A-LevelChemistryGroup 29701/42 · May/June 2022 · Q116 Marks

The solubility of the Group 2 hydroxides increases down the group. Explain this trend.

(a)The solubility of the Group 2 hydroxides increases down the group. Explain this trend.[3]
(b)(i)The solubility of Be(OH)2 in water is 2.40 × 10-6gdm⁻³ at 298 K. Write an expression for the solubility product, Ksp, of Be(OH)2 and state its units.[2]
(b)(ii)Calculate the numerical value of Ksp for Be(OH)2 at 298 K.[2]
(c)(i)Be(OH)2 is soluble in aqueous solutions containing an excess of hydroxide ions and forms the complex ion [Be(OH)4]²¯. This complex ion has a similar shape to that of [CuCl4]2-. Define the term complex ion.[1]
(c)(ii)Draw a three-dimensional diagram to show the structure of the complex ion [Be(OH)4]2-. Name the shape of the [Be(OH)4]²- complex ion.[1]
(d)(i)Explain why transition elements can form complex ions.[1]
(d)(ii)Complete Table 1.1 to show the coordination number of each metal ion, and the shapes and overall polarities of the complex ions listed. [Figure Table 1.1][2]
(e)(i)Define stability constant, Kstab.[1]
(e)(ii)Nickel can form complexes with the ligands en, H2NCH2CH2NH2, and tn, H2NCH2CH2CH2NH2, as shown. equilibrium 1 [Ni(H2O)6]2+ + 3en ⇌ [Ni(en)3]2+ + 6H2O Kstab = 6.76 × 1017 equilibrium 2 [Ni(H2O)6]2+ + 3tn ⇌ [Ni(tn)3]2+ + 6H2O Ketab = 1.86 x 1012 Construct an expression for the stability constant, Kstab, for equilibrium 1. State the units for Kstab.[2]
(e)(iii)Describe what the Kstab values indicate about the position of equilibrium for equilibrium 1 and 2. Use the Kstab values to deduce which complex, [Ni(en)3]2+ or [Ni(tn)3]2+, is more stable.[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 2022). The mark or mark type is shown in brackets.

[3] M1 ΔHlatt and ΔHhyd both become less exothermic / less negative
[3] M2 ΔHhyd changes less / becomes less exothermic by a smaller extent OR ΔHlatt changes more / dominant factor / changes faster
[3] M3 ΔHsol becomes more exothermic / more negative OR ΔHsol becomes less endothermic / less positive
[2] M1 Ksp = [Be2+][OH-]2
[2] M2 mol³ dm-9 ecf
[2] M1 solubility of Be(OH)2 = 2.40 × 10–6 / 43 = 5.58 × 10-8 (mol dm³) ecf (b)(i)
[2] M2 Ksp = 4 × (5.58 × 10−8)3 = 6.95 × 10–22 ecf min 2sf
[1] as a molecule or ion formed by a central metal atom / metal ion dative bonded to / surrounded by one or more ligands
[1] [Figure of tetrahedral Be(OH)4(2-) structure] tetrahedral and correct 3D structure
[1] d orbitals that are energetically accessible OR empty / vacant d orbitals AND form dative bonds / accept a lone pair from a ligand
[2] [Table values for complex ion: cis-[Pt(en)Cl2] shape: square planar coordination number: 4 polarity: polar; [Ag(NH3)2]+ shape: linear coordination number: 2 polarity: non-polar; [Fe(C2O4)3]3- shape: octahedral coordination number: 6 polarity: non-polar] three for one mark, six for two marks
[1] the equilibrium constant for the formation of the complex ion in a solvent (from its constituent ions or molecules)
[2] M1 Kstab = [Ni(en)32+] / [Ni(H2O)62+][en]3
[2] M1 units = mol-3 dm9 ecf
[1] the equilibria lie to the right / product side OR equilibrium 1 lies more to the right AND en complex is more stable / [Ni(en)3]2+ is more stable