Pre-Lab Rate Law Determination of the Crystal Violet Reaction Objective: To use UV-Visable Spectroscopy to determine the order of the reaction with respect to crystal violet.

Reaction being Studied CV+(aq) + OH-  CVOH(aq) Violet Colorless

Reaction changing over time: CV+(aq) + OH-  CVOH(aq)

CV+(aq) + OH-  CVOH(aq) Kinetics The study of the rate of a reaction (how fast it goes from reactants to products) CV+(aq) + OH-  CVOH(aq)

How do we determine this? Question to be answered in this lab: How does changing the concentration of CV+ effect the rate of the reaction? How do we determine this? Determine the order (m) of the reaction with respect to crystal violet (CV+) rate = k [CV+]m[OH-]n where [OH-] is held constant

How? Data: Concentration [CV+] Time Look at Graphs:

How do we get the concentration over time. Use Spectroscopy How do we get the concentration over time? Use Spectroscopy! Absorbance of light is directly related to concentration according to Beer’s Law A = abc a = molar absorptivity, M-1cm-1 b = path length, cm c = concentration, M

How Spectroscopy Works

Spectroscopy Colorimeter – UV-Visible Spectrophotometer Attached to a computer Limited wavelengths UV-Visible Spectrophotometer

What Wavelength? Yellow, the complimentary color of violet is absorbed and the violet light is not absorbed, so that is what color we see. So this is a good choice for the wavelength setting on the spectrometer.

What wavelength?

What Wavelength? Yellow-green, the complimentary color of violet is absorbed and the violet light is not absorbed, so that is what color we see. So a good choice for the wavelength setting on the spectrometer would be between 500-600nm

What Wavelength? Best results use wavelengths that give absorbance values between 0.5 and 1.0 If using a particular wavelength and the absorbance is greater than 1.0 must dilute the sample to a concentration that lowers the absorbance to between 0.5 and 1.0

Procedure See handout

Processing the Data Use the graphs to determine the order with respect to crystal violet for this reaction. Look at the “linear fit” data. “R” closest to 1.0 means the data is close to a true fit for the equation for a line: y = mx+b Y-axis will actually be absorbance vs time, but that’s OK, because: A = abc

Goal – graph with best straight line will correlate to order of reaction C L R

Processing the Data rate = k[CV+]m[OH-]n pseudo rate = k*[CV+]m 2. Rate constant, k Background: rate = k[CV+]m[OH-]n pseudo rate = k*[CV+]m where k* = k[OH-]n You are solving for the pseudo rate, k* as we do not know (n) the order with respect of [OH-]

Processing Data k will be equal to the slope of your linear graph This will be in the box when you click on “linear fit” Justify using the following information -

Processing the Data If 1st Order, recall that the Integrated Rate Law: ln[A]t – ln[A]0 = -kt rearranges to ln[A]t = (-k)t + ln[A]0 y = mx + b Slope will be –k! Software will calculate the slope when you click “Linear Fit” button! You just need to identify it and justify your answer.

Processing the Data If 2nd Order, recall that the Integrated Rate Law 1 - 1 = kt rearranges [A]t [A]0 1 = kt + 1 [A]t [A]0 y = mx + b Slope will be k! Software will calculate the slope when you click “Linear Fit” button! You just need to identify it and justify your answer.

Processing the Data 3. Write the correct rate law in terms of crystal violet (omit OH-) Remember, you will be determining the pseudo rate law because we do not know the order with respect of OH-. pseudo Rate = ?

Processing the Data 4. Half-life of the reaction? Remember, half-life is a unit of time. It answers the question: What time does it take for the concentration of CV+ to be reduced to ½ it’s initial concentration value (or 50% of what you started with) or ½ it’s initial Absorbance.

Processing the Data Half-Life What equation for half-life is given in the AP Constant Chart? Does that apply to this reaction?