalpy change of solution, ΔH°sol, of KCl +15 lattice energy, ΔH°latt, of KCl(s) -701 standard enthalpy change of hydration, ∆H°hyd, of K+ -322 standard enthalpy change of hydration, ∆H°hyd, of Cl- -364 standard enthalpy change of solution, ΔH°sol, of MgCl2 -155 lattice energy, ∆H°latt, of MgCl2(s) -2493

1. (a)Complete the energy cycle involving the enthalpy change of solution and the lattice energy of potassium chloride, KCl, and the relevant enthalpy changes of hydration. Label your diagram. State symbols should be used. [Figure incomplete energy cycle diagram with empty boxes and KCl(s) at bottom right, requiring K+(g) + Cl-(g) and K+(aq) + Cl-(aq) or KCl(aq) to be filled][2]
2. (b)Use the data in Table 1.1 to calculate the enthalpy change of hydration of magnesium ions, Mg2+. Show your working. AHhyd of magnesium ions, Mg2+ = ...................................... kJ mol-1 [2][2]
3. (c)Explain the reasons why the lattice energy of MgCl2 is more exothermic than the lattice energy of KCl. [2][2]
4. (d)(i)Define the following terms. enthalpy change of atomisation [1][1]
5. (d)(ii)first electron affinity [1][1]
6. (e)(i)Explain what is meant by entropy, S. [1][1]
7. (e)(ii)Potassium chloride is very soluble in water at 20°C. Explain the solubility of potassium chloride by reference to change in entropy, ΔS. [1][1]
8. (e)(iii)Use the Gibbs equation and your answer to (e)(ii) to predict whether potassium chloride is more soluble in water at 20 °C or at 80°C. Explain your answer. [1][1]

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