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OCR A Jun 2010 Paper 5 Q6
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![6 Chromium shows typical properties of a transition element. The elements name comes from the Greek word Chroma meaning colour because of its many colourful compounds. 15 (a) Write down the electron configuration of (i) a Cr atom,[1] (ii) a Cr3+ ion.[1] (b) An acidified solution containing orange Cr2O7 form a solution containing Zn2+ ions and blue Cr2+ ions. 2 ions reacts with zinc in a redox reaction to The unbalanced half-equations are shown below. Zn Zn2+ + e Cr2O7 2 + H+ + e Cr2+ + H2O Balance these equations and construct an overall equation for this reaction.[3] (c) Aqueous solutions of Cr3+ ions contain ruby-coloured [Cr(H2O)6]3+ complex ions. If an excess of concentrated ammonia solution is added, the solution changes to a violet colour as the hexaammine chromium(III) complex ion forms. (i) What type of reaction has taken place?[1] (ii) Suggest an equation for this reaction.[2]OCR 2010 Turn over<br />
(d) Chromium picolinate, Cr(C6H4NO2)3, is a bright red complex, used as a nutritional supplement to prevent or treat chromium deficiency in the human body. 16 In this complex,chromium has the +3 oxidation state, picolinate ions, C6H4NO2 , act as bidentate ligands. The structure of the picolinate ion is shown below. O Cr(C6H4NO2)3 exists as a mixture of stereoisomers. (i) What is meant by the term ligand ?[1] (ii) How is the picolinate ion able to act as a bidentate ligand?[2] (iii) Why does Cr(C6H4NO2)3 exist as a mixture of stereoisomers? Draw diagrams of the stereoisomers as part of your answer.[3]OCR 2010<br />
(e) Compound A is an orange ionic compound of chromium with the percentage composition by mass N, 11.11%; H, 3.17%; Cr, 41.27%; O, 44.45%. Compound A does not have water of crystallisation. 17 On gentle heating, compound A decomposes to form three products, B, C and water. B is a green oxide of chromium with a molar mass of 152.0 g mol1. C is a gas. At RTP, each cubic decimetre of C has a mass of 1.17 g. In the steps below, show all your working.Calculate the empirical formula of compound A.Deduce the ions that make up the ionic compound A.Write an equation for the decomposition of compound A by heat. Identify substances B and C. TURN OVER FOR QUESTION 7OCR 2010 [8] [Total: 22] Turn over<br />](https://learnah.org/wp-content/uploads/2018/08/OCR_5_QP_JUN10_question_6-0.jpg)
![F325 Mark Scheme June 2010 Question 6 a i 1s22s22p63s23p63d54s1 (cid:57) Expected Answers ii 1s22s22p63s23p63d3 (cid:57) Zn (cid:127)(cid:111) Zn2+ + 2e (cid:57) 2 + 14H+ + 8e (cid:127)(cid:111) 2Cr2+ + 7H2O (cid:57) Marks Additional Guidance ALLOW 1s22s22p63s23p64s13d5 (i.e. 4s before 3d) ALLOW [Ar]4s13d5 OR [Ar]3d54s1 ALLOW [Ar]3d3 ALLOW 1s22s22p63s23p63d34s0 OR [Ar]3d34s0 ALLOW multiples WATCH for balancing of the equations printed on paper IF printed equations and answer lines have different balancing numbers OR electrons, IGNORE numbers on printed equations (i.e. treat these as working) and mark responses on answer lines only NO ECF for overall equation i.e. the expected answer is the ONLY acceptable answer ALLOW ligand exchange 1 mark is awarded for each side of equation ALLOW equilibrium sign ALLOW 1 mark for 2+ shown instead of 3+ on both sides of equation ALLOW 1 mark for substitution of 4 NH3: [Cr(H2O)6]3+ + 4NH3 (cid:127)(cid:111) [Cr(NH3)4(H2O)2]3+ + 4H2O ALLOW donates an electron pair to a metal ALLOW dative (covalent) bond for coordinate bond First mark is for the idea of two coordinate bonds ALLOW lone pair on O and N DO NOT ALLOW lone pairs on COO (could involve C) Second mark is for the atoms that donate the electron pairs Look for the atoms with lone pairs also on response to (d)(iii) and credit here if not described in (d)(ii) Cr2O7 4Zn + Cr2O7 2 + 14H+ (cid:127)(cid:111) 4Zn2+ + 2Cr2+ + 7H2O (cid:57) i Ligand substitution (cid:57) ii [Cr(H2O)6]3+ + 6NH3 (cid:127)(cid:111) [Cr(NH3)6]3+ + 6H2O (cid:57) (cid:57) i Donates an electron pair to a metal ion OR forms a coordinate bond to a metal ion (cid:57) ii Donates two electron pairs OR forms two coordinate bonds (cid:57) Lone pairs on two O atoms (cid:57) 14<br />
F325 Mark Scheme June 2010 Question Expected Answers iii Forms two optical isomers OR two enantiomers OR two non-superimposable mirror images (cid:57) Marks Additional Guidance IGNORE any charges shown ALLOW any attempt to show bidentate ligand. Bottom line is the diagram on the left. 1 mark for 3D diagram with ligands attached for ONE stereoisomer. Must contain 2 out wedges, 2 in wedges and 2 lines in plane of paper: Cr Cr Cr OR Cr 2nd mark for reflected diagram of SECOND stereoisomer. The diagram below would score the 2nd mark but not the first (cid:57)(cid:57) For each structure Cr Cr 15<br />
Expected Answers Marks + (cid:57) 2 (cid:57) N : H : Cr : O 11.1/14 : 3.17/1 : 41.27/52 : 44.45/16 OR 0.793 : 3.17 : 0.794 : 2.78 (cid:57) A: N2H8Cr2O7 (cid:57) Ions: NH4 Cr2O7 B: Cr2O3 (cid:57) Correctly calculates molar mass of C = 1.17 (cid:117) 24.0 = 28.08 (g mol1) (cid:57) C: N2 (cid:57) Equation: (NH4)2Cr2O7 (cid:127)(cid:111) Cr2O3 + 4H2O + N2 (cid:57) Additional Guidance ANNOTATIONS MUST BE USED ALLOW A: (NH4)2Cr2O7 IF candidate has obtained NH4CrO4 for A, ALLOW NH4 + DO NOT ALLOW CrO4ALLOW: (relative) molecular mass ALLOW: 28 ALLOW: C is 28 ALLOW N2H8Cr2O7 in equation. F325 Question Mark Scheme June 2010 Total 22 16<br />](https://learnah.org/wp-content/uploads/2018/08/OCR_5_QP_JUN10_answer_6-0.jpg)
