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Edexcel Jan 2013 Paper 4 Q18
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![18 Bromate(V) ions, BrO3 shown in the equation below. , oxidize bromide ions, Br, in the presence of dilute acid, H+, as BrO3 (aq) + 5Br(aq) + 6H+(aq) o 3Br2(aq) + 3H2O(l) Three experiments were carried out using different initial concentrations of the three reactants. The initial rate of reaction was calculated for each experiment. The results are shown in the table below. Experiment number (aq)] / [BrO3 mol dm3 [Br(aq)] / mol dm3 [H+(aq)] / mol dm-3 0.050 0.050 0.15 0.25 0.25 0.50 0.30 0.60 0.30 Initial rate of reaction / mol dm3 s1 1.68 x 105 6.72 x 105 1.01 x 104 *(a) (i) This reaction is first order with respect to BrO3 (aq). State, with reasons, including appropriate experiment numbers, the order of reaction with respect to (5) H+(aq)Br(aq)(ii) Write the rate equation for the reaction. (1) 22 *P41214A02228*<br />
(iii) Use the data from experiment 1 and your answer to (a)(ii) to calculate the value of the rate constant. Include units in your answer. (b) What evidence suggests that this reaction proceeds by more than one step? (3) (1)(c) The initial rate of reaction was obtained from measurements of the concentration of bromine at regular time intervals. How is the initial rate of formation of bromine calculated from a concentration-time graph? (2)(Total for Question 18 = 12 marks) TOTAL FOR SECTION B = 52 MARKS *P41214A02328* 23 Turn over<br />](https://learnah.org/wp-content/uploads/2018/08/l_smalljpgJanuary_2013_QP_-_Unit_4_Edexcel_Chemistry_A-level_18-0.jpg)
![Reject Mark Reject Mark Question Number 18(a)(i) Acceptable Answers ] triples (withIn experiments 1 and 2, [H+] doubles (whilst keeping other concentrations constant) and the rate quadruples / rate increases x 4 (1)Second order (with respect to H+) (1)In experiments 1 and 3, [Br] doubles and [BrO3 [H+] constant) (1)Rate increases by 3 x 2 / rate increases x 6 / rate increases to 5.04 x 10-5 (then to 1.01 x 10-4 stated or implied) (1)First order with respect to Br (1) ORIn experiments 2 and 3,[Br] doubles and [BrO3 [H+] halves (1)Rate increases by 3 x 0.25 x 2 / rate increases x 1.5 (1)First order with respect to Br (1) Penalise OMISSION of Experiment Numbers once only Mark each point independently ] triples and Question Number 18(a)(ii) Rate = k [BrO Acceptable Answers 3] [Br] [H+]2 Mark CQ on (a)(i) Allow "r" or "R" for "rate" in the rate equation. IGNORE If k appears to be in upper case.<br />
Question Number 18(a)(iii) IGNORE sf except 1 sf THROUGHOUT Acceptable Answers Reject Mark 3] [Br] [H+]2 FIRST, CHECK THE FINAL ANSWER IF answer k = 1.49 x 102 dm9 mol3 s1 award (3) marks k = rate . [BrO = 1.68 x 105 . 0.05 x 0.25 x (0.30)2 (1) = 0.014933333 = 0.0149 (1) dm9 mol3 s1 / mol3 dm9 s1 (1) IGNORE sf except 1 sf Mark CQ from (a)(ii) or, if no rate equation in (a)(ii), then any rate equation stated in (a)(iii) NOTE: IF the rate equation in (a)(ii) is given as Rate = k [BrO CQ k = 3.73 x 103 dm6 mol2 s1 scores (3) IF [H+] is not squared in the correct rate equation: k = 4.48 x103 dm9 mol3 s1 OR k = 4.48 x103 dm6 mol2 s1 scores (2) ALLOW Correct answers derived from the data in the table for Experiment 2 or Experiment 3 3] [H+]2<br />
Reject Mark Question Number 18(b) Acceptable Answers The number(s) (of particles) in the rate equation / rate-determining step do not match those in the equation for the reaction OR The chance of (simultaneous) collision of 12 particles is unlikely OR The chance of (simultaneous) collision of 4 particles is unlikely OR The chance of (simultaneous) collision of 3 reactants is unlikely ALLOW molecules / substances for particles NOTE ALLOW AS A CQ from (a)(ii) Br ions not in rate equation / Br ions not in rate-determining step / Zero order with respect to Br / (Only) two reactants in the rate- determining step / (only) two reactants in the rate-equation/ particles are in the equation (for the reaction) that are not in the rate equation<br />
Mark Acceptable Answers Reject Answers relating to half-life score (0) overall If sketch-graph or comments suggest that gradient is measured at other than t = 0 or at several values of t then max (1) REMEMBER TO SCROLL DOWN BELOW THE SPACE LEFT FOR A SKETCH-GRAPH TO SEE WHAT CANDIDATE HAS WRITTEN ON THE DOTTED LINES(Calculate) gradient (of tangent) (1) ALLOW slope for gradientAt t = 0 / at the start / at the beginning / when reaction is at its fastest / at the origin (1) Each mark is stand-alone NOTE: Answer may be annotated on a suitable sketch-graph IGNORE any sketch-graph that shows an increase in concentration with time MAX (1) if sketch-graph shows a decrease in the concentration of a reactant / Br2 (Total 12 marks) Question Number 18(c)<br />](https://learnah.org/wp-content/uploads/2018/08/January_2013_MS_-_Unit_4_Edexcel_Chemistry_A-level_18-0.jpg)
