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Edexcel Jun 2015 (IAL) Paper 4 Q14
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![Acceptable Answers Reject Mark Diagram of apparatus that will not work eg delivery tube starting in solution or apparatus not sealed for first mark only Measure rate at which gas is produced The first two marks can be scored from a diagram or a written account Suitable reaction vessel e.g. side arm conical flask / flask with delivery tubing attached via bung / side arm boiling/test tube / boiling/test tube with delivery tubing attached via bung (1) Method of gas collection e.g. gas syringe / upturned measuring cylinder/burette over water Allow this as a label on a poorly drawn diagram (1) Measure volume collected at time intervals / time taken to collect fixed volume Allow mention of volume and time Allow amount of gas and time Ignore measure time taken for reaction to go to completion (1) OR A suitable open reaction vessel (but plugged with cotton wool) (1) Use of balance (1) Measure the mass at various time intervals / at a fixed time on a balance Allow mention of mass (loss) and time (1) Ignore heating Question Number 14(a)<br />
Mark is dependent on the 1st mark being awarded. EITHER Reaction is endothermic /energy taken in / temperature falls Allow just lower temperature Ignore room temperature falls (1) Decreases rate of reaction (1) OR There is loss of product/gas before the apparatus is sealed (1) This is greater because the reaction is at a higher concentration (of A) (1) OR Active sites/surface (area) on catalyst full/blocked/saturated (1) Because the reaction is at a higher concentration (of A)/ decreases rate of reaction (1) Ignore references to experimental error Ignore comparisons of concentrations of A and B Ignore any reference to side-reactions Question Number 14(b)(i) Any linked pair of responses. In each pair, the 2nd mark Acceptable Answers<br />
Mark Mark Question Number 14(b) (ii) Acceptable Answers 0 order (1) As increase/change in concentration does not affect the rate /rate is independent of [A] Allow graph is a horizontal line / has zero gradient (1) Ignore graph is a straight line Ignore just there is no change in the rate / rate is constant / gradient remains constant Question Number 14(c)(i) EITHER Acceptable Answers increases reliability improves validity (of the data obtained) / confirms the initial result / to check for anomalous results Ignore References to average/precision/accuracy OR to determine order w.r.t B and/or X / to determine order w.r.t reactants / substances / to find overall order / to see the effect of B and/or X on the rate/ to see the effect of reactants/ substances on the rate/ to determine rate equation / to calculate k Allow to find out which species are in the rate determining step<br />
Mark Acceptable Answers Question Number 14(c)(ii) 2nd order w.r.t B (1) (Compare expt 1 & 2 when [X] is constant), as [B] triples so rate increases by a factor of 9 (1) First order w.r.t X (1) EITHER (using experiments 1 and 3 or 1 and 4) as [B] quadruples so rate should increase by a factor of 16 but increases by a factor of 32 / additional increase of x 2 due to doubling of [X] (hence first order w.r.t X) OR (using experiments 2 and 3 or 2 and 4) as [B] x4/3 (1.333) so rate should increase by a factor of 16/9 (1.778) but increases by 3.556 / additional increase of x2 due to doubling of [X] (hence first order w.r.t X) Allow these explanations shown as equations If C used instead of X, allow both marks for order and explanation (1) Allow TE on order w.r.t A and B Question Number 14(c) (iii) Question Number 14(c) (iv) Mark Mark Acceptable Answers Rate=k[B]2[X] / Rate=k[A]0[B]2[X] Allow r/R for rate and K for k Allow TE from b(ii) and c(ii) Acceptable Answers k = rate/[B]2[X] = 0.08/(0.1x0.1x0.2) = 40 (1) dm6mol2s1 Allow units in any order (1) Allow use of data from experiments 1, 2 & 4 Allow TE from c(iii)<br />
Mark Acceptable Answers Correct featuretwo from Mechanism does involve (formation of) a transition state Allow mechanism does involve the (formation of) an intermediate Allow transition/intermediate step (1) Second order overall / SN2 /both halogenoalkane and hydroxide ions involves in slow step/rds/1st Step (1) Correct curly arrow from C-Br bond to Br (1) Transition state has a negative charge / correct charge Or Charges on all species are correct (1) Ignore references to stereochemistry Ignore references to final product correct/ lone pairs correct Incorrect featurestwo from Curly arrow should go from OH to carbon (attached to Br as it represents movement of a lone pair of electrons) / OH- should give electrons rather than accept them Allow the arrow between C and O should be in the opposite direction (1) Bonds to OH and Br should be partial bonds /dotted lines (in transition state as insufficient electrons for (five) complete bonds) / carbon can only form four full bonds (1) Allow Dipole/partial charges on C-Br not shown (1) Ignore Mechanism should be 1 step not 2 steps for SN2 Ignore there should be a curly arrow from C-Br bond to Br in the transition state Total for Question 14 = 19 marks Question Number 14(d)<br />](https://learnah.org/wp-content/uploads/2018/08/June_2015_IAL_MS_-_Unit_4_Edexcel_Chemistry_A-level_14-0.jpg)
