Skip to content
Edexcel Jun 2015 Paper 4 Q14
Answers available below
Show answer
Question
13
14
15
16
17
![14 The kinetics of the fast reaction below were investigated in a series of experiments. catalyst X A + B C + D (cid:507)H is negative (a) Outline a titrimetric method that could be used to measure the change in concentration of compound A with time. Compound A is an alkali, whereas compounds B, C and D are neutral. (3)16 *P42978A01628*<br />
(b) The rate of the reaction was measured at several different concentrations of A, in the presence of a large excess of compound B and a constant amount of catalyst X, to find the order of reaction with respect to A. The results are shown on the graph below. anomalous results Rate / mol dm3s1 [A] / mol dm3 (i) Explain how the graph confirms that the reaction is first order with respect to A. (1)(ii) Suggest an explanation, other than human error, for the two anomalous results circled on the graph. (3)*P42978A01728* 17 Turn over<br />
(c) In a second series of experiments, further data were collected using an initial rates method. These results are summarised in the table below. Experiment Initial concentration / mol dm3 A X Initial rate / mol dm3 s1 0.020 0.040 0.060 0.080 0.005 0.005 0.010 0.010 0.500 0.500 0.500 0.250 2.1103 4.2103 6.3103 4.2103 (i) Give one reason why obtaining these further data may be considered useful. (1)18 *P42978A01828*<br />
(ii) Deduce the rate equation for this reaction, explaining how you arrived at your answer. (5)(iii) Use your answer from (c)(ii), and appropriate data from Experiment 4, to calculate the value of the rate constant, k. Include units in your answer. (2) *P42978A01928* 19 Turn over<br />
(d) A student carried out a similar investigation into the kinetics of the reaction between 2-bromomethylpropane and hydroxide ions. A summary of the students findings is shown below. Kinetics Investigation - Summary of Key Findings Reaction is first order with respect to 2-bromomethylpropane Suggested Mechanism SN2 - as two steps in process Br CH3 CH3 CH3 H3C H3C C+ OHCH3 Step 1 Fast Step 2 Slow H3C H3C BrOH CH3 C+ CH3 CH3 CH3 Use your knowledge of the mechanism of nucleophilic substitution reactions to suggest one feature of the summary, including the students mechanism, that you agree with and two features you think are incorrect. (3) One feature you agree with.Two features you think are incorrect.(Total for Question 14 = 18 marks) 20 *P42978A02028*<br />](https://learnah.org/wp-content/uploads/2018/08/l_smalljpgJune_2015_QP_-_Unit_4_Edexcel_Chemistry_A-level_14-0.jpg)
![Correct Answer Reject Mark 1st mark: Take samples (of reaction mixture) at various times OR Using of different mixtures (e.g. in separate conical flasks) (1) THEN: EITHER Quench (with ice) / remove the catalyst (1) Titrate with acid of known concentration/standard (using a suitable indicator) NaHCO3 NaHCO3 Reject Mark OR Quench with acid Titrate with alkali of known concentration (using a suitable indicator) (1) (1) (1) If no quenching, M3 can only be awarded if titrate with acid of known concentration Correct Answer (As) rate is (directly) proportional to concentration / as [A] doubles so does rate / rateconcentration / rate[A] ALLOW Just straight line through origin/(0,0) IGNORE References just to a constant gradient References to just it is a straight line References to positive correlation Question Number 14 (a) Question Number 14 (b) (i)<br />
Reject Mark Correct Answer 1st mark: Rate higher than expected / rate unusually high / higher rate (for the anomalous points on the graph) (1) 2nd mark: Reaction is exothermic / (heat) energy is released during the reaction (1) 3rd mark: EITHER (So) there are more particles/collisions with energy > Ea ALLOW Higher proportion of successful collisions / just more successful collisions IGNORE Just more collisions / more frequent collisions OR At higher concentrations of A, the effect of the reaction being exothermic is greater (1) Question Number 14 (b) (ii)<br />
Question Number 14 (c) (i) Question Number 14 (c) (ii) Reject Mark Correct Answer Increases reliability / improves validity (of the data obtained) / confirms the initial results / to check for anomalous results IGNORE References to average / precision / accuracy OR To determine order w.r.t. B and X / to see the effect of B and X (on the rate) / enables order of other reagents to be determined / to determine order w.r.t. B / find overall order / determine rate equation / to calculate k Correct Answer 0 order w.r.t. B 1st order w.r.t. X Reject Mark (1) (1) Rate =k [A][X] OR Rate = k [A][X][B]0 ALLOW TE for CQ correct rate equation on incorrect order(s) (1) Correct reasoning using data from table to deduce the CORRECT order w.r.t. B NOTE that there must be reference to TWO relevant concentrations changing Eg (Expt 1 & 3) [A] triples, so does rate AND [B] doubles so order w.r.t. B is 0 (Expt 2 & 3) [A] x 1.5, rate x 1.5 AND [B] doubles so order w.r.t. B is 0 This mark can only be awarded if the reasoning shows that order w.r.t B is zero. Not enough just to say as [B] doubles, rate unchanged Correct reasoning using data from table to (1)<br />
deduce the CORRECT order w.r.t. X NOTE that there must be reference to TWO relevant concentrations changing E.g. (Expt 1 & 4) [A] x 4 (and [B] x 2) AND [X]2 rate doubles so order w.r.t. X is 1 (Expt 2 & 4) [A] x 2 (and [B] x 2) AND [X]2 rate stays the same so order w.r.t. X is 1 (Expt 3 & 4) [A] x 4/3 (and [B] stays the same) AND [X]2 rate decreases by 2/3, so order w.r.t. X is 1 This mark can only be awarded if the reasoning shows that order w.r.t X is one. Not enough just to say as [X] doubles, rate doubles (1) IGNORE Any justification not concluded from data in the table Working to confirm order w.r.t. A = 1 (already given in question) NOTE Correct rate equation alone scores M1, M2 and M3<br />
Reject Mark Correct Answer k = rate / [A][X] = 4.2 x 103(0.08 x 0.25) = 0.21 (1) dm3 mol1 s1 / mol1 dm3 s1 ALLOW units in any order (1) Comment Unit mark is independent of the value Allow use of data from experiments 1, 2 & 3 Allow TE from an incorrect rate equation given in answer to Q14(c)(ii) or a new rate equation given at the start of answer to Q14(c)(iii), if of the form rate = kQuestion Number 14 (c) (iii)<br />
Correct Answer Question Number 14 (d) Reject Mark Correct feature ANY one of: First step does involve carbocation formation / carbocation is correct / two electrons taken by Br atom in CBr bond / CBr bond breaks (heterolytically) (Second step does involve) attack of hydroxide ion First order wrt the halogenoalkane / 1st order wrt 2-bromomethylpropane (SN1) is a two-step process Curly arrows are correct (1) Incorrect features ANY two of:Should be SN1 (not SN2)First step is slowSecond step is fast(It is not) SN2C+ Br - not shown / dipole on CBr bond not shown (2) TOTAL FOR QUESTION 14 = 18 marks)<br />](https://learnah.org/wp-content/uploads/2018/08/June_2015_MS_-_Unit_4_Edexcel_Chemistry_A-level_14-0.jpg)
