A-LevelChemistryChemical periodicity9701/42 · May/June 2021 · Q115 Marks

An aqueous solution of chromium(III) contains the green [Cr(H2O)6]3+ complex ion.

(a)(i)Complete the electronic configuration of an isolated, gaseous Cr3+ ion. 1s2 ................................................................................................................................................[1]
(a)(ii)Define the term complex ion. ........................................................................................................................................................... .........................................................................................................................................................[1]
(b)[Cr(H2O)6]3+(aq) shows some similar chemical properties to [Co(H2O)6]2+(aq). Samples of [Cr(H2O)6]3+ are reacted separately with either NaOH(aq), H2O2(aq), or excess NH3(aq). Use this information and the Data Booklet to suggest the formula of the chromium species formed. State the type of reaction taking place in each case. reagent added to [Cr(H2O)6]3+(aq) NaOH(aq) H2O2(aq) an excess of NH3(aq) formula of chromium species formed type of reaction[5]
(c)[Cr(H2O)6]2+ and [Cr2(O2CCH3)4(H2O)2] are both complexes of chromium(II) and have different colours. Explain why the colours of these complexes are different. ........................................................................................................................................................... ........................................................................................................................................................... ...........................................................................................................................................................[2]
(d)The structure of [Cr2(O2CCH3)4(H2O)2] is shown. Ethanoate ions act as ligands in this complex. The ethanoate ligand, CH3CO2-, is shown as O O- H2O- Cr O Cr OH2
(d)(i)Water and ethanoate ions behave as different types of ligand in this complex. Suggest an explanation for this statement. ...........................................................................................................................................................[1]
(d)(ii)Deduce the coordination number of Cr and the geometry around each Cr atom in this structure. coordination number ....................................................................................................................... geometry around Cr atom ...............................................................................................................[1]
(d)(iii)State the type of bond between the two atoms in the Cr-Cr bond. ...........................................................................................................................................................[1]
(e)The [Cr2(O2CCH3)4(H2O)2] complex reacts with aqueous acid to form Cr2+(aq) ions. Cr2+(aq) ions react with O2(aq) under acidic conditions. Cr3+(aq) ions are formed. Use the Data Booklet to answer the following questions.
(e)(i)Construct an ionic equation for the reaction of Cr2+(aq) with O2(aq) under acidic conditions.[2]
(e)(ii)Calculate E°cell for the reaction in (e)(i). E°cell = ...................................................................................................................... V[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 2021). The mark or mark type is shown in brackets.

[1] [1s²] 2s22p63s23p63d3
[1] (a molecule or ion formed by a central) metal atom/ion surrounded by / bonded to one or more ligands
[5] chromium species: one mark for each correct species type of reaction: two correct for one mark and three correct for two marks + NaOH(aq) -> Cr(OH)3 or Cr(OH)3(H2O)3 (precipitation) + H2O2(aq) -> Cr2O72-/CrO42- (redox/oxidation) + excess NH3(aq) -> Cr(NH3)63+ (ligand substitution)
[2] M1: ΔE is different OR energy gap between d-orbitals is different M2: different frequency / wavelength is absorbed OR different energy / light in visible region is absorbed
[1] ethanoate ions are bidentate whereas as H2O are monodentate OR ethanoate ions form two dative bonds whereas H2O forms one (dative) bond OR ethanoate ions donate two lone pairs whereas H2O donates one (lone) pair
[1] (coordination number) six AND (geometry around Cr) octahedral
[1] coordinate / (dative) covalent
[2] 4Cr2+ + O2 + 4H+ → 4Cr3+ + 2H2O OR 2Cr2+ + O2 + 2H+ → 2Cr3+ + H2O2 M1: correct species M2: balancing
[1] E°cell = 1.23 – (−0.41) = (+)1.64 V OR E°cell = 0.68 − (−0.41) = (+)1.09 V value linked to (e)(i)