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 MicroLab Interface colorimeter will be used to monitor the change in concentration of crystal violet as a function of time.  The reactant and product structures and the reaction stoichiometry are shown in Figure 1.

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 reacted with the OH−OH− size 12{ ital "OH" rSup { size 8{ - {}} } } {}.

Figure 1

Figure 1. Net ionic reaction between Crystal Violet and NaOH 

The color that crystal violet exhibits 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. The color is due to continuous movement of electrons between single and double bonds. When crystal violet reacts with a base (−OH)(−OH) size 12{ \( - ital "OH" \) } {}, the conjugation is disrupted and the color is lost. Note that in the reaction product, the three rings are no longer in conjugation with one another, and hence the material is colorless. This reaction has kinetics slow enough that the change in color can be observed over time as the molecules are being changed. In today’s experiment, you will trace the loss of conjugation in the crystal violet structure by using colorimetry.

Kinetic rate laws

The rate of the reaction of crystal violet with NaOH is given by the generalized rate expression

Rate=k[OH−]x[CV]yRate=k[OH−]x[CV]y size 12{ ital "Rate"=k \[ ital "OH" rSup { size 8{ - {}} } \] rSup { size 8{x} } \[ ital "CV" \] rSup { size 8{y} } } {}(1)

In Equation (1), k is the rate constant for the reaction, CV is an abbreviation for crystal violet, C25H30N3+C25H30N3+ size 12{C rSub { size 8{"25"} } H rSub { size 8{"30"} } N rSub { size 8{ {} rSub { size 6{3} } } } rSup {+{}} } {}, x is the order of reaction with respect to OH−OH− size 12{ ital "OH" rSup { size 8{ - {}} } } {}, and y is the order of reaction with respect to CV. The values of x and y will be determined experimentally. Possible values are 0, 1, or 2 (zeroeth order, first order or second order).

In the experiment you will perform, the [ OH−OH− size 12{ ital "OH" rSup { size 8{ - {}} } } {}] will always be much greater than [CV]. Thus the change in [ OH−OH− size 12{ ital "OH" rSup { size 8{ - {}} } } {}] has a negligible effect on the initial [ OH−OH− size 12{ ital "OH" rSup { size 8{ - {}} } } {}]. For this reason, [OH−]x[OH−]x size 12{ \[ ital "OH" rSup { size 8{ - {}} } \] rSup { size 8{x} } } {} can be treated as a constant and Equation (1) can be rewritten

Rate=k'[CV]yRate=k'[CV]y size 12{ ital "Rate"=k' \[ ital "CV" \] rSup { size 8{y} } } {}(2)

where k'=k[OH]xk'=k[OH]x size 12{k'=k \[ ital "OH" \] rSup { size 8{x} } } {}. k' is termed a pseudo rate constant.

The integrated form of the rate law depends on the order of reaction with respect to the concentration of CV. The integrated rate laws for y = 0, 1, and 2 are given in Equations 3 through 5.

[CV]t=−k't+[CV]o[CV]t=−k't+[CV]o size 12{ \[ ital "CV" \] rSub { size 8{t} } = - k't+ \[ ital "CV" \] rSub { size 8{o} } } {}(zero order)(3)

ln[CV]t=−k't+ln[CV]oln[CV]t=−k't+ln[CV]o size 12{"ln" \[ ital "CV" \] rSub { size 8{t} } = - k't+"ln" \[ ital "CV" \] rSub { size 8{o} } } {}(first order)(4)

1 = k’t+ 1 (second order)(5)

[ CV ] t [ CV ] t size 12{ \[ ital "CV" \] rSub { size 8{t} } } {} [ CV ] o [ CV ] o size 12{ \[ ital "CV" \] rSub { size 8{o} } } {}

