EXPERIMENT 27
KINETICS BY COLORIMETRY
INTRODUCTION
One of the most interesting aspects of chemical reactions is the rate at which they occur, along with a
consideration of the variables which influence the rate. In this experiment, you will examine the effect of reactant
concentrations and temperature on the reaction rate.
To understand this experiment, it is important to review the section on chemical kinetics in your chemistry
textbook. Be sure you understand how the rate is expressed in terms of the change in concentration with time and
be able to distinguish between the average rate over a finite time interval and the instantaneous rate at a given
moment in time. For this experiment it is most important to be familiar with rate laws (which show how rates vary
with concentrations) and integrated rate laws (which show how concentrations vary with time).
The reaction you will study is that between crystal violet and sodium hydroxide. Crystal violet is an intensely
colored dye compound with the molecular formula C25H30N3Cl. In aqueous solution it exists as C25H30N3+ and Cl
ions. The net ionic equation for the reaction that occurs is
N(CH3)
N(CH3)
2
2
+
N(CH3)
+
OH
+ OH

N(CH3)
2
2
N(CH3)
N(CH3)
2
2
cation, purple - one of the
possible resonance forms
neutral, colorless
Figure 1. Reaction between crystal violet cation and hydroxide ion.
which we will abbreviate as CV+ + OH  CVOH. Note the extensive pattern of alternating single and
double bonds throughout the structure for the cationic form of crystal violet. This pattern, common in organic dye
compounds, leads to molecular orbitals spread over the entire molecule with energy spacings corresponding to
photons in the visible region of the spectrum, hence the deep coloration. The reaction with hydroxide ion causes a
change in geometry around the central carbon atom. This disrupts the pattern and results in the loss of the
coloration. You will use this loss of coloration to monitor the progress of the reaction.
The rate law for this reaction can be written
 d CV   
   k CV   x OH   y
rate    

 



dt


(27-1)
The expression in parentheses is, in calculus terms, a derivative. It is the instantaneous rate of reaction at any
given point in the reaction. Your task will be to determine the exponents x and y, the order of the reaction with
respect to each of the reactants. In addition, you will determine the rate constant k for the reaction at room
temperature, thereby determining the complete rate law. Finally, you will examine the effect of temperature on the
rate constant and calculate the activation energy for the reaction.
The activation energy is the term Ea in the Arrhenius equation,
k  Ae
 Ea
RT
(27-2)
When k is determined at two different temperatures, the value of Ea can be calculated using
ln
E a  R
k2
k1
1
1
  
