ASRJC N2017, 2019, 2020, 2021 Planning Solutions

Solution to N2017/P4/Q2c Part (i) Since the question stated that “The procedure you followed in 2(a)(i) can be modified”, the table below shows the modifications that are made from the procedures in 2(a)(i) of the original exam question to study the effect of temperature on rate of decomposition. You may take reference of the original procedure from Experiment 21 that you have done in JC2. You will only need to give the answer in the right-hand side column. Original procedure in Expt 2(a) Planning Procedure to answer 2(c)(i) 1. Fill the burette with FA 3 (KMnO4) Fill the burette with KMnO4. 2. Using a measuring cylinder, add 100.0 cm3 of FA 4 to the conical flask labelled reaction mixture. Using a measuring cylinder, add 100.0 cm3 of H2O2 to the conical flask labelled “reaction mixture”. Place this conical flask labelled “reaction mixture” into a thermostatically controlled water bath maintained at 30oC. Insert a thermometer into the reaction mixture. Allow the temperature of the reaction mixture to equilibrate to within 1°C of the water bath. Record the temperature of the reaction mixture. 3. Using a measuring cylinder, add 2.0 cm3 of FA 2 (iron(III) nitrate, Fe(NO3)3, is an effective catalyst) to the same conical flask. Start the stopwatch and swirl the mixture thoroughly to mix its content. Note: in 2022 JC2 experiment 21, the volume of FA2 used is 4.0 cm3. Using a scalpel, cut a small piece of liver with dimension 1cm x 1cm x 1cm. Add the small piece of liver to the same conical flask. Start the stopwatch and swirl the mixture thoroughly to mix its content. 4. Using a measuring cylinder, add 50.0 cm3 of 0.2 mol dm-3 sulfuric acid to a second conical flask. Using a measuring cylinder, add 50.0 cm3 of 0.2 mol dm-3 sulfuric acid to a second conical flask. 5. Transfer a 10.0 cm3 aliquot (portion) of the reaction mixture to a 10 cm3 measuring cylinder, using a dropping pipette. Transfer a 10.0 cm3 aliquot (portion) of the reaction mixture to a 10 cm3 measuring cylinder, using a dropping pipette. 6. Immediately transfer this aliquot into the second conical flask and vigorously swirl the mixture. Read and record the time of transfer in minutes and seconds, to the nearest second, when the aliquot is added. Immediately transfer this aliquot into the second conical flask and vigorously swirl the mixture. Read and record the time of transfer in minutes and seconds, to the nearest second, when the aliquot is added.

Original procedure in Expt 2(a) Planning Procedure to answer 2(c)(i) 7. Immediately titrate the H2O2 in the second conical flask with FA 3. The end-point is reached when a permanent pale pink colour is obtained. Record the titration results. Immediately titrate the H2O2 in the second conical flask with KMnO4. The end-point is reached when a permanent pale pink colour is obtained. Record the titration results. 8. Wash out the second conical flask with water. Wash out the second conical flask with water. 9. Repeat steps 4 to 8 until a total of five aliquots have been titrated and their results recorded. Repeat steps 4 to 8 until a total of five aliquots have been titrated and their results recorded. 10. Vary the temperature of the thermostatically controlled water bath and repeat steps 2 to 9, with 4 other temperatures (25oC, 35oC, 40 oC, 45oC) 11. Plot all 5 graphs of VolKMnO4 against time. Draw a tangent to each graph at t= 0. Find the initial rate of each reaction, which is the gradient of tangent at t=0

