|In this experiment, you will observe the reaction between crystal violet
and sodium hydroxide. One objective is to study the relationship between
concentration of crystal violet and the time elapsed during the reaction.
The equation for the reaction is shown here:
A simplified (and less intimidating!) version of the equation is:
CV+ + OH- ®
The rate law for this reaction is in the form:
rate = k[CV+]m[OH-]n,
where k is the rate constant for the reaction, m is the
order with respect to crystal violet (CV+), and n is
the order with respect to the hydroxide ion. Since the hydroxide ion concentration
is more than 1000 times as large as the concentration of crystal violet,
[OH-] will not change appreciably during this experiment. Thus,
you will find the order with respect to crystal violet (m), but
not the order with respect to hydroxide (n).
As the reaction proceeds, a violet-colored reactant will be slowly
changing to a colorless product. Using the computer-interfaced spectrophotometer,
you will monitor the absorbance of the crystal violet solution with time.
We will assume that absorbance is proportional to the concentration of
crystal violet (Beer’s law).
Absorbance = e [CV+]
Therefore, absorbance will be used in place of concentration in plotting
the following three graphs:
• Absorbance vs. time: A linear plot indicates a zero
(k = –slope).
• ln(Absorbance) vs. time: A linear plot indicates a first
(k = –slope).
• 1/(Absorbance) vs. time: A linear plot indicates a second
(k = slope).
Once the order with respect to crystal violet has been determined, you
will also be finding the rate constant, k, and the half-life for
Solutions of crystal violet and NaOH, spectrophotometer, cuvet, two
small (10 mL) beakers, 25 mL beaker.
Each student in a group will measure kinetics of the reaction
at different concentration of sodium hydroxide. Before starting kinetic
measurements, you should measure the spectrum of the crystal violet in
the aqueous solution and establish the wavelength suitable for absorption
measurements. Of course, this wavelength is the same for the whole class,
so there is no reason to establish it once and once again.
1. Prepare spectrophotometer for kinetics measurements (separate
instructions will be provided for each group).
2. Pipet 5.0 mL of NaOH solution into 10 mL beaker. CAUTION:
Sodium hydroxide solution is caustic. Avoid spilling it on your skin or
Pipet 5.00 mL of 2.0 X 10-5 M crystal violet solution.
CAUTION: Crystal violet is a biological stain. Avoid spilling it on your
skin or clothing.
3. To initiate the reaction, simultaneously pour the 5-mL portions
of crystal violet and sodium hydroxide into a 25-mL beaker and stir the
reaction mixture. Rinse the cuvette with ~1-mL amounts of the
reaction mixture and then fill it 3/4 full. Place the cuvette in the cuvette
slot of the spectrophotometer, and take click on "Collect" button. THe
program will collect the absorbance data for you.
4. Analyze the data graphically to decide if the reaction is zero,
first, or second order with respect to crystal violet. You will see three
windows on display.
• Zero Order: If the current graph of absorbance vs. time is linear,
the reaction is zero order.
• First Order: To see if the reaction is first order, it is necessary
to plot a graph of the natural logarithm (ln) of absorbance vs. time. If
this plot is linear, the reaction is first order.
• Second Order: To see if the reaction is second order, plot a graph
of the reciprocal of absorbance vs. time. If this plot is linear, the reaction
is second order.
PROCESSING THE DATA
1. Was the reaction zero, first, or second order, with respect to the
concentration of crystal violet? Explain.
2. Calculate the rate constant, k, using the slope of the linear regression
line for your linear curve (k = –slope for zero and first order and k =
slope for second order). Be sure to include correct units for the rate
constant. Note: This constant is sometimes referred to as the pseudo rate
constant, because it does not take into account the effect of the other
3.Estimate the half-life of the reaction.
4. Compare the reaction orders a nd rate constants received with your
group for various concentrations of NaOH.
5. Write the correct rate law expression for the reaction