OCR A Specimen Paper 3 Q6

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6 Hydroxylamine, NH2OH, is a strong reducing agent. 18 (cid:127) Fe2+ + N2 + H+ + H2O When heated in aqueous solution, NH2OH reduces Fe3+ ions to Fe2+ ions. A student suggests the three possible equations for the reaction, shown below. Equation 1 NH2OH + Fe3+ Equation 2 NH2OH + 2Fe3+ (cid:127) 2Fe2+ + N2O + 2H+ + H2O Equation 3 NH2OH + 3Fe3+ (cid:127) 3Fe2+ + NO + 3H+ The student plans to carry out an investigation to determine which equation is correct. The method is outlined below. Stage 1 Using a pipette, add 25.0 cm3 of 4.32102 mol dm3 NH2OH to a conical flask. Add 10 cm3 of 1 mol dm3 H2SO4 to the conical flask followed by an excess of a solution containing 0.0400 mol dm3 Fe3+(aq). Boil the mixture for 5 minutes and allow to cool. Titrate the cooled mixture with 2.00102 mol dm3 KMnO4(aq). (a) Determine the minimum volume of 0.0400 mol dm3 Fe3+(aq) that the student should plan to use in Stage 2 Stage 3 Stage 1. Explain your reasoning. Specimen volume =cm3 explanation:OCR 2014 H432/03 [4]<br /> 19 (b) In the students titration, 21.6 cm3 of KMnO4(aq) is required to reach the end point. MnO4 (aq) + 8H+(aq) + 5Fe2+(aq)Mn2+(aq) + 5Fe3+(aq) + 4H2O(l) The equation that takes place during the titration is shown below. Analyse the students results to determine which of the three equations is correct. Show all your working. Specimen (c) The student intends to repeat the procedure to check their results. [3] There is insufficient time for the student to repeat all three stages and the student decides to omit Stage 2, the boiling stage. Unfortunately the resulting titre is much less than the original titre. The student rejects the results from the repeated procedure. (i) Suggest the purpose of the boiling in Stage 2 and reasons for the second titre being much less than the original titre.[2]OCR 2014 H432/03 Turn over<br /> (ii) The main reason for insufficient time is the need to boil and cool the mixture for each titration. 20 Suggest how the procedure could be modified so that Stage 2 does not need to be carried out repeatedly. Give your reasoning.[1] END OF QUESTION PAPER Specimen Copyright Information: OCR is committed to seeking permission to reproduce all third-party content that it uses in the assessment materials. OCR has attempted to identify and contact all copyright holders whose work is used in this paper. To avoid the issue of disclosure of answer-related information to candidates, all copyright acknowledgements are reproduced in the OCR Copyright Acknowledgements booklet. This is produced for each series of examinations and is freely available to download from our public website (www.ocr.org.uk) after the live examination series. If OCR has unwittingly failed to correctly acknowledge or clear any third-party content in this assessment material, OCR will be happy to correct its mistake at the earliest possible opportunity. For queries or further information please contact the Copyright Team, First Floor, 9 Hills Road, Cambridge CB2 1GE. OCR is part of the Cambridge Assessment Group; Cambridge Assessment is the brand name of University of Cambridge Local Examinations Syndicate (UCLES), which is itself a department of the University of Cambridge.OCR 2014 H432/03<br />

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