March 20 th 2005
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Reaction Chemistry
Chemical kinetics is the study of reaction rates. In this experiment, the kinetics of the reaction between crystal violet and NaOH will be studied. The LabWorks Interface colorimeter will be used to monitor the crystal violet concentration as a function of time. The reactant and product structures and the reaction stoichiometry are shown in Figure 1 on the next page.
All of the reactants and products shown in Figure 1 are colorless except for crystal violet which has an intense violet color. Thus, during the course of the reaction, the reaction mixture color becomes less and less intense, ultimately becoming colorless when all of the crystal violet has been consumed.
The crystal violet color is due to the extensive system of alternating single and double bonds which extends over all three benzene rings and the central carbon atom. This alternation of double and single bonding is termed conjugation, and molecules which have extensive conjugation are usually highly colored. Trace the conjugation in the crystal violet structure and note that in the reaction product, the three rings are no longer in conjugation with one another, and hence the material is colorless.
Kinetic Rate Laws
The rate of the crystal violet/NaOH reaction is given by the following generalized rate law.
1)
In equation 1, k is the rate constant for the reaction, CV is an abbreviation for crystal violet, C25H30N3+, x is the reaction order with respect to OH-, and y is the reaction order with respect to CV. The values of x and y will be determined experimentally. Possible x values are 0, 1 or 2 (zero, first or second order). Possible y values are also 0, 1 or 2.
In this experiment, the initial [OH-] is made much greater than the initial [CV]. Thus, the [OH-] change, during the time that the CV is consumed, is negligible. For this reason, [OH-]x can be treated as a constant and Equation 1 can be written as follows,
(2)
where k’ = k [OH-]x . k’ is termed a pseudo rate constant.
The integrated form of the pseudo rate law (2) depends on the reaction order with respect to CV. The integrated rate laws for y = 0, 1 and 2 are given in Equations 3 and 4. Compare each with the general form of a linear equation, y = mx + b.
(3)
4)
In Equations 3 and 4, [CV]0 is the concentration of crystal violet in the reaction mixture at time zero, before any reaction has occurred; [CV]t is the concentration at any time t during the course of the reaction. If a plot of ln [CV]t versus time is linear, y = 1 and the reaction is first order in CV. Similarly, a linear plot of 1/[CV]t versus time indicates a second order reaction in CV. Only one of these plots will be linear. For the one that is linear, the resulting straight line slope (its absolute value) equals the pseudo rate constant, k’.
In order to do the graphing just described, we need to have data showing how the CV concentration changes with time. This data will be obtained using the LabWorks Interface colorimeter with its green LED light source. The light from the LED will pass through the solution containing CV and NaOH and then fall on the system photocell. The photocell circuit will then produce a current, measured in microamps (I), which is directly proportional to the light intensity striking the photocell surface.
Crystal violet solutions obey Beer’s law. Thus, the relationship between the observed current and the CV concentration is given by:
(5)
In Equation 5, At is the reaction solution absorbance at any time t; I0 is the blank photocell current (observed for pure water); It is the current observed for the CV/NaOH reaction mixture at time t; µ is the CV molar absorptivity, 5.0 x 104 L/(cm mol); b is the cell path length (1.00 cm); and c is the CV molar concentration at time t, [CV]t. Thus, Beer’s law can be used to calculate [CV]t from each photocell current reading (It) during the kinetic run.
Link to the Report Form here. This report is due at the beginning of your next lab period.
SAFETY PRECAUTIONS
Crystal violet solutions may cause skin and eye irritation. Sodium hydroxide solutions are caustic and will cause skin burns. Any skin contact with either chemical should be immediately washed. Safety goggles must be worn in the lab at all times. Wash hands with soap and water before leaving the lab.
Part 1
Measurements
Obtain a colorimeter (wait for on-screen instructions before making its connections to the interface). Load and start the experiment program. Follow each of the on-screen instructions for conducting this experiment. Supplemental information for the on-screen instruction steps are give below.
1. Connect the colorimeter to the interface as directed by the on-screen instructions. Five connections need to be made. These connections are color coded. Be sure that DAC 2 (blue) and I 2 (yellow), located on the interface right side, are used.
