A-LevelChemistryGroup 29701/41 · May/June 2021 · Q17 Marks

The carbonates and hydroxides of Group 2 elements show similar trends in thermal stability. Suggest and explain the variation in the trend in the thermal stability of the Group 2 hydroxides.

(a)The carbonates and hydroxides of Group 2 elements show similar trends in thermal stability. Suggest and explain the variation in the trend in the thermal stability of the Group 2 hydroxides.[3]
(b)(i)Write an equation to show the dissociation of calcium hydroxide, Ca(OH)2(s), in aqueous solution. Include state symbols.[1]
(b)(ii)Calculate the solubility, in moldm¯³, of Ca(OH)2. [Ksp: Ca(OH)2, 5.02 × 10-6 mol³ dm-] solubility = moldm³[2]
(b)(iii)Suggest how the solubility of Ca(OH)2 in aqueous NaOH compares to its solubility in water. Explain your reasoning.[1]

Mark scheme

Marking points from the official mark scheme (May/June 2021). The mark or mark type is shown in brackets.

[1] M1: increases down the group
[1] M2: radius / size of (cat)ion / M2+ increases
[1] M3: less polarisation / distortion of anion / hydroxide ion / hydroxide group / OH- / OH
[1] Ca(OH)2(s) ⇌ Ca2+(aq) + 2OH-(aq)
[1] M1: Ksp = [Ca2+][OH-]2 OR Ksp = 4x3
[1] M2: x = 3√5.02 × 10–6 / 4 = 0.0108 / 0.011 / 1.08 × 10–2 / 1.1 × 10–2 (mol dm³) min 2 sf
[1] less soluble / decreases due to the common ion effect OR decreases as equilibrium in (b)(i) has shifted to the left OR decreases as [OH-] increases causing [Ca2+][OH-](2) to exceed its Ksp

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