A-LevelChemistryGroup 29701/42 · Feb/Mar 2020 · Q211 Marks

Group 2 metals form stable carbonates and sulfates.

(a)(i)State and explain the trend in the thermal stability of the Group 2 carbonates down the group.[3]
(a)(ii)The sulfates of Group 2 elements become less soluble down the group. Explain this trend.[3]
(b)(i)The half-equation for the reaction at the anode is shown. O²⁻ + C → CO + 2e⁻ Use this half-equation to write the ionic equation for the electrolysis of Al₂O₃.[1]
(b)(ii)Aluminium oxide is electrolysed for 3.0 hours using carbon electrodes and a current of 3.5 × 10⁵ A. Calculate the mass of aluminium that is formed. mass of aluminium =[3]
(b)(iii)Cryolite can be made from SiF₄. The first step in this conversion is the reaction of SiF₄ with H₂O, forming H₂SiF₆ and SiO₂. Write an equation for this reaction.[1]

Mark scheme

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

[3] M1 (thermal stability) increases (down the group)
M2 size /radius of metal ion/M2+ increases
M3 polarisation / distortion of anion / CO32- decreases
[3] M1 lattice energy AND hydration enthalpy become less exothermic
M2 hydration enthalpy / ΔHhyd becomes less exothermic more
M3 enthalpy change of solution / ΔHsol becomes less exothermic / more endothermic
[1] 2Al3+ + 3O2- + 3C → 2Al + 3CO
[3] M1 Q = It = 3.5 × 105 × 3 × 602 = 3.78 × 109 C
M2 no. of mol e¯ = 3.78 × 109 / 96500 = 3.92 × 104
M3 mass Al = 27 x 3.92 × 104 / 3 = 3.5(3) × 105 g
[1] 3SiF4 + 2H2O → 2H2SiF6 + SiO2

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