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Edexcel Jun 2013 (R) Paper 4 Q13
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![SECTION C Answer ALL the questions. Write your answers in the spaces provided. 13 This question is about the elimination of hydrogen bromide from bromoalkanes by reaction with alcoholic potassium hydroxide. To investigate the kinetics of this reaction the following apparatus was used: C4H9Br + KOH o C4H8 + KBr + H2O Gas syringe Rubber seal HEAT A solution of concentrated potassium hydroxide in ethanol was refluxed and the gas syringe connected as shown. 0.6 cm3 of 1-bromobutane was added to the solution with a hypodermic syringe through a rubber seal. A stop clock was started and the volume of gas, Vt, measured at 2 minute intervals, for 12 minutes. When there was no further evolution of gas the volume of gas, Vfinal, was 76.5 cm3. (a) (i) Calculate the number of moles of 1-bromobutane used. You will need the values of the density and molar mass of 1-bromobutane from your Data booklet. (2) *P42984A01924* 19 Turn over<br />
(ii) Calculate the maximum volume of gaseous but-1-ene, in cm3, that could form. [Molar volume of a gas 24 000 cm3 under reaction conditions] Suggest two reasons why this volume is unlikely to form. (3)(b) The results obtained are shown in the table below. Time t/min Volume of but-1-ene Vt/cm3 (VfinalVt)/cm3 0 2 4 6 8 10 12 31.5 51.0 62.5 68.5 72.0 74.0 76.5 45.0 25.5 14.0 8.0 4.5 2.5 (i) Explain why a large excess of potassium hydroxide is used in this experiment. (1)20 *P42984A02024*<br />
(ii) Plot a graph of (VfinalVt)/cm3 against t/min. (3) (iii) Suggest why the value of (VfinalVt) was plotted on your graph. (1)*P42984A02124* 21 Turn over<br />
(iv) Measure two successive half lives from your graph. First half lifemin Second half lifemin (v) Deduce the order of reaction with respect to 1-bromobutane. Justify your answer. (2) (2)(c) In another experiment, an excess of 1-bromobutane is reacted with varying concentrations of hydroxide ions. The results for the initial rate of the reaction are shown in the table below. Experiment Number [C4H9Br] /102 mol dm3 [OH] /103 mol dm3 Initial rate /105 mol dm3 min1 1 2 3 2.50 2.50 2.50 2.50 1.25 0.50 (i) Deduce the order of reaction with respect to hydroxide ions. Justify your answer using the data in the table. 5.00 2.50 1.00 (2)22 (ii) Write the rate equation for the reaction using your answers to parts (b)(v) and (c)(i). (1) *P42984A02224*<br />
(iii) Give the units of the rate constant. *(iv) It is suggested that the reaction begins with the slow attack by a hydroxide ion on a hydrogen atom in the 1-bromobutane, as shown below. Complete the electron pair movement for this reaction using curly arrows and explain why this step is consistent with the rate equation for the reaction you have given in (c)(ii). Br C2H5 HO: H oC2H5CH=CH2 + H2O + Br (1) (3)(Total for Question 13 = 21 marks) TOTAL FOR SECTION C = 21 MARKS TOTAL FOR PAPER = 90 MARKS *P42984A02324* 23<br />](https://learnah.org/wp-content/uploads/2018/08/l_smalljpgJune_2013_R_QP_-_Unit_4_Edexcel_Chemistry_A-level_13-0.jpg)
![Section C Question Number 13(a)(i) Question Number 13(a)(ii) 5.5883 x 10-3 x 24 000 Acceptable Answers Acceptable Answers Mass of bromobutane = 0.6 x 1.276 (=0.7656 (g)) (1) Amount of bromobutane = 0.6 x 1.276 137.0 = 5.5883 x 10-3 = 5.59 x 10-3 / 0.00559 (mol) OR Amount of bromobutane = 0.6 x 1.276 136.9 = 5.5924 x 10-3 = 5.59 x 10-3 / 0.00559 (mol) TE on incorrect mass ALLOW 6 x 10-3 (mol) (1) Correct answer with no working scores 2 marks = 134.12 (134.22 from 136.9)=134 cm3 (1) ALLOW answer from (i) x 24000 IGNORE SF except 1 Any two from: Formation of butan-1-ol / other / side reactions Incomplete reaction Some but-1-ene may remain in solution (2) IGNORE Reaches equilibrium / reaction reversible But-1-ene reacts with ethanol/ solvent Reject Mark Mark Reject Transfer losses Gas escapes Gas reacts with water But-1-ene condenses 6CH0401R 1306<br />
Question Number 13(b)(i) So [OH] remains (effectively) constant Acceptable Answers OR [1-bromobutane] is the only variable IGNORE So [OH] is not the limiting factor Question Number 13 (b)(ii) Acceptable Answer Reject Ensure that all C4H9Br reacts [OH] is in excess [OH] does not affect the rate Just Only [1-bromobutane] affects the rate Mark Mark Reject Axes plotted wrong way round Volume Axes correct with sensible scales to use at least half of graph paper on both axes (1) Labels ((Vfinal - Vt) and t) fully correct with units (1) All 7 points correctly plotted and smooth curve drawn (1) Acceptable Answers Question Number 13(b)(iii) (Vfinal - Vt) is proportional to the concentration of 1-bromobutane Reject Mark 6CH0401R 1306<br />
Question Number 13(b)(iv) Two values 2.50.3 (min) Acceptable Answers (each scores one mark) (2) Question Number 13(b)(v) Answer must be consistent with values in Acceptable Answers (iv) Because half lives are constant / similar (1) The reaction is first order (1) If values in (iv) are 2.5 and 5, then: Reaction is 2nd order because half lives are increasing scores both marks. Reaction is 1st order because half lives are constant scores 1 mark Question Number 13(c)(i) Order one (1) Acceptable Answers Any one of: (Exp 1 and 2) [OH] halves and rate halves. (Exp 1 and 3) [OH] 1/5 and rate 1/5 (Exp 2 and 3) [OH] 2/5 and rate 2/5 ALLOW reverse logic (1) Question Number 13(c)(ii) Rate = k[C4H9Br][OH-] Acceptable Answers IGNORE case of K/k TE on b(v) and c(i) Question Number 13(c)(iii) dm3 mol-1 min-1 Acceptable Answers ALLOW dm3 mol-1 s-1 any sequence of units TE on (ii) Reject Reject Mark Mark Reject Mark Reject Reject Mark Mark 6CH0401R 1306<br />
Mark Reject Both are involved in the reaction Mechanism described as SN2 Arrows from OH-to H and from C-H bond to make additional bond between carbons (1) Third arrow from bond between carbon and bromine to bromine (1) (Because) both 1bromobutane and hydroxide ion appear in the RDS ALLOW Attack of OH- on H is slow, therefore this is the RDS (Because) both 1bromobutane and hydroxide ion appear in the slow step (1) IGNORE mention of rate equation Question Number 13(c)*(iv) Acceptable Answers C2H5 Total for Section C = 21 Marks Total for Paper = 90 Marks 6CH0401R 1306<br />](https://learnah.org/wp-content/uploads/2018/08/June_2013_R_MS_-_Unit_4_Edexcel_Chemistry_A-level_13-0.jpg)
