A-LevelChemistryChemical energetics9701/42 · May/June 2022 · Q414 Marks

Calcium carbonate decomposes on heating. CaCO3(s) → CaO(s) + CO2(g) Table 4.1 shows the values of the Gibbs free energy change, AG°, for this reaction at various temperatures. [Figure Table 4.1] Assume the standard enthalpy change, ∆Η°, and the standard entropy change, AS, for this reaction remain constant over this temperature range.

(a)(i)Use the data in Table 4.1 to plot a graph of AG against T on the grid.[2]
(a)(ii)Calculate the gradient of your graph. Determine the ∆S® in JK-1mol-¹ for this reaction. Show all working.[2]
(b)(i)Group 1 hydrogencarbonates, MHCO3, decompose on gentle heating to give the corresponding metal carbonate, carbon dioxide and water vapour. Write an ionic equation for the decomposition of the hydrogencarbonate ion.[1]
(b)(ii)The thermal stability of Group 1 hydrogencarbonates increases down the group. Suggest an explanation for the trend in thermal stability of the Group 1 hydrogencarbonates.[2]
(c)(i)The buffer system in seawater contains a mixture of HCO3 and H2CO3. equilibrium 5 H2CO3 + H2O = HCO3 + H3O+ Define a buffer solution.[2]
(c)(ii)Construct two equations to show how equilibrium 5 acts as a buffer solution.[2]
(c)(iii)The [HCO3¯]/[H2CO3] ratio in a sample of seawater is 14.1. Calculate the pH of this sample. [pK: H2CO3, 6.35][3]

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Mark scheme

Marking points from the official mark scheme (May/June 2022). The mark or mark type is shown in brackets.

[2] M1 all five points plotted correctly
[2] M2 best-fit straight line (ruler) with negative gradient drawn
[2] M1 gradient correctly calculated OR gradient working seen
[2] M2 gradient = −ΔS° ΔSº evaluated correctly ecf ΔS° = (+)160 ± 10 (JK-1 mol-1)
[1] 2HCO3 → CO32- + CO2 + H2O
[2] M1 ionic radius of M+ / cationic radius increases OR charge density of ion / M+ decreases down Group 1
[2] M2 less distortion / polarisation of the anion / HCO3- OR CO bond / C-O / C=O less weakened
[2] M1 solution which resists changes in pH when opposes / resists change in pH
[2] M2 when small amount of acid / H+ or alkali/base/ OH- is added
[2] M1 (with acid) HCO3¯ + H+ → H2CO3_OR_HCO3¯ + H3O+ → H2CO3 + H2O
[2] M2 (with alkali) H2CO3 + OH- → HCO3¯ + H2O
[3] M1 Ka = 10-6.35 = 4.47 × 10-7
[3] M2 [H+] = 4.47 × 10–7 / 14.1 = 3.17 × 10-8 ecf
[3] M3 pH = -log [H+] = 7.5 ecf from a calculated [H+] min 2sf

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