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OCR A Jan 2012 Paper 5 Q3
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![3 Cobalt is a transition element. Solid compounds of cobalt are often complexes and in solution, complex ions are formed. (a) In its complexes, the common oxidation numbers of cobalt are +2 and +3. Complete the electron configurations of cobalt as the element and in the +3 oxidation state: cobalt as the element: 1s2 2s2 2p6cobalt in the +3 oxidation state: 1s2 2s2 2p6[2] (b) State one property of cobalt(II) and cobalt(III), other than their ability to form complex ions, which is typical of ions of a transition element.[1] (c) Complex ions contain ligands. State the meaning of the term ligand.[1] (d) Aqueous cobalt(II) sulfate, CoSO4(aq), takes part in the following reactions. For each reaction, state the formula of the transition element species formed and the type of reaction taking place. (i) Aqueous cobalt(II) sulfate, CoSO4(aq), reacts with aqueous sodium hydroxide. transition element species formed:type of reaction:[2] (ii) Aqueous cobalt(II) sulfate, CoSO4(aq), reacts with concentrated hydrochloric acid. transition element species formed:type of reaction:[2]OCR 2012 Turn over<br />
(e) Cobalt(III) chloride, CoCl3, reacts with ammonia to form a range of complexes. These complexes contain different amounts of ammonia. Information about these complexes is summarised below. The complex ions C and D are stereoisomers. complex formula CoCl3(NH3)6 CoCl3(NH3)5 CoCl3(NH3)4 CoCl3(NH3)4 formula of complex [Co(NH3)6]3+ 3Cl[Co(NH3)5Cl ]2+ 2Cl[Co(NH3)4Cl2]+ Cl[Co(NH3)4Cl2]+ Cl(i) Complete the diagrams below to suggest possible structures for the complex ion in complexes A to D. 3+ Co Co complex ion in A complex ion in B Co Co 2+ complex ion in C complex ion in D [4]OCR 2012<br />
(ii) Chemists provided evidence for the formulae of these complexes from their reactions with aqueous silver nitrate. Aqueous silver nitrate reacts with aqueous halide ions in a precipitation reaction. An excess of silver nitrate solution was reacted with 0.0100 mol of one of the complexes A to D. 2.868 g of a precipitate was formed. Determine which complex was reacted. In your answer you should explain how the result of the experiment would allow the formula of the complex to be identified.[3] [Total: 15]OCR 2012 Turn over<br />](https://learnah.org/wp-content/uploads/2018/08/OCR_5_QP_JAN12_question_3-0.jpg)
![F325 Mark Scheme January 2012 Question Expected answers 3 a Co: (1s22s22p6)3s23p63d74s2 (cid:57) Co3+: (1s22s22p6)3s23p63d6 (cid:57) Marks Additional guidance ALLOW (1s22s22p6)3s23p64s23d7 (i.e. 4s before 3d) ALLOW upper case D, etc. and subscripts, e.g. [Ar]4S23D7 If included, ALLOW 4s0 IGNORE forms different oxidation states catalyst OR coloured (cid:57) Donates an electron/lone pair to a metal ion OR forms a coordinate bond to a metal ion (cid:57) ii Co(OH)2 (cid:57) precipitation (cid:57) CoCl4 ligand substitution (cid:57) 2 (cid:57) 1 1 2 2 10 ALLOW donates an electron pair/lone pair to a metal/transition element ALLOW dative (covalent) bond for coordinate bond Mark independently ALLOW Co(OH)2(H2O)4 ALLOW precipitate (reaction) Mark independently ALLOW ligand exchange DO NOT ALLOW just substitution<br />
F325 Mark Scheme January 2012 Question Expected answers 3 e 3+ NH3 H3N H3N NH3 NH3 Co NH3 H3N H3N Cl Co Cl NH3 NH3 (cid:57) (cid:57) H3N H3N Cl Co NH3 NH3 NH3 H3N H3N Cl Co NH3 Cl NH3 2+ (cid:57) (cid:57) Marks Additional guidance ANNOTATIONS MUST BE USED CARE: Cl can be on any position, e.g. for B H3N H3N NH3 Co NH3 Cl NH3 2+ complex ions in C and D can be other way around In one complex ion, the 2 Cls must be opposite one another In the other complex ion, the 2 Cls must be next to one another CARE: Cl atoms can be on any position, e.g. for C and D H3N Cl NH3 Co NH3 Cl NH3 H3N H3N NH3 Co NH3 Cl Cl Marking sequence See also Appendix 2 for examples 1. Mark any correct complex ions first Do not look at these complex ions again 2. Mark with crosses any complex ions with incorrect ligands. This could include Cl in complex A, and NH3Cl and NH3 but NOT NH3------ connectivity on the LEFT only and NOT Cl and NOT just NH3 Do not look at these complex ions again 3. In the remaining complex ions, identify errors in ligands (See Appendix 2): e.g. (cid:120) NH3 ligands bonded to an H on the LEFT only: NH3----- (connectivity error) (cid:120) Cl + (cid:120) NH3 Mark these complex ions to maximise errors but treat any incorrectly bonded NH3, Cl and NH3 as ECF +Cl, 11<br />
F325 Mark Scheme January 2012 Question Expected answers 3 e ii Marks Additional guidance SEE APPENDIX 2 FOR EXAMPLES 143.4 OR 107.9 + 35.5 (g mol1) used i.e. molar mass AgCl OR amount of AgCl = 0.02(000) mol (cid:57) Ratio ratio complex : Cl = 1 : 2 OR 0.01 : 0.02 (cid:57) Identificationavailable from 1 : 2 ratio OR 2Cl Therefore the complex is B (cid:57) 0.01 linked to AgCl, not complex DO NOT ALLOW AgCl2 DO NOT ALLOW 2.868 0.01 ALLOW this mark ONLY for evidence of Cl Quality of Written Communication Identification as B is dependent on correct 1 : 2 ratio OR 2Cl for this mark Total 15 12<br />](https://learnah.org/wp-content/uploads/2018/08/OCR_5_QP_JAN12_answer_3-0.jpg)
