A-LevelChemistryGroup 29701/41 · Oct/Nov 2025 · Q115 Marks

Solutions of Group 2 hydrogencarbonates, M(HCO3)2, decompose on heating to give the corresponding metal carbonate, carbon dioxide and water.

(a)(i)Write an equation for the decomposition of strontium hydrogencarbonate, Sr(HCO3)2.[1]
(a)(ii)The thermal stability of Group 2 carbonates increases down the group. Explain this trend.[2]
(b)The hydroxides and fluorides of Group 2 elements show similar trends in solubility. Describe the trend in the solubility of the fluorides of calcium, strontium and barium. Explain your answer. least soluble __________________________________ most soluble __________________________________ explanation ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________[4]
(c)(i)Define enthalpy change of hydration, ∆Hhyd.[1]
(c)(ii)State the main factors that affect the magnitude of enthalpy change of hydration. Explain your answer.[2]
(d)Table 1.1 shows various energy changes. Table 1.1 energy change value/kJ mol-1 lattice energy of MgF2 -2957 enthalpy change of hydration, ∆Hhyd, of Mg2+ -1926 enthalpy change of hydration, ∆Hhyd, of F- -505 Use data from Table 1.1 to calculate the enthalpy change of solution, ∆Hsol, for MgF2(s). It may be helpful to draw a labelled energy cycle. Show your working. ∆Hsol of MgF2(s) = _________________________ kJ mol-1 [2][2]
(e)(i)Mercury(I) fluoride, Hg2F2, is sparingly soluble in water. The cation in Hg2F2 exists as the diatomic ion Hg22+ with a covalent Hg-Hg bond. Write the expression for the solubility product, Ksp, of Hg2F2. Include the units. Ksp = _________________________ units _________________________ [2][2]
(e)(ii)The solubility of Hg2F2 is 9.20 × 10-3 mol dm-3 at 298 K. Calculate the value of Ksp of Hg2F2 at 298K. Ksp = _________________________ [1][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 (Oct/Nov 2025). The mark or mark type is shown in brackets.

[1] Sr(HCO3)2 → SrCO3 + CO2 + H2O
[1] M1: radius of cation / M2+ increases OR charge density of cation / M2+ decreases
[1] M2: less polarisation / less distortion of the anion / carbonate ion
[1] M1: least soluble CaF2 < SrF2 < BaF2 most soluble
[1] M2: ∆Hlatt and ∆Hhyd both become less exothermic / less negative
[1] M3: ∆Hhyd changes less / becomes less exothermic by a smaller extent OR ∆Hlatt changes more / becomes less exothermic by a larger extent
[1] M4: ∆Hsol becomes more exothermic / more negative
[1] enthalpy change when one mole of gaseous ions dissolve in water to form a solution
[1] M1: ionic radii AND ionic charge
[1] M2: (ionic) radii increase / charge density decreases AND ∆Hhyd decreases/less exothermic AND less attraction between water molecules and (gaseous) ions OR as ionic charge increases / charge density increases AND ∆Hhyd increases/more exothermic AND more attraction between water molecules and (gaseous) ions
[1] M1: use of -2957 / –1926 / −505 AND 2 × (-505)
[1] M2: correct signs and evaluation ΔΗsol of MgF2(s) = −1926 + (2 × −505) – (−2957) = (+)21 kJ mol-1
[1] M1: Ksp = [Hg22+] [F-]2
[1] M2: units = mol³ dm-9
[1] Ksp = 22 × (9.20 × 10−3)3 = 3.11 × 10-6 min 2sf