An experiment was done to to determine the rate law for the reaction between crystal violet and sodium hydroxide.

We have several difficulties we must overcome when we enter the lab and try to determine a rate law in this experiment. The first involves the measurement of crystal violet concentration. We will be measuring absorbance of light as it passes through the reaction mixture; we will not be measuring crystal violet concentration. So when we attempt to produce a linear plot, it is the absorbance that is being plotted relative to time. This is not a problem due to Beers Law (see Equation 9) which shows that absorbance is directly proportional to concentration. (The quantity s represents molar absorptivity which is dependent on wavelength and the substance being analyzed. The term b is the pathlength, and c represents molar concentration.) Since crystal violet is the only colored species in the reaction mixture, absorbance can be substituted for concentration in our plots.

absorbance = sbc (9)

A second, and more substantial difficulty involves the effect of OH^- concentration on our plots. The OH^- concentration will also be changing over time, and unless the reaction is zero order with respect to OH^-, this change in concentration will affect the overall reaction rate. This invalidates the graphic method as a means of analysis. How do we overcome this obstacle? We essentially make the change in concentration for OH^- meaningless. That is, while we plot the change due to crystal violet concentration, the OH^- concentration is barely changing. Even though the two reactants combine in a 1:1 ratio, the OH^- concentration is several orders of magnitude greater than the crystal violet concentration. During the reaction, the crystal violet concentration will be reduced by at least 50%. The OH^- concentration will be reduced by less than 0.01%. Graphically, OH^- will be pseudo-zero order.

Procedure:

(Note: Use the same cuvette for your blank and all trials.) After starting the Microlab software, select Spectrokinetics. Then click on Advanced Spectrophotometer Settings. Select only wavelength 590 nm. Run distilled water as a blank. Be sure to wipe the cuvette before placing in the spectrophotometer. After running the blank, switch to absorbance. Change the time interval to 5 seconds. Do not press Start at this time!

Pipet 10.00 mL of 0.040 M NaOH into a 50-mL beaker. Using your 10-mL graduated cylinder, measure 10.0 mL of 1.5 x 10^-5 M crystal violet. Pour the crystal violet into the 50-mL beaker and mix well. Rinse the cuvette two times with a small portion of the solution and fill the cuvette. Place the cuvette in the spectrophotometer and start data collection. Collect data for 10-12 minutes.

After stopping data collection, select File Export Data As to move data to Excel. The spreadsheet should open automatically with columns for time, concentration, natural log of concentration, and inverse concentration. Generate plots (as in the prelab) to find the one that produces a straight line. Add a trendline with at least 3 significant (non- leading zero) digits. (To increase the number of digits shown, right-click on the trendline equation and then select Format Trendline Label. Click on General and change to Number. Then change digits to 6) Make the plot look pretty by adding an appropriate title and axes labels with units. Ask the instructor to verify the plot before continuing - he will grade your plot at this time. Record the equation o the trendline, as the ratio o slopes will be used or the ratio o rates in the initial rate method.

Perform a second trial as above, but pipet 7.50 mL of distilled water and 2.50 mL of

0.040 M NaOH before the crystal violet addition of 10.00 mL.

Perform a third trial as above, but pipet 5.00 mL of distilled water and 5.00 mL of 0.040

M NaOH before the crystal violet addition of 10.00 mL. WASTE: Any remaining solutions can be rinsed down the drain.

Question: The rate constant cannot be determined from the data that was collected in this experiment. To find the rate constant, one needs an actual rate and reactant concentrations (so that you could insert the rate and associated concentrations into the rate law to find k). Another option for finding k is using the slope from the integrated rate law. Explain why this second option also does not work in this experiment.