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 />

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