0.13 Crystal violet kinetics  (Page 12/2)

Crystal violet kinetics

Objective

• To study the reaction rate of crystal violet with NaOH using the LabWorks Interface colorimeter.
• To determine the reaction order with respect to each of the reactants.
• To calculate the room temperature rate constant for the reaction.

Grading

• Pre-Lab 10%
• Lab Report Form 80%
• TA Points 10%

Background information

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 ${\text{OH}}^{-}$ .

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 $(-\text{OH})$ , 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

$\text{Rate}=k[{\text{OH}}^{-}{]}^x[\text{CV}{]}^y$ (1)

In Equation (1), k is the rate constant for the reaction, CV is an abbreviation for crystal violet, $C_{\text{25}}{H}_{\text{30}}{N}_{{}_3}^{+}$ , x is the order of reaction with respect to ${\text{OH}}^{-}$ , 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 [ ${\text{OH}}^{-}$ ] will always be much greater than [CV]. Thus the change in [ ${\text{OH}}^{-}$ ] has a negligible effect on the initial [ ${\text{OH}}^{-}$ ]. For this reason, $[{\text{OH}}^{-}{]}^x$ can be treated as a constant and Equation (1) can be rewritten

$\text{Rate}=k\text{'}[\text{CV}{]}^y$ (2)

where $k\text{'}=k[\text{OH}{]}^x$ . 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.

$[\text{CV}{]}_t=-k\text{'}t+[\text{CV}{]}_o$ (zero order)(3)

$\text{ln}[\text{CV}{]}_t=-k\text{'}t+\text{ln}[\text{CV}{]}_o$ (first order)(4)