A-LevelChemistryChemical bonding9701/42 · May/June 2023 · Q212 Marks

Aqueous solutions of cobalt(II) salts contain the complex ion [Co(H2O)6]2+.

(a)State two typical chemical properties of a transition element. 1 2[1]
(b)(i)Define complex ion.[1]
(b)(ii)Samples of [Co(H2O)6]2+ are reacted separately with an excess of aqueous ammonia, with an excess of concentrated HCl and with an excess of aqueous sodium hydroxide, as shown in Fig. 2.1. [Figure 2.1] Complete Table 2.1 about the reactions shown by [Co(H2O)6]2+. [Table 2.1][4]
(c)(i)The ethanedioate ion, C2O42-, can act as a bidentate ligand. Explain what is meant by a bidentate ligand.[2]
(c)(ii)The complex [Co(H2O)2(C2O4)BrCl]¯ exists as stereoisomers. Complete the three-dimensional diagrams in Fig. 2.2 to show four stereoisomers of [Co(H2O)2(C2O4)BrCl]¯. The C2O42- ligand is represented using O Ο [Figure 2.2][3]
(c)(iii)State the oxidation state of cobalt in this complex and a type of stereoisomerism shown. oxidation state of cobalt type of stereoisomerism[1]

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Mark scheme

Marking points from the official mark scheme (May/June 2023). The mark or mark type is shown in brackets.

[1] they have variable / multiple oxidation states; they behave as catalysts; they form complex ions / complexes; they form coloured compounds/ions (any two for 1 mark)
[1] (is a molecule or ion formed by a central) metal atom / metal ion bonded / surrounded by (one or more) ligands
[1] Formula [Co(NH3)6]2+ AND colour brown solution ALLOW yellow-brown solution (for excess NH3(aq))
[1] Formula [CoC4]2- AND colour blue solution (for excess concentrated HCl)
[1] Formula Co(OH)2 OR Co(OH)2(H2O)4 AND colour blue ppt. ALLOW pink ppt (for excess NaOH(aq))
[1] Type of reaction ligand exchange (for NH3 reaction) AND type of reaction ligand exchange (for HCl reaction) AND type of reaction precipitation (for NaOH reaction)
[1] M1 (a species) that donates / uses two lone pairs
[1] M2 to form a two dative / coordinate bond to a metal atom / metal ion
[1] Structure 1 of [Co(H2O)2(C2O4)BrCl]- (e.g. trans-H2O, trans-BrCl, cis-C2O4)
[1] Structure 2 of [Co(H2O)2(C2O4)BrCl]- (e.g. trans-H2O, cis-BrCl, cis-C2O4)
[1] Structure 3 of [Co(H2O)2(C2O4)BrCl]- (e.g. cis-H2O, trans-BrCl, cis-C2O4)
[1] Structure 4 of [Co(H2O)2(C2O4)BrCl]- (e.g. cis-H2O, cis-BrCl, cis-C2O4)
[1] Structure 5 of [Co(H2O)2(C2O4)BrCl]- (e.g. cis-H2O, cis-Br, cis-C2O4, optical isomer of structure 4)
[1] Structure 6 of [Co(H2O)2(C2O4)BrCl]- (e.g. cis-H2O, cis-Cl, cis-C2O4, optical isomer of structure 4)
[1] oxidation state of cobalt (+)3 AND type of stereoisomerism cis-trans OR geometric(al) OR optical

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