CHEM 251 Laboratory

1. CHEM 251 Laboratory Chemical Kinetics Lab

2. Agenda For the week of November 29th • Experiment: Iodine Clock, handout • Prelab • Quiz: Material in the handout

3. Overview • When it comes to experiments involving kinetics, the first thing one must do is to determine the order of a reaction. • Order of reaction: the sum of the exponents on the concentration terms for a rate law. • Rate = k[A]x[B]y, where reaction order = x+y • What the reaction order tells us: • It illustrates the functional relationship between concentration and rate. • Ex. a reaction order of three means that the rate of reaction increases as the cube of the concentration while a reaction order of -1 means the compound actually slows down the rate of reaction.

4. Overview, con’t • The reaction order becomes a part of the rate law for the reaction: Rate = -D[A]/Dt = k[A]x • The rate law tells of the change in the quantity of a reactant or product that takes place in a period of time. • In this experiment, you will monitor by spectroscopy the disappearance of color of one of the reactants and use that information to determine the reactant behavior.

5. Overview, con’t • You can assume that the disappearance of color is due to the consumption of the reactant and the appearance of the product. • The intense color of crystal violet is due to the system of alternating single and double bonds of the 3 benzene rings and the central carbon atom in the dye structure. • This is called conjugation. • When the dye is added to NaOH, the OH group is added to the central C-C double bond, breaking it and disrupting the conjugation between the rings. • This is why the product is colorless. • You will determine the order of reaction with respect to this consumed reactant, and from there, determine the rate law for the reaction. • You will first have to find the order of the reaction with respect to [dye] by graphical determination, then use the method of integrated rate laws to find the order of the reaction with respect to [NaOH].

6. Methodology • You will first have to make three graphs of your data: • Absorbance vs. time (zero order) • Ln(absorbance) vs. time (first order) • 1/absorbance vs. time (second order) • Only one of these will be linear, and the one that is linear determines the order of your reaction with respect to [dye].

7. Methodology, con’t • Using the integrated rate laws, you can find the reaction with respect to [NaOH]. • Because the [NaOH] is in such great access, its concentration will not change much during the reaction. Therefore, the rate law can be rewritten in the following manner: Rate = k’1[dye]m where k’1 = k[OH-]n • In order to establish the reaction order with respect to the NaOH concentration, we have to take into account the effect of different [NaOH].

8. Methodology, con’t • Therefore, we get the two equations below: k’1A = k[OH1-A]n k’1B = k[OH1-B]n where k’1A and k’1B are the slopes of the lines • Therefore, in order to determine the value of n, you can use the two above equations to obtain the ratio between k’1A and k’1B. • Hint: solve for n using logarithms!

9. Methodology, con’t • Using logarithms, your two equations become: lnk’A = lnk + nln[OH-]A lnk’B = lnk + nln[OH-]B • Using basic algebra and subtracting B from A, you get the simplified version of this equation: ln(k’A/k’B) = mln([OH-]A/[OH-]B) • By solving for m, you can determine the reaction order with respect to [NaOH]. • Afterwards, you can plug in this value into any of the two above equations and solve for k.

10. Procedure • Turn on the Spectronic 20 and set the wavelength to 540 nm. Take a clean cuvette and fill it 2/3 full with one of the two NaOH solutions to use as a blank to calibrate the machine. • When you are ready to start the reaction, add one single drop of crystal violet dye solution, invert to mix the solution, and place inside the spectrometer (this is time zero). One minute after you placed it in the spec, you will begin taking the absorbance values every minute for 15 minutes. • Once you have taken the 15th absorbance value, remove the sample tube from the instrument and re-zero the spec with the second NaOH solution. Repeat steps 1-2 with the second NaOH solution. • When you finish collecting the data, turn off the instrument, discard samples down the drain, and clean the test tubes you used.

11. Outside of class Include the following in your lab report: • Sample calculations on how you found the reaction order and the k value. • Make three graphs: • Absorbance vs. time • ln (absorbance) vs. time • 1/absorbance vs. time • For your discussion conclusion, answer the following questions along with the usual error and data analysis: • Prior to placing the sample inside the spectrometer, it takes a bit of time to completely mix the reactants. Would a delay of 2-3 minutes cause an error in the data? If so, how would it influence the graph of absorbance vs. time? • If the temperature was increased while the reaction was occurring, how would this effect the graph of absorbance vs. time? • Why do you think initial rates are used to study a chemical reaction? • Sometimes linearity could not be seen in any of the three graphs. Why do you think this happens? What do you think are some possible sources of error that could have occurred in this experiment?

12. Before you leave • Clean up your workstations before you leave • Don’t forget to turn in your carbon copies to me before you go • There will be no class data this week • You are responsible for your data only