A-LevelChemistryChemical periodicity9701/41 · Oct/Nov 2022 · Q611 Marks

An aqueous solution of copper(II) sulfate is a blue colour due to the presence of [Cu(H2O)6]2+ complex ions.

(a)(i)Write an equation for the reaction between [Cu(H2O)6]2+ ions and NaOH(aq). [1][1]
(a)(ii)Write an equation for the reaction between [Cu(H2O)6]2+ ions and an excess of conc. HCl. [1][1]
(b)(i)If an excess of ammonia is added to a solution of [Cu(H2O)6]2+ a deep blue solution containing [Cu(NH3)4(H2O)2]2+ complex ions is formed. There are two possible stereoisomers with the formula [Cu(NH3)4(H2O)2]2+. Complete the diagrams to show the two stereoisomers in the boxes below. [Figure two incomplete diagrams for isomer 1 and isomer 2][1]
(b)(ii)Use your answer in (b)(i) to deduce whether each of these isomers is polar or non-polar. polarity of isomer 1 polarity of isomer 2 [1][1]
(c)(i)The numerical value of the stability constant, Kstab, of the [Cu(NH3)4(H2O)2]2+ complex ion is 1.40 × 1013. Define stability constant. [1][1]
(c)(ii)Compare the stabilities of the [Cu(H2O)6]2+ and [Cu(NH3)4(H2O)2]2+ complex ions. Explain your answer. [1][1]
(c)(iii)Write an expression for the stability constant, Kstab, of the [Cu(NH3)4(H2O)2]2+ complex ion. State the units of the stability constant. Kstab = units = [2][2]
(c)(iv)In a particular solution the concentration of the [Cu(NH3)4(H2O)2]2+ complex ion is 0.0074 mol dm-3 and the concentration of NH3 is 0.57 mol dm-3. Use your expression in (c)(iii) and the Kstab value of 1.40 × 1013 to calculate the concentration of the [Cu(H2O)6]2+ complex ion in this solution. concentration of [Cu(H2O)6]2+ = ...................................... mol dm-3 [1][1]
(d)Phenanthroline, C12H8N2, and ethanedioate ions, C2O42-, are bidentate ligands. Ruthenium(III) ions, Ru3+, form an octahedral complex with phenanthroline and chloride ions. The complex ion contains two phenanthroline molecules. Iron(III) ions, Fe3+, form an octahedral complex with ethanedioate ions only. Deduce the formula and charge of each of these complex ions. Ru3+ complex Fe3+ complex [2][2]

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 2022). The mark or mark type is shown in brackets.

[1] [Cu(H2O)6]2+ + 2OH- → [Cu(OH)2(H2O)4] + 2H2O OR [Cu(H2O)6]2+ + 2NaOH → [Cu(OH)2(H2O)4] + 2H2O + 2Na+
[1] [Cu(H2O)6]2+ + 4Cl- → [CuCl4]2- + 6H2O OR [Cu(H2O)6]2+ + 4HCl → [CuCl4]2− + 6H2O + 4H+
[1] cis and trans drawn (as shown in the mark scheme)
[1] cis identified as polar AND trans identified as non-polar conditional on diagram
[1] the equilibrium (constant) for the formation of the complex ion in a solvent from its constituent ions or molecules
[1] [Cu(NH3)4(H2O)2]2+ AND as Kstab is large OWTTE
[2] Kstab = [[Cu(NH3)4(H2O)2]2+] / [[Cu(H2O)6]2+] [NH3]4 [1] mol-4 dm12 [1] ecf M1
[1] [Cu(H2O)6]2+] = (0.0074) ÷ (1.4 × 1013 × 0.574) [Cu(H2O)6]2+] = 5.01 × 10–15 min 2sf ecf 6(c)(iii)
[2] [Ru(C12H8N2)2Cl2]+ [1] [Fe(C2O4)3]3- [1]