Chemical Kinetics 1 Overview Getting Started Techniques Procedure FAQ Full Lab Manual Introduction & Goals Chemistry & Background Key Questions Prelab Problems Safety Procedure In Your Write-up Experiments Index ChemLab Home		Procedure: Week 1 First, complete the Safety Quiz and Safety Agreement at the beginning of this manual. Bring a watch with a second hand to lab this week. The rate dependence on the ketone and hydrogen ion concentrations can be conveniently assessed using the following reaction mixtures. Note that the precise concentration of the triiodide stock solution will probably vary somewhat from this value. The posted value should be recorded, and then used in all calculations. Dependence on Ketone Run Cyclohexanone HCl H20 I2 + I3- Total (0.23 M) (0.50 M) (~4 x 10-2 M) Volume (i) 7.0 mL 5.0 mL 11.0 mL 2.0 mL 25 mL (ii) 10.0 5.0 8.0 2.0 25 (iii) 13.0 5.0 5.0 2.0 25 Dependence on Hydrogen Ion (iv) 10.0 mL 3.0 mL 10.0 mL 2.0 mL 25 mL (ii) 10.0 5.0 8.0 2.0 25 (v) 10.0 7.0 6.0 2.0 25 Note that the second reaction mixture in each series, Run (ii), is the same and need be measured only once. If there are no major mishaps and the work is done efficiently, it should be possible to complete all five runs in one afternoon. Plot the data as you perform each run, to insure that interpretable data are being obtained. You should determine the reaction order with respect to iodine from your first kinetic run. This dependence of reaction rate on the iodine/triiodide concentration can be determined from plots of I2 + I3- absorbance vs. time. Check the predictions of zero, first, and second order rate laws to determine the reaction order, before continuing to the subsequent reactions. For subsequent reactions, only one function of absorbance vs. time need be plotted, to check for linearity. Reaction mixtures should be prepared by measuring the appropriate amounts of H2O, HCl, and cyclohexanone into a 50 mL Erlenmeyer flask. Then, when the temperature of all reactants has stabilized, the iodine solution can be added to initiate the reaction. Five different reaction mixtures will be observed, as described in the table, above. The HCl solution and ketone can be conveniently measured from burets and the water from a Mohr pipet. By using two 10 mL burets, a Mohr pipet, and a 2 mL pipet, a pair of students can have separate measuring capabilities for the HCl, ketone, H2O, and triiodide, respectively. It is then unnecessary to rinse out the burets and pipets, except at the start of a series of runs. Note that 10 mL burets are graduated differently than 50 mL burets. Divisions are marked every 0.05 mL. When reading the buret, you should estimate the volume to the nearest 0.01 mL. Temperature control is achieved by using a water bath in a 2 L insulated beaker. The beaker should be filled about three-quarters full with a suitable mixture of hot and cold water to achieve a constant temperature near 25 °C, measured as accurately as possible with the thermometer in your drawer. If your water bath is full when you arrive, measure its temperature and check the temperature stability, rather than starting over with fresh water. The bath is best kept at a uniform temperature by magnetic stirring. Do not turn on the heater portion of your hotplate-stirrer. If a reaction is being carried out substantially above room temperature, the flask should be stoppered to prevent evaporation while reactants are coming to temperature. Record the water bath temperature, since you will need it next week. The 50 mL Erlenmeyer flask containing all the reagents except the triiodide solution should be suspended in the bath on a loop of copper wire. About ten minutes is sufficient to assure good temperature equilibration. A stoppered 16 x 150 mm test tube containing about 15 mL of triiodide solution should also be thermostated in your water bath. Once all the reagents are at constant temperature, the necessary volume of triiodide solution should be added to the 50 mL flask with a 2 mL pipet, using a pipet bulb. The addition of the triiodide defines the starting time (t = 0) for the reaction. Be sure to stir the solution thoroughly with a glass stirring rod after addition of the triiodide. An aliquot of the solution should be transferred to a cuvette that is also suspended in the bath on a loose loop of copper wire. The cuvette should be easily removable for readings. Readings should be taken every one to two minutes until the absorbance stops decreasing. A measurement should be made as quickly as possible, removing the cuvette from the bath, wiping it dry, taking the reading at 565 nmand returning it to the bath, all in a few seconds. Another cuvette with water in it should be used to re-zero the colorimeter between readings. In principle the terminal absorbance of a run should be zero, since the halogen is the only absorbing component in the solution and it is the limiting reagent of the reactions. A terminal absorbance slightly different from zero may occur, however, due to differences between the cuvettes containing the reaction mixture and the one used to zero the instrument. Furthermore, the final mixture in your reaction is not pure water. You need not follow any reaction longer than 20 to 30 minutes, since the final time can be determined by extrapolation. More than one run can be carried out simultaneously, taking the readings alternately, since you will have three cuvettes. Careful planning and record keeping are essential. Judicious division of labor between lab partners is required for smooth operation of the work. A sensible approach starts with the fastest run by itself. This strategy allows efficient development of experimental technique before attempting simultaneous runs. The procedure is taken with minor modification from J.A. Bell, Chemical Principles in Practice, AddisonWesley, Reading, MA, 1967, pp. 189-194.
Trustees of Dartmouth College, Copyright 1997–2004