A-LevelChemistryGroup 29701/43 · Oct/Nov 2025 · Q115 Marks

(a) Solutions of Group 2 hydrogencarbonates, M(HCO3)2, decompose on heating to give the corresponding metal carbonate, carbon dioxide and water. (i) Write an equation for the decomposition of strontium hydrogencarbonate, Sr(HCO3)2. [1] (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] (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, ∆Ηsol, for MgF2(s). It may be helpful to draw a labelled energy cycle. Show your working. ΔΗsol of MgF2(s) = .............................................. kJ mol⁻¹ [2] (e) 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. (i) Write the expression for the solubility product, Ksp, of Hg2F2. Include the units. Ksp = units [2] (ii) The solubility of Hg2F2 is 9.20 × 10⁻³ moldm⁻³ at 298 K. Calculate the value of Ksp of Hg2F2 at 298K. Ksp = [1]

(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, ∆Ηsol, for MgF2(s). It may be helpful to draw a labelled energy cycle. Show your working. ΔΗsol of MgF2(s) =[2]
(e)(i)Write the expression for the solubility product, Ksp, of Hg2F2. Include the units. Ksp = units[2]
(e)(ii)The solubility of Hg2F2 is 9.20 × 10⁻³ moldm⁻³ at 298 K. Calculate the value of Ksp of Hg2F2 at 298K. Ksp =[1]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2025). The mark or mark type is shown in brackets.

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

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