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.] [H2O2] in Solution B of example = 0.750 mol dm–3 (because equal volumes of H2O2 and ethanoic acid were mixed) [H2O2] in reaction mixture of example = 0.375 mol dm–3 (because equal volumes of Solution A and B were mixed) Hence Experiment 1 will be expected to take significantly less than 20 s since [H2O2] used is twice that of the example. (i.e. It will take about 10 s if the order with respect to H2O2 is 1)

©2013AndersonJC/CHEM 3 Analysis of results 1. For each of the experiments, calculate the reaction rate, where rate is represented by time 1 . 2. Plot a graph of the reaction rate against [H2O2]. 3. For a zero order reaction, the bes t fit line will be a horizontal straight line. For a first order reaction, the best fit line will be a straight line passing through the origin. [1]: use of appropriate apparatus (e.g. burette, standard / graduated flask) [1]: essential details (e.g. 250 cm3 flask, 50 cm3 measuring cylinder) [1]: appropriate [H 2O2] chosen for the stock solution (i.e. any concentration higher than 1.5 mol dm–3) [1]: correct procedure in the preparation of the standard stock solution (including the correct choice of volume of the provided H 2O2 solution to be used) [1]: appropriate volumes of H 2O2 proposed (such that at least one experiment takes less than 20 s and there are sufficient points to draw a best fit line – minimum 3 points for Chemistry) [1]: maintain proportion of solution A and aq. CH 3CO2H (keep ing the total volume constant by adding deionised water) [1]: start stopwatch at the same time when A and B are mixed [1]: graph of rate against [H2O2] proposed [1]: correct interpretation of proposed graph Alternatively for the iodine clock experiment, you may prepare 3 /5 sets of Solution B, each with a different [H 2O2] and then follow the steps in the given example. e.g. Solution B1 – 500 cm 3 of 3.0 mol dm –3 stock H2O2 prepared + 500 cm 3 of 1.0 mol dm–3 ethanoic acid Solution B2 – 400 cm 3 of 3.0 mol dm –3 stock H2O2 prepared + 100 cm 3 of deionised water + 500 cm3 of 1.0 mol dm–3 ethanoic acid Solution B3 – 300 cm 3 of 3.0 mol dm –3 stock H2O2 prepared + 200 cm 3 of water + 500 cm3 of 1.0 mol dm–3 ethanoic acid Then you mix 50 cm 3 of Solution A with 50 cm 3 of Solution B1/B2/B3 and record the time taken for the mixture to turn dark blue . (+ the other 2 parts given above)

©2013AndersonJC/CHEM 4 Comments: Preparation of stock solution of H2O2 Students were expected to state clearly that they intended to dilute the given 7.5 mol dm –3 solution of H 2O2. Descriptions of this procedure must be precise with essential details (e.g. stating the accurate concentration of this solution) . Apparatus such as measuring cylinders, beakers and conical flasks used for measuring volumes are inappropriate here. Iodine clock experiment Many students did not realise that [H2O2] is the independent variable . They needed to ensure a suitable range of concentrations such that [H2O2] was high enough for one experiment to take l ess than 20 s , and that the proportions of solution A and aqueous ethanoic acid should also be the same as described in the example . It is also essential that at the point when solutions A and B are mixed and swirled the stopwatch is started at the same time. Analysis of results Students had to state clearly what graph they would draw and give a correct interpretation of their proposed graph. They are not to use numerical changes in concentration and time to determine order. (d) The concentrated hydrogen peroxide solution used is corrosive. Safety goggles and gloves should be worn when handling it. [1]

©2013AndersonJC/CHEM 5 2 (a) (i) hydrocarbon [1] (ii) the high temperature in the internal combustion engine allows the atmospheric nitrogen and oxygen to react. [1] (iii) protein / amino acid from the fossil remains [1] (b) (i) By Hess’ Law, Horeaction 1 = –Hof (SO2) + Hof (SO3) = –396 – (–297) = –99.0 kJ mol–1 Note: Drawing of energy cycle is not necessary. [1] [1] (ii) Goreaction 1 = Horeaction 1 – TSoreaction 1 = – 99.0 – 298(–93.5 x 10–3) = –71.137 kJ mol–1 = –71.1 kJ mol–1 Note: Students are reminded to take note of the difference in magnitude of entropy (J mol K–1) and enthalpy (kJ mol–1) and their respective units. [1] [1] ecf (c) (i) –71.137 x 103 = –2.303(8.31)(298)log10(Kc) log10(Kc) = 12.47 Kc = 1012.47 = 2.97 x 1012 (mol dm–3)–½ Note: No units are required for this question but s tudents are encouraged to work out the unit of Kc based on the stoichiometry equation. [1] [1] (ii) The large Kc value calculated in (i) suggests that the position of equilibrium lies very much to the right and the forward reaction is very spontaneous and the reaction would be effectivel