A-LevelChemistryChemical energetics9701/43 · May/June 2025 · Q48 Marks
Table 4.1 gives the enthalpy changes of hydration, ΔHhyd, of three ions, F¯, K+ and Ca2+. Table 4.1 ion ΔHhyd/kJ mol-1 F- -506 K+ -322 Ca2+ -1650
- (a)(i)Define enthalpy change of hydration.[1]
- (a)(ii)Explain the relative magnitudes of the enthalpy changes of hydration of K+ and Ca2+.[2]
- (a)(iii)Define lattice energy.[1]
- (a)(iv)The lattice energy, ∆Hlatt, of calcium fluoride, CaF2, is –2602 kJ mol−1. Calculate the enthalpy change of solution, ∆Hsol, in kJ mol-1, of CaF2. ΔΗ sol of CaF2 =[2]
- (b)The formation of CaF2 at 298 K is shown. Ca(s) + F2(g) → CaF2(s) ΔΗ° = –1214 kJ mol-1 AG° = −1162 kJ mol-1 Calculate the entropy change, ∆Sº, in JK−1 mol¯¹, for this reaction. = SV[2]
Mark scheme
Marking points from the official mark scheme (May/June 2025). The mark or mark type is shown in brackets.
- [1] energy change when one mole of gaseous ions dissolves in water
- [2] M1: K+ has larger radius AND K+ has smaller charge
- [2] M2: Ca2+ has larger attraction for H2O
- [1] energy change when one mole of ionic solid forms from gaseous ions
- [2] M1: -1650 - (2 × 506) + 2602
- [2] M2: -60
- [2] M1: ΔG = ΔH - TΔS AND T = 298
- [2] M2: -174.5 [1]
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