A-LevelChemistryChemical energetics9701/42 · May/June 2020 · Q710 Marks

Define the term electron affinity.

(a)(i)Define the term electron affinity.[2]
(a)(ii)Define the term lattice energy.[2]
(b)Use the following data and relevant data from the Data Booklet to calculate a value for the enthalpy change of formation of zinc bromide, ZnBr₂(s). You might find it helpful to construct an energy cycle. electron affinity of Br(g) = -325 kJ mol⁻¹ enthalpy change of atomisation of Zn(s) = +131 kJ mol⁻¹ enthalpy change of vaporisation of Br₂(l) = +31 kJ mol⁻¹ lattice energy of ZnBr₂(s) = -2678 kJ mol⁻¹ enthalpy change of formation of ZnBr₂(s) = kJ mol⁻¹ [4][4]
(c)(i)Explain why there is a difference between the lattice energies of ZnBr₂ and ZnCl₂.[1]
(c)(ii)Explain why there is a difference between the lattice energies of ZnCl₂ and ZnO.[1]

Mark scheme

Marking points from the official mark scheme (May/June 2020). The mark or mark type is shown in brackets.

[2] • energy change • when one electron is added • to each atom /ion in one mole of • gaseous atoms /ions Award one mark for two correct statements. Award two marks for four correct statements
[2] M1 energy change when 1 mole of an ionic compound is formed M2 from gas phase ions/ gaseous ions
[4] M1 use of data (with no multipliers) 31, 131, -2678 M2 extraction of data 908, 1730, 193 M3 use of (2 x -325) M4 evaluation of their expression correctly, as shown ΔH(ZnBr₂) = 131 + (908 +1730) + 193 + 31 + (2x-325) + (-2678) = -335 kJ mol⁻¹
[1] Br⁻ is a largest ion/larger ion than Cl⁻ so attraction between Br⁻ and Zn²⁺ is smaller
[1] O²⁻ is a smallest ion/smaller ion than Cl⁻ AND O²⁻ has the highest charge/ higher charge than Cl⁻ (so attraction between O²⁻ and Zn²⁺ is larger)

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