A-LevelChemistryChemical periodicity9701/41 · Oct/Nov 2023 · Q512 Marks

Transition elements behave as catalysts and can form complex ions.

(a)Explain why transition elements behave as catalysts.[2]
(b)Silver forms the linear complex ion [Ag(CN)2]¯. Copper forms the tetrahedral complex ion [Cu(CN)4]3-. Titanium forms the complex [TiCl4(diars)2], where diars is a neutral bidentate ligand.
(b)(i)State the oxidation state and the coordination number of titanium in [TiCl4(diars)2]. oxidation state coordination number[1]
(b)(ii)Draw three-dimensional diagrams to show the shapes of [Ag(CN)2]⁻ and [Cu(CN)4]³⁻, in the boxes. Label one bond angle on each diagram. [Figure with two empty boxes, labeled Ag and Cu][2]
(c)The numerical value of the stability constant, Kstab, of the copper(I) complex [Cu(CN)4]3– is 2.0 x 1027.
(c)(i)Write an expression for the Kstab of [Cu(CN)4]3-. Kstab =[1]
(c)(ii)In a solution the concentrations of CN⁻ and [Cu(CN)4]³⁻ are both 0.0010 moldm⁻³. Use your expression from (c)(i) and the value of Kstab to calculate the concentration of Cu⁺(aq) in this solution. concentration of Cu⁺(aq) =[1]
(d)A piece of a copper-containing alloy has a mass of 0.567 g. It is dissolved in an acid giving 100.0 cm³ of a blue solution in which all the copper is present as Cu2+ ions. An excess of KI(aq) is added to a 25.0 cm³ sample of this solution. All of the copper is precipitated as white CuI(s). Cu2+ ions are the only component in the solution that react with KI(aq). This is reaction 1. reaction 1 2Cu2+ + 4I⁻ → 2CuI + I2 The liberated I₂ is then titrated with 0.0200 moldm⁻³ S2O32-. This is reaction 2. reaction 2 I2 + 2S2O32- → 2I⁻ + S4O62- The titration requires 20.10 cm³ of 0.0200 moldm⁻³ S2O32- to reach the end-point.
(d)(i)Calculate the number of moles of I2 that are reduced in this titration. number of moles of I2 =[1]
(d)(ii)Calculate the number of moles of copper in the original piece of alloy. number of moles of copper =[1]
(d)(iii)Calculate the percentage of copper in the alloy. percentage of copper =[1]
(d)(iv)Suggest why a solution of Cu2+ is coloured but solid CuI is white.[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 2023). The mark or mark type is shown in brackets.

[1] more than one (stable) oxidation state
[1] empty / vacant (d) orbitals are energetically accessible OR empty / vacant (d) orbitals can form dative bonds with ligands
[1] oxid. no. = + 4 AND coord. no. = 8
bond angle must go from bond to bond OR CN group to CN group
diagram
• for Ag: NC—Ag—CN
• angle = labelled 180°
• for Cu: (tetrahedral diagram)
• angle = labelled 109–110°
any two [1] all four [2]
[1] Kstab = [[Cu(CN)4]³-] / [Cu+] [CN-]4 ALLOW use of [[Cu(H2O)6]+]
[1] [Cu+] = 5.0 × 10–19 (mol dm¬³) min 1sf ECF from reversed ratio 5(c)(i)
[1] mol of I2 = 0.5 × 0.02 × 20.1 / 1000 = 2.01 × 10⁻⁴ min 2sf
[1] mol of Cu = 2.01 × 10⁻⁴ × 2 × 4 = 1.608 / 1.61 × 10⁻³ ecf min 2sf
[1] % of Cu = 100 × (1.608×10⁻³ × 63.5) / 0.567 = 18.0 ecf
[1] Cu2+ is d⁹ / d shell / sub-shell / orbitals is / are not full AND Cu⁺ is d¹⁰ / d shell / sub-shell / orbitals is / are full
[1] d-d* transitions / d electron promotion not possible / possible (linked correctly)