In Equations 3 - 5, [CV]o[CV]o size 12{ \[ ital "CV" \] rSub { size 8{o} } } {} is the concentration of crystal violet in the reaction mixture at time zero before any reaction occurs; [CV]t[CV]t size 12{ \[ ital "CV" \] rSub { size 8{t} } } {} is the concentration at any time during the course of the reaction. Equations 3, 4b, and 5 are each an equation of a straight line. If a plot of [CV]t[CV]t size 12{ \[ ital "CV" \] rSub { size 8{t} } } {} versus time is linear, y = 0 and the reaction is zero order in CV. Similarly, a linear plot of ln [CV]t[CV]t size 12{ \[ ital "CV" \] rSub { size 8{t} } } {} versus time indicates a first order reaction in CV, and a linear plot of 1 / [CV]t[CV]t size 12{ \[ ital "CV" \] rSub { size 8{t} } } {} versus time indicates second order behavior. In every case, the slope of the resulting straight line would be the pseudo rate constant, k'. All three of these plots will be made to determine the actual value of y and the value of k'.

In order to do the graphing just described, we will need to have data showing how the concentration of CV changes with time. This data will be obtained using the MicroLAB colorimeter at the 590 nm wavelength and the Kinetics program. 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 in microamps (I) which is proportional to the light intensity striking the photocell surface. This current is divided by the current obtained with the blank, and the result is termed Transmittance.

Solutions of crystal violet obey Beer's Law. Thus, the relationship between the observed current and the concentration of CV is given by

At=−log(It/Io)=εbcAt=−log(It/Io)=εbc size 12{A rSub { size 8{t} } = - "log" \( I rSub { size 8{t} } /I rSub { size 8{o} } \) =ε ital "bc"} {}(6)

In Equation (6), AtAt size 12{A rSub { size 8{t} } } {} is the reaction solution absorbance at any time t; IoIo size 12{I rSub { size 8{o} } } {} is the photocell current observed for pure water (the blank value); It is the current observed for the CV reaction mixture at time t, εε size 12{ε} {} is the molar absorptivity of crystal violet; b is the cell path length (2.54 cm for the MicroLAB colorimeter) vial; and c is the molar concentration of CV at time t, [CV]t[CV]t size 12{ \[ ital "CV" \] rSub { size 8{t} } } {}. Since εε size 12{ε} {} and b are constants at a given wavelength, it should be clear that the absorbance, AtAt size 12{A rSub { size 8{t} } } {}, is directly proportional to the concentration of CV at any time during the reaction and can be used in place of [CV]t[CV]t size 12{ \[ ital "CV" \] rSub { size 8{t} } } {} in preparing the graphs described above.

Experimental Procedure

Part I.

Measurements

Open the MicroLAB program by selecting it, then click on the Kinetics Experiment Icon. Enter the experiment filename (CV then your name) when requested, then click OK. Data will come from the interface. Be sure the MicroLAB interface is connected to the computer and turned on. From that point follow the procedures listed below.

Data Analysis

Click on the Linear - Zero Order tab at the bottom of the graph. If the reaction you just did was zero order on the concentration of crystal violet, this will show a horizontal straight line. Print this screen as follows:

Part II.

Measurements

Recall that the k' just obtained is a pseudo rate constant, whose value depends upon the concentration of OH−OH− size 12{ ital "OH" rSup { size 8{ - {}} } } {}, i.e. k'=k[OH−]xk'=k[OH−]x size 12{k'=k \[ ital "OH" rSup { size 8{ - {}} } \] rSup { size 8{x} } } {}. In this part of the experiment, the value of x will be determined as well as the value of the true rate constant, k.

In Part I of the experiment, 9.00 mL of 1.50×10−51.50×10−5 size 12{1 "." "50" times "10" rSup { size 8{ - 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 concentration of NaOH will be doubled to 0.10 M.

Data Analysis

Note: The value of x should be an integer. If your value is not an integer, it is probably due to experimental error (probably in measuring and adding the NaOH solutions). If necessary, round your value to the nearest integer.