OCR A Specimen Paper 3 Q4 (with explained solutions) |

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OCR A Specimen Paper 3 Q4

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10 4 Methanoic acid and bromine react as in the equation below. Br2(aq) + HCOOH(aq)2H+(aq) + 2Br(aq) + CO2(g) A student investigates the rate of this reaction by monitoring the concentration of bromine over time. The student uses a large excess of HCOOH to ensure that the order with respect to HCOOH will be effectively zero. From the experimental results, the student plots the graph below. Specimen (a) Suggest how the concentration of the bromine could have been monitored.[1]OCR 2014 H432/03<br />
(b) Suggest a different experimental method that would allow the rate of this reaction to be followed over time. 11(c) Why would use of excess HCOOH ensure that the order with respect to HCOOH is effectively zero?[1][1] (d)* Using the graph, determine the initial rate of reaction the rate constant.Your answer must show full working using the graph and the lines below as appropriate. Specimen[6]OCR 2014 H432/03 Turn over<br />

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Measure reduction of colour of bromine (cid:57) Answer Measure volume of CO2 (produced) (cid:57) Mark Scheme June 20XX Marks Guidance H432/03 Question 4 (a) (b) (c) (d)* Concentration of HCOOH would be constant (cid:57) Specimen Please refer to the marking instructions on page 4 of this mark scheme for guidance on how to mark this question. Level 3 (56 marks) A comprehensive conclusion which uses quantitative data from the graph to correctly identify and calculate initial rate AND half lives and reasoned order of Br2 AND determination of k with units There is a well-developed conclusion showing a line of reasoning which is clear and logically structured. The working for initial rate, half life and order are clearly shown. Determination of k is clear and correct. Level 2 (34 marks) Reaches a sound, but not comprehensive, conclusion based on quantitative data from the graph. Correctly identifies and calculates initial rate AND half lives and reasoned order of Br2 The conclusion has a line of reasoning presented with some structure. The initial rate and order is relevant and supported by correct evidence from the graph. There may be errors in the calculations which prevent the correct determination of k. half-life: k = Indicative scientific points may include: Initial rateEvidence of tangent on graph drawn to line at t = 0 s AND gradient determined in range 41105 initial rate expressed as gradient value with units of mol dm3 s1, e.g. initial rate = 4105 mol dm3 s1 Half lives and reasoned order of Br2Half life measured on graph OR within text OR stated in range 180200 sConstant half life OR two stated half lives within 20 s AND conclusion that Br2 is 1st order Determination of k with unitsRate constant k clearly linked to initial rate OR OR k =k determined correctly from measured initial rate or measured half life with units of s1, e.g. k = 4103 s1 from initial rate of 4105 mol dm3 s1 OR t1/2 of 175 s 14 P M T<br />
H432/03 Question Mark Scheme June 20XX Answer Marks Guidance Level 1 (12 marks) Reaches a simple conclusion using at least one piece of quantitative data from the graph. Attempts calculation of initial rate OR half lives and reasoned order of Br2. The information selected from the graph is basic and communicated in an unstructured way. The calculations may not be clear and the evidence used from the graph may not be clearly shown. 0 marks No response or no response worthy of credit. Total Specimen 15 P M T<br />

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