AJC H2 Chemistry 9647 N2012 P2 Suggested Solutions

©2013AndersonJC/CHEM 1 H2 Chemistry 9647 N2012 P2 Suggested Solutions 1 (a) H2O2 + 2 I– + 2 H+ I2 + 2 H2O [1] (b) To create a buffer solution so as to keep the [H +] effectively constant since changes in [H +], as reaction proceeds, will a ffect the rate of reaction. [1] (c) Preparation of stock solution of H2O2 Intended concentration of stock solution of H2O2 = 3.0 mol dm–3 [In order to ensure that at least one of the experiments would take significantly less than 20 s , the concent ration of H 2O2 to be used for Solution B is doubled compared with the info given in the question] volume of 7.5 mol dm–3 H2O2 solution used for dilution = o 11 C VC = 7.5 2503.0  = 100 cm3 Procedure: 1. Using a burette, place 100 cm3 of the 7.5 mol dm –3 H2O2 solution into a 250 cm3 graduated flask. 2. Top up with deionised water to the 250 cm 3 mark. Add dropwise when near to the mark. 3. Shake well to obtain a homogeneous solution. [9] Iodine clock experiment [50 cm3 of solution A and 25 cm 3 of 1.0 mol dm –3 ethanoic acid are used in each experiment. The volumes of the H2O2 stock solution added are to be varied in each experiment and the total volume is kept constant (100 cm 3) by adding appropriate amounts of deionised water. 25 cm3 of ethanoic acid is used to ensure that the same proportions of solution A and aqueous ethanoic acid are used in every experiment.] Procedure: 1. Using a burette, transfer 50 cm 3 of solution A to a 250 cm 3 conical flask labelled A. 2. Fill a 2nd burette with the 3.0 mol dm –3 H2O2 stock solution prepared and add 25 cm 3 of H 2O2 into another 250 cm 3 conical flask labelled B. 3. Using a 50 cm 3 measuring cylinder, add 25 cm3 of ethanoic acid to the same conical flask labelled B and mix the contents thoroughly by swirling the conical flask.

©2013AndersonJC/CHEM 2 4. Pour the contents in flask B rapidly into flask A. Start the stopwatch immediately. 5. Mix the contents thoroughly by swirling the flask. 6. Stop the stopwatch when the solution turns dark blue. Record the time taken. 7. Repeat steps 1 to 6 using different concentrations of H 2O2 by varying the volume of H 2O2 stock solution used . The total volume of solutions is kept constant by adding deionised water as required. 8. The results are recorded in the table below. expt vol of solution A / cm3 vol of ethanoic acid / cm3 vol of H2O2 / cm3 vol of deionised water / cm3 [H2O2] in reaction mixture / mol dm–3 time taken, t / s rate ( t 1 ) / s–1 1 50.00 25.0 25.00 0.0 0.750 2 50.00 25.0 20.00 5.0 0.600 3 50.00 25.0 15.00 10.0 0.450 4 50.00 25.0 10.00 15.0 0.300 5 50.00 25.0 5.00 20.0 0.150 e.g. [H2O2] in experiment 1 = 100 0.300.25  = 0.750 mol dm–3 [Some processing of data is required here, by using the example given in question, to check if at least one of the experiments will take significantly less than 20 s.]