OCR A AS Jun 2016 Paper 2 Q5 (with explained solutions) |

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OCR A AS Jun 2016 Paper 2 Q5

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5 A student investigates the reaction between strontium carbonate and dilute nitric acid. 12 SrCO3 + 2HNO3 Sr(NO3)2 + CO2 + H2O The rate of reaction is determined from the loss in mass over a period of time. (a) (i) Explain why there is a loss in mass during the reaction.[1] (ii) An excess of strontium carbonate, SrCO3, is mixed with 20.0 cm3 of 1.25 mol dm3 nitric acid, HNO3. Calculate the mass of SrCO3 that reacts with the HNO3. (b) The student plots a graph of total mass (reagents + container) against time. mass =g [3] mass / g 96.40 96.30 96.20 96.10 96.00 95.90OCR 2016 50 100 150 200 250 300 350 400 time / s<br />
(i) Describe and explain the change in the rate of the reaction during the first 200 seconds of the experiment. 13[2] (ii) Using the graph, calculate the rate of reaction, in g s1, at 200 seconds. Show your working on the graph. rate of reaction =g s1 [2] (c) Outline a method that could be used to obtain the results that are plotted on the graph. Your answer should include the apparatus required and the procedure for the experiment.[3]OCR 2016 Turn over<br />

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Answer (i) carbon dioxide lost/evolved/given off/or produced as a gas(ii) FIRST CHECK ANSWER ON THE ANSWER LINE IF answer = 1.85 OR 1.845 g award 3 marks ------------------------------------------------------------------------ n(HNO3) = 1.2520.0 1000 = 0.0250 moln(SrCO3) m(SrCO3) = 0.0250 = 0.0125 mol= 0.0125147.6 = 1.845 g OR 1.85 grate of reaction decreases AND concentration decreases/reactants are used upless frequent collisionsH032/02 Question 5 (a) (b) (i) Mark scheme June 2016 AO element AO3.1 DO NOT ALLOW water or steam or CO2 evaporates Guidance AO2.83 AO2.7 AO2.3 If there is an alternative answer, check to see if there is any ECF credit possible ALLOW ECF from incorrect n(HNO3) molar mass of SrCO3 = 147.6 (g mol1) ALLOW ECF from incorrect n(SrCO3) ALLOW reaction slows down ALLOW concentration of reactants decreases. ALLOW fewer collisions per unit time OR collisions less often OR decreased rate of collision IGNORE less successful collisions/ less collisions less chance of collisions Marks 15<br />
H032/02 Mark scheme June 2016 Answer Marks Question (ii) Attempted tangent on graph drawn to line at approximately t = 200 sGradient (y/x) e.g. 0.20 290 = 6.9104AO element AO3.1 AO3.2 1 1 Guidance ALLOW 1 SF up to calculator value, in range 5104 to 8104 IGNORE units IGNORE sign DO NOT ALLOW round-bottomed flask. IGNORE weighing scales ALLOW weigh at time intervals Flask OR beaker AND balance AND stopwatch OR stop clock OR other timing deviceAO3.3 Records mass at time intervalsTime interval quoted between 10-50s1 1 12 Total 11 16 (c)<br />

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