Edexcel Jun 2016 Paper 4 Q14

Answers available below

14 This is a question about how clock reactions are used to study reaction kinetics. (cid:3) (cid:3) The bromine clock involves a reaction between bromide ions and bromate(V) ions in acid solution: 5Br (aq) + BrO3 (aq) + 6H+(aq) (cid:108) 3Br2(aq) + 3H2O(l) In order to monitor this reaction, phenol and methyl orange are added to the reaction mixture. (cid:404)(cid:3) A small fixed amount of phenol is present which reacts immediately with the bromine as it is produced, thus removing it from solution. (cid:404)(cid:3) Once the bromine produced has reacted with all of the phenol present, then any further bromine produced will bleach the methyl orange solution providing a means to monitor the reaction rate. (a) It is assumed that the initial rate of reaction is proportional to 1/time taken for the methyl orange to be bleached. Explain why it is essential for the amount of phenol to be small compared to the amounts of the reactants for this assumption to be valid. (1)(b) A series of experiments was carried out where only the concentration of bromide ions present was varied and the solution contained a large excess of BrO3and H+ ions. The total volume of the mixture was kept constant. (i) Why was it important that the solution contained a large excess of BrO3 H+ ions?and (1)A E R A S I H T N I E T I R W T O N O D A E R A S I H T N I E T I R W T O N O D A E R A S I H T N I E T I R W T O N O D *P46660RA01928* 19 Turn over<br /> (ii) The following results were obtained. Complete the table and use the results to plot a graph of 1/time on the vertical axis against the volume of bromide ions. Volume of Br (aq) / cm3 Time / s (1/time) / 103 s1 10.0 180 5.56 8.0 226 4.42 6.0 300 3.33 5.0 364 4.0 444 2.25 (4) 2.0 900 1.11 20 *P46660RA02028* D O N O T W R I T E I N T H I S A R E A D O N O T W R I T E I N T H I S A R E A D O N O T W R I T E I N T H I S A R E A<br /> (iii) Deduce the order of the reaction with respect to bromide ions. Justify your answer. (2)(iv) The reaction is first order with respect to bromate(V) ions and second order with respect to hydrogen ions. Write the overall rate equation for the bromine clock reaction and deduce the units of the rate constant. Rate equation: (2) Units of rate constant(cid:3) (cid:3) (c) Another clock reaction is the iodine clock reaction, where hydrogen peroxide solution is mixed with a solution containing sodium thiosulfate, potassium iodide and starch. (cid:3) (cid:3) The main reaction is H2O2(aq) + 2I(aq) + 2H+(aq) (cid:108) I2(aq) + 2H2O(l) (cid:404)(cid:3) The thiosulfate ions present react immediately with the iodine as it is produced, thus removing it from solution. (cid:404)(cid:3) Once all of the thiosulfate ions are used up, further iodine produced reacts with the starch present. (i) Why are the potassium ions omitted from the above equation? (1)(ii) State the observation made after all of the thiosulfate ions are used up and more iodine is produced. (1)A E R A S I H T N I E T I R W T O N O D A E R A S I H T N I E T I R W T O N O D A E R A S I H T N I E T I R W T O N O D *P46660RA02128* 21 Turn over<br /> (d) 'Iodine clock' reactions can be used to determine the activation energy of a reaction using the equation: ln rate =E a R 1 T constant (i) State the experimental measurements you would make to provide the numerical data for the calculation of the activation energy. (1)D O N O T W R I T E I N T H I S A R E A (cid:3) (cid:3) (cid:3) (cid:3) (cid:3) (cid:3) (ii) Describe how you would use your experimental measurements to obtain a value for the activation energy. (cid:3) (cid:3) (cid:3) You should include (cid:404)(cid:3) how the data is processed (cid:404)(cid:3) the graph you would plot and its expected shape (cid:404)(cid:3) how the activation energy of the reaction can be determined from the graph produced. (6) D O N O T W R I T E I(Total for Question 14 = 19 marks) TOTAL FOR SECTION B = 49 MARKS 22 *P46660RA02228* N T H I S A R E A D O N O T W R I T E I N T H I S A R E A<br />

Show answer