2. Before beginning the kinetic measurements, a blank current reading (I0) with pure water needs to be obtained. Fill a clean, rinsed cuvet about ¾ full with distilled (or deionized H2O, dry the outside of the cuvet thoroughly with a KimWipe (being careful to remove any fingerprints), and insert the cuvet into the colorimeter. Note carefully the positioning of the cuvet for future reference. Cover the cuvet and indicator lights with the cuvet canister and lid. Follow the on-screen instructions to obtain the value of I0. This value will be stored as the first (and only) entry in column C of your data file.
3. Check the next on-screen instruction to be certain that the experiment timer will start as soon as you click OK or press the Enter key (but do not start the timer yet). Empty the cuvet and dry it thoroughly inside and out. Using the buret provided, dispense 9.00 mL of 1.5 x 10-5 M crystal violet solution into a clean, dry 50 mL beaker. Using a calibrated plastic dropper, add 1.00 mL of 0.050 M NaOH to the CV solution as rapidly as possible without splashing. At the same instant of NaOH addition, click OK (or press the Enter key) to start the timer.
4.Thoroughly mix the CV/NaOH solution in the beaker and then fill the cuvet ¾ full. Position the cuvet in the colorimeter in exactly the same manner as was used for pure H2O. As soon as the cuvet (with covers) is in place, click OK or press the Enter key. The first current measurement on the CV/NaOH solution will automatically be made 15 seconds later. All of the operations in this step should be completed as quickly as possible. This will insure that the first measurement will be made as close to the reaction start as possible.
The program will automatically take current readings at two minute intervals for a period of 30 minutes and then stop. If there is a need to stop data collection prior to the end of 30 minutes, press switch W on the interface and the program will terminate. Otherwise, wait for the automatic data collection to be completed.
Data Analysis
1.Enter the LabWorks spreadsheet and retrieve your data file. The value listed in Row 1 of Column C is I0, obtained using pure H2O. Using this I0 value, compute the solution absorbance (At) in column D or E for each data point. Then, in the next column, compute [CV]t for each data point using Beer’s law. Format the data in this column to be in scientific notation. Be sure to properly label all columns.
2.Plot a graph of 1/[CV]t (y-axis) versus time in minutes (x-axis). If the reaction between crystal violet and NaOH is second order in crystal violet, this plot will be linear. If it is not second order, this plot will be curved. Test for linearity with a first order (or linear regression) curve fit. Look at the actual data points. If they clearly follow a curved departure from the regression line it should be concluded that the reaction is not second order in crystal violet.
3.Repeat step (2), substituting ln [CV]t for 1/[CV]t. A linear graph in this instance would indicate a first order dependence on crystal violet. Closely examine your data as done in step 2 above.
4.Prepare and print a carefully labeled graph for the plot which exhibited the best linear relationship. Include the first order (or linear regression) curve fit line. With this plot you have identified y, the reaction order with respect to CV. Record the value of y on the report sheet. The absolute value of the slope for the straight line (shown in the regression equation at the top) is the best value of k’. Record this value with proper units and to the correct number of significant figures on the report sheet. Also obtain a spreadsheet printout. Attach both your graph and spreadsheet to the report sheet.
Part 2
Measurements
Recall that the k’ just obtained is a pseudo rate constant, whose value depends upon the OH- concentration, i.e. k’ = k[OH-]x. In this part of the experiment, the value of x will be determined as well as the value of the true constant, k.
In part 1 of the experimental procedure, 9.00 mL of 1.50 x 10-5 M crystal violet and 1.0 mL of 0.050 M NaOH were combined to form the reaction mixture. A second kinetic run will now be made in exactly the same way except that the NaOH concentration will be doubled to 0.10 M.
Repeat each of the four experimental steps on pages 4 and 5 using 1.0 mL of 0.10 M NaOH in place of 0.050 M NaOH.
Data Analysis
1.Repeat data treatment steps 1, through 4 as listed on pages 5 and 6, and again record the k’ value on the report sheet.
2.From the ratio of the two k’ values, determine the reaction order with respect to OH- (the value of x). Clearly show your calculation of x on the report sheet. You will need to use the NaOH concentrations after dilution with CV. Note: The value of x should be an integer. If your value is not an integer, it is because of experimental error (probably in measuring and adding the NaOH solutions). If necessary, round your x value to the nearest integer.
3.Calculate the true rate constant (k) value from each of the k’ values. Be sure to use OH- concentrations that have been adjusted for dilution. Finally, average the two k values obtained. Again, be sure to watch significant figures and use proper units.