A-LevelChemistryChemical periodicity9701/42 · Feb/Mar 2022 · Q319 Marks

Titanium is a transition element in Period 4. It is commonly found as TiO2 in minerals.

(a)(i)Define transition element.[1]
(a)(ii)Identify two typical properties of transition elements. 1 2[1]
(b)(i)TiO2+(aq) is a colourless ion. Suggest why.[2]
(b)(ii)Give the electronic configuration of an isolated Ti³+ ion.[1]
(b)(iii)Write an ionic equation for the reduction of TiO2+ by zinc metal in acidic conditions.[1]
(c)(i)Acidified Ti³+(aq) reacts with oxygen dissolved in water as shown. 4Ti³+ + O2 + 2H2O → 4TiO2+ + 4H+ ∆G = -436.1 kJ mol⁻¹ The standard reduction potential, Eº, of O2 + 4H+ + 4e⁻ ⇌ 2H₂O is +1.23V. Calculate the standard reduction potential, Eº, in V, of the TiO2+(aq)/Ti³+(aq) half-cell. Show your working.[3]
(c)(ii)When aqueous citrate ions, C6H5O7³⁻, are added to Ti³+(aq), the [Ti(C6H5O7)2]³⁻(aq) complex forms. Explain, in terms of d-orbitals, why Ti³+ is able to form complex ions.[1]
(c)(iii)Acidified [Ti(C6H5O7)2]³⁻(aq) does not react with oxygen dissolved in water, unlike acidified Ti³+(aq). Suggest what this means for the value of the standard reduction potential, Eº, of the following half-cell. [Ti(C6H5O7)2]²⁻(aq) + e⁻ ⇌ [Ti(C6H5O7)2]³⁻(aq) Explain your answer.[1]
(d)(i)The titanium ions in TiF6²⁻ and Ti(acac)2Cl2 have a coordination number of 6. State what is meant by coordination number.[1]
(d)(ii)Write an equation for the formation of TiF6²⁻ from TiO2.[1]
(d)(iii)State what is meant by bidentate ligand.[2]
(d)(iv)Ti(acac)2Cl2 shows both optical and geometrical (cis/trans) isomerism. Ti(acac)2Cl2 exists as three stereoisomers. The structure of one stereoisomer of Ti(acac)2Cl2 is shown in Fig. 3.2. [Figure 3.2] Complete the structures of the other two stereoisomers of Ti(acac)2Cl2.[2]
(d)(v)The acac anion is symmetrical. Deduce which, if any, of stereoisomers 1, 2 and 3 in (d)(iv) are polar. Explain your answer.[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 (Feb/Mar 2022). The mark or mark type is shown in brackets.

[1] (d-block) element that forms one or more stable ions with incomplete d subshell / incomplete d orbitals
[1] • variable oxidation states ; • behave as catalysts ; • form coloured compounds / ions ; • form complex ions / complexes (Any two for one mark)
[2] Ti is in +4 oxidation state so no d electrons / dº OR Ti in TiO2+ has no d electrons / dº [1] ; cannot absorb photons / light in visible spectrum OR no wavelength / frequency absorbed in visible spectrum [1]
[1] (1s²) 2s2 2p6 3s2 3p6 3d1 (4s°)
[1] 2TiO2+ + 4H+ + Zn → 2Ti3+ + 2H2O + Zn2+
[3] M1: ΔG = – nEocellF AND n = 4 ; M2: ∴ Eocell = −436100 / –4(96500) = 1.13 V ecf ; M3: Eocell = E(O2,4H+|H2O) – E(TiO2+|Ti3+) = 1.23 – E(TiO2+|Ti3+) ∴ E(TiO2+|Ti3+) = (+)0.1 (V) ecf
[1] Ti³+ empty / vacant d orbitals can form dative bonds / accept a lone pair from a ligand OR Ti³+ has vacant d-orbitals which are energetically accessible
[1] the E of the half-cell must be greater than +1.23 V / E of the O2|H+ half-cell ; as Ecell < 0 and the reaction does not occur
[1] the number of co-ordinate bonds being formed by the metal atom/ion
[1] TiO2 + 6HF → TiF62- + 2H2O + 2H+ OR TIO2 + 6HF → TiF62- + 2H3O+
[2] species with two lone pairs (of electrons) [1] ; that form dative covalent/co-ordinate bonds to a central metal atom/ion [1]
[2] [mirror image of isomer I and trans isomer diagram]
[2] isomer I AND cis isomer drawn by candidate ecf [1] ; dipoles do not cancel / partial charges do not cancel [1]