A-LevelChemistryGroup 29701/42 · May/June 2023 · Q113 Marks

Group 2 carbonates decompose when heated to form the metal oxide and carbon dioxide.

(a)(i)Suggest a mechanism for the decomposition of the carbonate ion by adding two curly arrows in Fig. 1.1. [Figure 1.1][1]
(a)(ii)Describe the variation in the thermal stability of Group 2 carbonates. Explain your answer.[3]
(b)(i)Define lattice energy.[2]
(b)(ii)The lattice energy of the Group 2 carbonates, ΔHlatt (MCO3), becomes less exothermic down the group. The lattice energy of the Group 2 oxides, ΔHlatt(MO), also becomes less exothermic down the group. ΔHlatt(MCO3) and ΔHlatt(MO) change by different amounts going down the group. Suggest how the standard enthalpy change of the decomposition reaction for Group 2 carbonates changes down the group. Explain your reasoning in terms of the relative sizes of the anions and the relative changes in lattice energy down the group.[2]
(c)(i)Potassium sulfite, K2SO3, is used as a food additive. The concentration of sulfite ions, SO32-, can be determined by titration using aqueous acidified manganate(VII) ions, MnO4¯. A 250 cm³ solution contains 3.40g of impure K2SO3. 25.0cm³ of this solution requires 22.40cm³ of 0.0250 moldm¯³ acidified MnO4¯ to reach the end-point. All the SO32– ions are oxidised. None of the other species in the impure K2SO3 are oxidised. The reaction occurs as shown by the two half-equations. H2O + SO32- → SO42- + 2H+ + 2e- MBO4¯ + 8H+ + 5e¯ → Mn2+ + 4H2O Give the ionic equation for the reaction between SO32- and acidified MnO4¯.[1]
(c)(ii)Calculate the percentage purity of the sample of K2SO3. Show your working. percentage purity of K2SO3 =[3]
(d)Potassium disulfite, K2S2O5, is another food additive. The disulfite ion, S2O52-, has the displayed formula shown in Fig. 1.2. [Figure 1.2] Deduce the geometry (shape) around the S(α) atom in S2O52-. geometry around S(α)[1]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (May/June 2023). The mark or mark type is shown in brackets.

[1] ARROW 1 starts from O- to the C-O bond
[1] ARROW 2 starts at the C-O bond to other O- ion
[1] M1 increases (down the group)
[1] M2 (cat)ionic radius / ion size increases (down the group) OR charge density of M2+ decreases
[1] M3 less polarisation / distortion of anion / carbonate ion / CO3(2)-
[1] M1 energy released when one mole of a ionic solid / compound is formed
[1] M2 from gas (phase) ion(s) / gaseous ion(s) (under standard conditions)
[1] (∆Hdecomp / it) becomes more positive/less negative (down the group)
[1] size /(ionic) radii of oxide ion is smaller (than carbonate ion) ORA
[1] so Hlatt of oxides becomes ORA less exothermic faster OR less negative faster OR changes more OR changes faster
[1] 2MnO4- + 6H+ + 5SO32- → 2Mn2+ + 3H2O + 5SO42-
[2] M1 M2: moles MnO4 = 0.025 × 22.40 / 1000 = 5.6 × 10-4; moles SO32- = 5.6 × 10–4 × 5/2 = 1.4 × 10–3 (in 25 cm³) ecf from (c)(i) and bullet 1; moles SO32- = 1.4 × 10-2 (in 250 cm³) ecf bullet 2; mass K2SO3 = 1.4 × 10-2 × 158.3 = 2.2162 g ecf bullet 3 OR moles K2SO3 (if 100% pure) = 3.40 ÷ 158.3 = 0.02148
[1] M3: % purity = 100 × 2.2162/3.40 = 65.2 / 65.3 % ecf min 2sf OR % purity = 100 × 0.014/0.02148 = 65.2 / 65.3%
[1] S(a) tetrahedral