Edexcel Jun 2016 (IAL) Paper 6 Q

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4 Vanadium is a transition metal and forms ions in which it has oxidation numbers +2, +3, +4 and +5. Part 1 Two students want to make solutions containing ions in which vanadium has oxidation numbers of +3 and +4. Both students start with solid ammonium vanadate(V), NH4VO3. The first student decides to make an aqueous solution containing vanadium(III) ions, V3+(aq). (a) First, the student heats solid ammonium vanadate(V) to make vanadium(V) oxide, ammonia and steam. (i) Write a balanced equation for this decomposition of ammonium vanadate(V). State symbols are not required. (1) (1) 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 D O N O T W R I T E I N T H I S A R E A (ii) How would the student confirm that this decomposition is complete?(b) The student then reduces the vanadium(V) oxide to vanadium(III) oxide, V2O3, by passing hydrogen over the heated oxide. Identify the hazard associated with this procedure. (1)18 *P46666A01824*<br />
 (c) The student reacts 1.498g of vanadium(III) oxide, V2O3(s), completely with an excess of sulfuric acid, H2SO4(aq), to make 250cm3 of a solution, T, containing aqueous vanadium(III) ions, V3+(aq). (i) Calculate the concentration, in mol dm3, of vanadium(III) ions, V3+(aq), in the solution T. (2) A E R A S I H T N I E T I R W T O N O D Concentration =mol dm3 (ii) Give the formulae of two ions that would be present in high concentration in solution T, apart from vanadium(III) ions, V3+(aq) ions. (1)ions andions 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 *P46666A01924* 19 Turn over<br />
 (d) The second student decides to make a solution containing VO2+(aq) ions, in which vanadium has an oxidation number of +4. The student dissolves the ammonium vanadate(V) in dilute sulfuric acid. In this +(aq). The student needs to reduce the VO2 solution, the vanadium species is VO2 ions to VO2+(aq) ions. +(aq) The teacher suggests using sulfur dioxide as the reducing agent and supplies the student with the following standard electrode potential values. +(aq) + 2H+(aq) + e (cid:85) VO2+(aq) + H2O(l) E (cid:57) = +1.00 V VO2 VO2+(aq) + 2H+(aq) + e (cid:85) V3+(aq) + H2O(l) E (cid:57) = +0.34 V 2 (aq) + 4H+(aq) + 2e (cid:85) SO2(aq) + 2H2O(l) E (cid:57) = +0.17 V SO4 D O N O T W R I T E I N T H I S A R E A (i) These data suggest that a step-wise reduction of an acidified solution of +(aq) ions, using sulfur dioxide, SO2(aq), will initially produce VO2+(aq) ions VO2 and then V3+(aq) ions in solution. By calculating the relevant E (cid:57) occur, show that such a step-wise reduction is possible. cell values for any reactions that you predict will (2) 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 20 *P46666A02024*<br />
 A E R A S I H T N I E T I R W T O N O D (ii) When the student bubbles sulfur dioxide into a solution containing acidified VO2 +(aq) ions, VO2+(aq) ions are obtained, rather than V3+(aq) ions. Suggest why VO2+(aq) ions are produced, rather than the V3+(aq) ions predicted in (d)(i). (1)(iii) Construct the overall equation for the reduction of acidified VO2 +(aq) ions to VO2+(aq) ions by aqueous sulfur dioxide, SO2(aq). State symbols are not required. (1) 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 *P46666A02124* 21 Turn over<br />
 Part 2 Having made solutions of VO2+(aq) and V3+(aq) ions, the students decide to measure the standard cell potential between the VO2+(aq) / V3+(aq) and Al3+(aq) / Al(s) half-cells. The students set up the apparatus below to measure this standard cell potential. The solutions were at 25C. high resistance voltmeter vanadium aluminium solution containing 1mol dm3 H +(aq), 1mol dm3 VO 2+(aq) and 1mol dm3 V3+(aq) platinum wire 1mol dm3 Al2(SO4)3(aq) (e) Identify three mistakes in the way the cell has been set up and state what modifications should be made to correct them. Write your answers in the table below. (3) Mistake in set-up Modification needed to correct mistake (Total for Question 4 = 13 marks) TOTAL FOR PAPER = 50 MARKS 22 *P46666A02224* 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 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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