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		Be sure to check the section in this manual that describes each section of your lab report. Your report should include all sections described in the info area (specifically, in How to Keep a Notebook and in the Sample Write-up), and provide complete data, results, and discussion. Consult your TA if you have questions about the details of your lab write-up. Before Lab Make sure your prelab write-up is complete, in your lab notebook. Your prelab should include an objective, reference, procedure, and sample calculations or analysis flowchart or outline. You also need to do the three prelab problems on pp. 61-62 in your lab manual. Consult the introduction to this manual for details about what should be in each section of your write-up. In Lab The experiment will be done with a partner, but the write-up must be done individually. The data should appear in each lab partner's notebook with a clear reference to the notebook containing the primary record. Record the name of your lab partner in your notebook. You should plot your data as you measure it, to see it evolve and to enable you to correct problems before it is too late. Before leaving the lab, all data must be plotted to check for linearity. If time permits, runs that are not consistent can be repeated. Before leaving the lab, have your TA check and sign your lab notebook. Data Analysis Your primary data will be the absorbance at 565 nm as a function of time. Since absorbance at this wavelength is directly proportional to the sum of the concentration of I2 + I3-, you can plot various functions of absorbance vs. time, to determine c, the reaction order with respect to I2 + I3-. You should begin your analysis of your first set of data by plotting the absorbance vs. time, the natural log of absorbance vs. time, and the inverse of absorbance vs. time for the first run. A linear plot will tell you whether the reaction is zero, first, or second order with respect to I2 + I3-. After you determine the reaction order with respect to I2 + I3-, you will need to evaluate the "flooding" of your reaction. By calculating the concentration of acid and cyclohexanone before and after the reaction, you can see how constant they are. This is a limiting reagent problem, familiar from Chem 3 or 5. Calculate the change in concentration of cyclohexanone and acid during the course of the reaction for all five runs. Once the reaction order with respect to I2 + I3- is determined, you must evaluate the reaction order with respect to acid and cyclohexanone. Use the results of runs i, ii, and iii to evaluate the reaction order with respect to cyclohexanone. For runs i, ii, and iii, with the same initial [H+], the slope of your linear plot for each run will vary with [cyclohexanone]o. For example, if b = 1 and the reaction is first order with respect to cyclohexanone, doubling the concentration of cyclohexanone should cause the slope of your linear plot to double, if all other concentrations remain the same. If b = 2, doubling the concentration of cyclohexanone would cause the slope of your linear plot to increase by a factor of 4. Compare the slope of your linear plot for the three runs with different [cyclohexanone]o and determine the value of b. Similarly, runs ii, iv, and v can be used to evaluate a, the reaction order with respect to acid. A convenient and accurate way to determine the slope of your line is to use the Kinetics Plot or Least Squares Applets on the ChemLab website. Instructions for using Least Squares are given at the beginning of this manual. You will need an accurate value of the slope for each run next week, when you calculate the rate constant for this week's data. After Lab A data sheet with your qualitative results will be turned in for Week 1. In your write up for next week, you will return to these results and analyze them more quantitatively. Your lab notebook for Week 1 should include the following in the results section: • For the first run, three plots to determine the reaction order with respect to total iodine. This includes plots of A vs. time, ln A vs. time, and 1/A vs. time. • Plots for each kinetic run that show a linear relationship between a function of absorbance and time. Several plots can be presented on a single graph. • Calculations of both the initial (t = 0) and final (t = ) values of [H+], ([I2]+[I3-]), and [cyclohexanone] for each run. Comments should be made on how nearly the [H+] and [cyclohexanone] remain constant. • Report the experimental rate law of the reaction and describe the determination of reaction order with respect to total iodine, acid, and cyclohexanone.
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