RI 2024 Kinetics Tutorial - Self Check Ans

10 Suggested Answers for Self-Check 1 (a) 5C 2O42(aq) + 2MnO4(aq) + 16H+(aq)  2Mn2+(aq) + 10CO2(g) + 8H2O(l) Method 1: Measuring change in colour intensity with time MnO4 is purple. Since colour intensity  [MnO 4], the change in the colour intensity of MnO4 over time may be used to follow the rate of reaction. 1. The reactants are mixed and the colour intensity of the reaction mixture may be measured using a colorimeter and the readings are taken at regular time intervals. 2. Samples of KMnO 4(aq) of various concentrations (e.g., 0.001, 0.005, 0.100 mol dm 3) are prepared and their colour intensities meas ured using the colorimeter. A calibration curve is obtained by plotting colour intens ity against concentration. The concentration of KMnO4 corresponding to each colour intensity obtained in Step 1 is found using the calibration curve. The concentration of KMnO4 is then plotted against time. 3. Instantaneous rate is found by drawing a tangent to the curve and finding its gradient g1, where rate = g1. (Note: If the graph of colour intensity is plotted against time, then instantaneous rate is proportional to the gradient, g1, of the tangent drawn to the curve at that particular time, where rate  g1.) Method 2: Sampling, quenching and titration 1. The reactants are mixed, and the stopwatch is started simultaneously. 2. Aliquots (same volume) of the reactant mixture are withdrawn and quenched at regular time intervals (e.g., two-minute intervals) using excess K I(aq). I reacts with MnO 4 instantaneously and quantitatively to form I2. 3. The amount of I2 formed in each aliquot is found by titrating against a standard solution of Na2S2O3. 4. Volume of Na 2S2O3(aq)  amount of I2 formed  amount of unreacted MnO4  [MnO4] remaining (since each aliquot has the same volume). Hence a graph of volume Na2S2O3(aq) against time is plotted, which is similar to a graph of [MnO4] against time. 5. Instantaneous rate is proportional to the gradient, g 1, of the tangent drawn to the curve at that particular time, where rate  g1. (Other possible method: Volume of CO2 evolved can be measured over time.) (b) 2H2O2(aq)  2H2O(l) + O2(g) Method 1: Measuring volume of oxygen gas evolved Since a gas is evolved, the change in volume of oxygen evolved over time may be monitored. 1. Set up the apparatus as shown and measure the volume of O2 gas collected in the syringe at regular time intervals. 2. A graph of volume of O 2(g) against time is plotted. 3. Instantaneous rate is proportional to the gradient, g 1, of the tangent drawn to the curve at that particular time, where rate  g1. Method 2: Sampling, quenching and titration 1. Aliquots of the H 2O2 solution are pipetted at suitable time intervals into conical flasks. 2. At a suitable time, ice-cold water is added to the conical flask to slow down or quench the reaction.

11 3. Each quenched aliquot is then titrated against a standard s