Edexcel Jan 2014 (IAL) Paper 4 Q17 (with explained solutions) |

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Edexcel Jan 2014 (IAL) Paper 4 Q17

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17 The decomposition of dinitrogen pentoxide in a suitable solvent produces nitrogen dioxide, which remains in solution, and oxygen gas which is given off. The overall equation for the reaction is: N2O5 (cid:111) 2NO2 + O2 (a) Draw a diagram of the apparatus you would use to follow the rate of this reaction and give the measurements you would make. (3)(b) (i) The rate equation for this reaction is: What are the units of the rate constant, k? Rate = k[N2O5] (1)*(ii) A suggested mechanism for the reaction is: 1st Step N2O5 (cid:111) NO2 + NO32nd Step NO2 + NO3 (cid:111) NO + O2 + NO23rd Step NO + N2O5 (cid:111) 3NO2Label these reactions, fast or slow, and explain how your labelling is consistent with the rate equation for the reaction. (3)*P44451A01324* 13 Turn over<br />
(c) The rate constant, k, was calculated at different temperatures. (i) Suggest a practical method for keeping the temperature constant. (1)(ii) The table shows the measurements of the rate constant, k, at different temperatures. Some of the corresponding values for reciprocal of temperature and ln k are also shown. Complete the table by calculating the missing values. T/K 280 290 300 310 320 3.80106 1.65105 6.87105 2.48104 8.65104 K/1 1 T 3.57103 3.45103 3.33103 3.23103 ln k 12.5 11.0 9.6 8.3 *(iii) Plot a graph of ln k on the vertical axis against 1/T on the horizontal axis. Calculate the gradient of your graph and use this to calculate the activation energy, Ea. Remember to include units with your answer, which should be given to three significant figures. The Arrhenius equation can be expressed as ln k =E a R 1 T + a constant [Gas constant , R = 8.31 J K1 mol1] (2) (7) 14 *P44451A01424*<br />
(Total for Question 17 = 17 marks) *P44451A01524* 15 Turn over<br />

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Acceptable Answers Reject Mark Method 1gas collection Diagram 2 marks stoppered/ sealed side arm test tube/ stoppered/ sealed test tube with delivery tube/ stoppered/ sealed side arm flask/ stoppered/ sealed flask with delivery tube (1) gas syringe OR collection of gas over water in a measuring cylinder/ upturned burette/ graduated gas tube (1) IGNORE heat Measurements volume of gas and time (1) Method 2mass loss Diagram 2 marks digital balance (1) flask with cotton wool/ mineral wool in neck OR open flask/ beaker (1) Measurements mass (loss) and time (1) Method 3colour change Diagram 2 marks colorimeter (1) light and filter shown (1) Measurements transmission/ absorbance and time (1) amount of gas Question Number 17(a)<br />
Acceptable Answers Question Number 17(b)(i) s-1 ALLOW 1/s sec-1 any actual unit of time to power -1 Acceptable Answers Question Number 17(b)*(ii) First mark Reject time-1 t-1 Reject Mark Mark 1st Stepslow 2nd Stepfast 3rd Stepfast (1) Second markstand alone the slow(est)/ first step is the rate determining step (1) Third markconsequential on correct first mark (1 mol) N2O5 is in the rate equation so the reaction with N2O5 is the slow/ rate determining step OR only the species in the rate equation is in the first/ slow/ rate determining step ALLOW (there is only 1 mol of) one species/ N2O5 in the first/ slow/ rate determining step (1) ALLOW 1st Stepfast 2nd Stepslow 3rd Stepfast (1) the slow(est) step/second step is the rate determining step (1) there is only (1 mol of) one species in the steps up to and including the rate determining step (1)<br />
Question Number 17(c)(i) Acceptable Answers Reject (thermostatically controlled) water bath/ ice bath ALLOW oil bath direct heating with flame electrical heater Mark Mark Acceptable Answers Question Number 17(c)(ii) (1/T) 3.13 x 10-3/ 3.125 x 10-3 (1) (lnk) 7.1/ 7.05/ -7.0528 (1) Reject 3.12 x 10-3 -7.0<br />
Acceptable Answers Mark Graph3 marks First mark axes correct with sensible scales i.e. points/line covering at least 3 large squares on the x axis and 5 on the y axis, with lnk values becoming more negative down the axis and the negative signs shown (1) Second mark both axes labelled, with units on x axis and no units on y axis x axis: 3.1-3.5 1/T /10-3K-1 OR 3.1-3.5 1/Tx103/K-1 OR 0.0031-0.0035 OR 3.1 x 10-3-3.5 x 10-3 1/T /K-1 ALLOW x axis labels at bottom of page (1) Third mark points correctly plotted and straight line drawn (1) Question Number 17(c) *(iii)<br />
Gradient2 marksthis may be shown on the graph gradient = 11550 to 12760 (K) negative sign (1) value (ignore sf) (1) maximum 1 mark if an incorrect unit is given ALLOW these marks if the correct sign and value for the gradient are shown in the calculation for Ea Calculation - 2 marks If Ea = (+)96.0 to (+)106 kJ mol-1 or (+)96000 to (+)106000 J mol-1, award 2 marks If Ea is in this range but is not given to 3 sf, or the units are incorrect or missing, award 1 mark If not, Ea = 8.31 x their gradient OR gradient = -Ea/R (1) value to 3 sf and units and consequential sign if negative (1) ALLOW correct answer to 3 sf, in range, with sign and units, but no working for gradient or Ea (3) Total for Question 17 = 17 marks<br />

Question 16 17 18 19

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