Week 2: Chemical Kinetics 2 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 This week, you and your partner will determine the best procedure for your experiments, using the guidelines given below. Meet with your partner before lab to plan your procedure. Determination of the Rate Constant Complete any unfinished measurements of reaction rate at last week's temperature, for the concentration conditions in last week's table on page 63. Work with your partner to do this as quickly and efficiently as possible. Plot your data, as you measure it. Temperature Dependence of the Iodination of Cyclohexanone For this experiment, your goal is to determine the activation energy for the iodination of cyclohexanone by determining the temperature dependence of the reaction rate constant. This temperature dependence can be obtained by carrying out a series of reactions at different temperatures. Maintenance of the bath temperature can be achieved by careful addition of ice or hot water depending on whether the desired temperature is above or below ambient. It is not advisable or necessary to alternate between hot water and ice if care is taken during the additions. For runs above room temperature, solutions should be thermostated in stoppered50 mL Erlenmeyer flasks, to avoid changes in concentration due to evaporation. Assuming that the rate law stays the same over the temperature range 0 - 50 °C, the concentrations of ketone and acid can be adjusted at each temperature to insure a reasonable time scale for the reaction. It is essential, however, to keep both [cyclohexanone] and [H+] at least twenty times larger than ([I2]+[ I3-]). Use the conditions in last week's experiments as a starting point. Which set of reaction conditions from last week would be best at high temperature? Which would be best at lower temperatures? Solutions will be available at the concentrations used last week, but the reaction conditions should be modified to improve the reaction conditions at each temperature. While it is possible to extrapolate your graphs to a t = much longer than the time that the reaction is monitored, the greater the extrapolation, the greater the error in the calculated reaction rate. Manipulate the reaction concentrations so that each reaction at each temperature comes to t = in 30 minutes, or less. If the rate of reaction is too slow for completion to occur in 30 minutes, stop the run and begin again with new reaction conditions. Reaction concentrations should also be adjusted so that they are not too fast. The total time of reaction should range from 10 to 30 minutes. For this reaction, the rate approximately doubles for every 10 ° increase in temperature. This approximate relationship should enable you to select reaction conditions to try for different temperatures. One effective technique is to change the concentrations of cyclohexanone and acid for different runs by increasing or decreasing the volume of water, while keeping the total volume of the reaction mixture at 25 mL and the initial concentration of iodine the same. You should use the Spectronic 20 just as you did last week and obtain a graph of absorbance vs. time. However you must be careful not to keep the cuvette out of the constant temperature bath any longer than absolutely necessary because the temperature of the solution in the cuvette will change quickly for runs at temperatures significantly different from room temperature. For example, the cooling rate for a cuvette at 40 °C is ~3 °C min-1. On the other hand, be sure that the outside of the cuvette is perfectly dry before placing it in the Spec 20. Droplets of water will scatter light and spoil your absorbance measurements. If an absorbance reading seems surprising, check for water droplets that may condense on a cold cuvet. Measurements at four different temperatures should give a reasonable value for Ea. The temperatures should be as widely spaced as possible, in the range of 0 to 50 °C. Since the procedure for this experiment is up to you, it is important to describe in detail what experimental conditions were used, in your lab notebook. A detailed experimental procedure should be written in your prelab write up, including a table of reaction conditions, like the one shown in this manual for Week 1. Work together with your partner on this prelab procedure before coming to lab.
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