Edexcel Jan 2016 (IAL) Paper 5 Q21

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21 This question is about the element chromium and some of its compounds. (a) (i) Complete the electronic configuration of the chromium atom, using the s, p, d notation. [Ar]*(ii) State how this electronic configuration of the chromium atom is unusual compared with most other transition metals. Give two reasons why chromium has this electronic configuration. (1) (2) D O N O T W R I T E I N T H I S A R E A16 *P46941A01628* D O N O T W R I T E I N T H I S A R E A D O N O T W R I T E I N T H I S A R E A<br /> (b) (i) A solution containing chromium(II) ions can be produced in the laboratory by reducing a solution of potassium dichromate(VI) using zinc in 50% hydrochloric acid. This reaction takes place in two steps. Dichromate(VI) ions are reduced to chromium(III) ions and the chromium(III) ions are then reduced to chromium(II) ions. Use the relevant standard reduction potentials from page 17 or standard electrode potentials on pages 14 and 16 of the Data Booklet to calculate E (cid:57) cell for each step. Use your E (cid:57) cell values to explain why both steps are spontaneous. (3)A E R A S I H T N I E T I R W T O N O D A E R A S I H T N I E T I R W T O N O D(ii) State the colour changes you would expect to see during this reaction. (1) A E R A S I H T N I E T I R W T O N O D(iii) A student suggests that the hydrogen produced by the reaction of the zinc with hydrochloric acid in this experiment is a serious risk. Evaluate the students suggestion. (1)*P46941A01728* 17 Turn over<br /> (c) Chromium(II) ions in aqueous solution are quickly oxidized by the oxygen in air. One method of stabilising chromium(II) ions is by adding a solution of sodium ethanoate, forming a complex, [Cr2(CH3CO2)4(H2O)2]. This complex may be represented by the structure below. CH3 O OH2 CH3 C Cr OC OC Cr CH3 H2O CH3 (i) What type of ligand is the ethanoate ion in this complex? D O N O T W R I T E I N T H I S A R E A (1)(ii) State the type of bonding which occurs between the ligands and the chromium(II) ions. (1)(iii) Suggest two unusual features in the structure and bonding of this complex. (2)(iv) This complex is red. Explain why the colour of chromium(II) ethanoate is different from that of Cr(H2O)6 2+(aq). (2)18 *P46941A01828* D O N O T W R I T E I N T H I S A R E A D O N O T W R I T E I N T H I S A R E A<br /> A E R A S I H T N I E T I R W T O N O D (v) Predict the number and relative areas of the peaks in the low resolution proton nmr spectrum of Cr2(CH3CO2)4(H2O)2. Justify your answers. (2)A E R A S I H T N I E T I R W T O N O D A E R A S I H T N I E T I R W T O N O D *P46941A01928* 19 Turn over<br /> (d) An experiment is carried out to check the oxidation number of chromium in chromium(II) ethanoate Cr2(CH3CO2)4(H2O)2. 1.00 g (2.66103 mol) of chromium(II) ethanoate is dissolved in 25 cm3 of 1.00 mol dm3 sulfuric acid. The solution is diluted with distilled water until the volume is 250 cm3. 25.0 cm3 portions of the diluted solution are titrated with 0.00750 mol dm3 potassium manganate(VII). Calculate the volume of potassium manganate(VII) needed to oxidize the chromium(II) ions present in each 25.0 cm3 portion to the +6 oxidation state. The manganese is reduced to the +2 oxidation state. Comment on your answer and suggest how the experiment could be improved to give a more suitable titre. D O N O T W R I T E I N T H I S A R E A (5) D O N O T W R I T E I N T H I S A R E A Comment(Total for Question 21 = 21 marks) TOTAL FOR SECTION B = 50 MARKS 20 *P46941A02028* D O N O T W R I T E I N T H I S A R E A<br />

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