Mark Scheme The following points are required by the question and the suggested marks distribution is as follows Question [7] Mark Scheme the reactants and conditions that you would use, [2] 1. Same quantity of H2O2, H2SO4 as used in 2(a)(i). 2. Fixed mass / volume of liver. 3. Vary temperature of the reaction mixture. the apparatus that you would use in addition to that specified in 2(a)(i), [1] 1. Thermostatically water bath or water bath adjusted with ice/ hot water with thermometer. 2. Scalpel to cut the liver. 3. Thermometer to measure the temperature of reaction mixture. the procedure that you would follow and the measurements that you would take, [3] 1. Record the temperature of the reaction mixture. 2. Ensure that the reaction mixture has been maintained at the required temperature BEFORE adding the liver and other reagents. 3. Prepare fixed mass of liver or piece of liver with fixed dimensions. 4. Start the stopwatch upon adding the liver. 5. Draw sample at appropriate time interval, quench and perform titration. Record volume of KMnO4 used. 6. Repeat experiments by varying the temperature of the water bath to a suitable temperature below 50oC. (Catalase will be denatured from temperatures above 50oC) Any 2 points in correct order – 1 mark how you would determine the initial rate of experiment. [1] 1. continuous method – plot 5 different graphs , 1 graph for each temperature. 2. draw a tangent at t=0 for each graph, gradient of tangent = initial rate. (Note that 5 graphs is necessary to obtain 5 rates and hence obtain 5 values of k’ at the corresponding T to obtain minimum of 5 plotted points on lnk’ vs T graph)

(ii) rate = k’[H2O2] Expt Temperature / oC Temperature / K 1 T / K-1 Gradient of tangent at t=0, = initial rate k’ = rate/[H2O2] In k’ 1 30.0 2 25.0 3 35.0 4 40.0 5 45.0 Calculate k’ = rate/[H2O2] and ln k’ -- [1] Calculate 1 T -- [1] (iii) (Using information from the question: Plotting ln k’ against 1 T gives a straight line of best fit.) The gradient of this line is − aE R [Note: Derivation of the gradient is shown in part (iv) answer below.] Since activation energy is always positive and R is positive, Gradient of line will be negative and hence downwards sloping. [1] Fig. 2.2 ln k’ 1 T [1] Graph line = downward sloping linear straight line

(iv) In the graph of ln k’ against 1 T gives a straight line of best fit, the gradient of this line is − aE R . − =' Ae aE RTk − =ln ' ln(Ae ) aE RTk Note: ln( ) ln lnAB A B=+ − = −=+ −=+ − = =  −  −− ln ' lnA + lne ln ' lnA -- [1] Note: lne =1 1ln ' ( ) lnA ( 1) [1 ] aE RT a a a a k Ek RT Ek RT E gradientR E gradient R y intercept = ln A A = ey-intercept [1]

Solution to N2019/P4/Q4 4(a) Suggested plan 1) Fill a burette with Na2S2O3. 2) Using a 50 cm3 measuring cylinder, measure 30 cm 3 of propanone followed by 30 cm3 of H2SO4 and transfer both solutions into a 250 cm3 conical flask. 3) Using another 50 cm3 measuring cylinder, measure 30 cm3 of iodine solution. 4) Transfer the iodine solution from step 3 into the conical flask from step 2. Start the stopwatch midway through pouring of iodine solution. Swirl to mix well. Label this as the “reaction mixture”. 5) About 1 minute after starting the stopwatch, use a 10 cm3 pipette to draw out 10 cm 3 of the reaction mixture and transfer it into another clean, dry 100 cm3 conical flask. 6) Using a 10 cm 3 measuring cylinder , measure 10 cm 3 of NaHCO3 solution. 7) At about 2 minutes after the start of the reaction, pour the 10 cm3 of NaHCO3 solution (in excess) into the sample from step 5 and swirl to quench the reaction. Record down the exact time of quenching. 8) Titrate the resulting solution against the standard Na 2S2O3 solution in the burette. When the solution turns from brown to pale yellow, add about 1 cm3 of starch solution using a dropper. Continue to titrate until the solution turns from blue-black to colourless. 9) Record the titration results. 10) Repeat steps 5-8 at about 4 minutes intervals (2, 6, 10, 14, 18 min) to obtain 5 sets of results in total to plot a graph.

Mark Scheme The following points are required by the qu