 T2 T1 
(27-3)
where R is the gas constant and the temperatures are expressed in Kelvins. (If you had a number of k values at a
series of temperatures, it would be preferable to combine them to determine Ea graphically. The slope of a plot of
lnk versus 1/T is equal to Ea/R, from which Ea is easily calculated.)
One final aspect of this reaction which should be mentioned briefly is that, as seen in Figure 1, the reaction is
between two ions. In this case the ionic charges are +1 and 1, respectively. The rate constants for such reactions
depend to some extent on the total concentration of all ions in solution, usually expressed in terms of a quantity
known as the “ionic strength” of the solution. (For this reaction, higher ionic strengths decrease the reaction rate,
since the attractive forces between the oppositely charged ions are partially “screened” by the higher concentrations
of ions in the solution.) While this is an interesting area that could be investigated in its own right, we will not take
the time to do so here. We will simply avoid this effect by arranging conditions so that the “ionic strength” is the
same in all of the reaction mixtures to be studied. The effect will be the same for all measurements and will not
influence the various comparisons we make.
TECHNIQUE
Colorimetry
You will follow the progress of the reaction by monitoring the disappearance of the color due to the crystal
violet. You can do this quantitatively, since the absorbance of a solution is proportional to the concentration of the
absorbing species it contains, as expressed in the Lambert-Beer Law,
A  bC
(27-4)
where A is the absorbance,  is the molar absorptivity, b is the path length of the light through the solution, and
C is the concentration of the absorbing species.
The device you will use to determine the absorbance is known as a colorimeter. Recall that you used the
networked spectrometer to determine the absorbance as a function of wavelength (the absorption spectrum) for
various solutions in other experiments. The light source used with the spectrometer provides light of all
wavelengths throughout the visible region. The diffraction grating in the spectrometer disperses the light onto a
diode array detector, thus allowing the absorbance to be determined for each diode element (therefore each tiny
wavelength range). The colorimeter does not have a grating to disperse the light, so all of the light passing through
the sample falls on a single detector. You can think of the colorimeter as behaving something like a single element
in a diode array. An absorbance value you determine with the colorimeter is therefore actually an average value
over the wavelength range of the light present.
Some selectivity is still possible, however, since the light source used in the colorimeter is a red/green/blue
three-element light-emitting diode (LED). You select which of the three colors to use in any particular experiment.
The best color is the one which is most strongly absorbed by the sample, since that allows the absorbance to be
determined with the greatest precision. Figure 2, below, displays the information needed to choose the best LED
color to use for crystal violet solutions. The emission spectra (y axis on the left) of the three LED elements are
shown, along with the absorption spectrum (y axis on the right) of a solution of crystal violet. All of these spectra
were measured with the networked spectrometer. Where the crystal violet curve is the highest, it is absorbing the
largest fraction of the light shining on it. Figure 2 shows the highest absorbance values for the wavelengths emitted
by the green LED. Therefore, the green LED is best for this experiment.
Pseudo-Order Reaction Kinetics
Equation 27-1 shows that the reaction rate depends on the concentrations of both reactants. In general, this
significantly complicates your task, since you must determine how both concentrations affect the rate. Since crystal
violet absorbs so strongly, however, absorbance values can be measured at in solutions containing very small
concentrations of crystal violet. Because of this, you can arrange things so that, in effect, you can isolate the effect
of the crystal violet concentration from that of the hydroxide ion concentration. In your experiments, you will have
hydroxide ion concentrations that are thousands of times as large as that of crystal violet. Since hydroxide ion and
Crystal Violet Absorption Spectrum &
LED Emission Spectra
Blue
LED
3500
1.6
Green
LED
CV
Red
LED
Intensity
3000
1.4
1.2
2500
1.0
2000
0.8
1500
0.6
1000
0.4
500
0.2
0
0.0
-500
400
450
500
550
600
650
Wavelength/nm
Figure 2. Spectra for choosing LED color.
-0.2
700
Absorbance
4000
crystal violet react in a one-to-one ratio (see Figure 1), this means that the hydroxide ion concentration remains
virtually constant throughout the reaction. Therefore the change in rate as the reaction proceeds is due only to the
changing crystal violet concentration. Under these circumstances Equation 27-1 can be written
rate  k[CV ] x [OH  ] y  k obs[CV ] x
(27-5)
k obs  k[OH  ] y
(27-6)
where
The expression kobs represents the observed, or pseudo, rate constant, and the reaction is said to be pseudo x
order in crystal violet. You will first determine the values of x and kobs as described below.
Integrated Rate Laws
You monitor concentration of CV+ versus time in this experiment. Equation 27-5 doesn’t directly apply,
since it gives rate versus concentration. You do not need to worry about the details, but the methods of calculus can
be used to convert Equation 27-5 to the equivalent concentration versus time expression for any assumed value of x.
We will only consider possible x values of 0, 1, and 2 for a pseudo zeroth-, first-, or second-order reaction,
respectively. The resulting expressions are
zeroth order:
[CV  ]  k obs t  [CV  ]0
(27-7)
first order:
ln[CV  ]  k obs t  ln [CV  ]0
(27-8)
second order:
1

[CV ]
 k obs t 
1
[CV  ] 0
(27-9)
where [CV+]0 represents the initial concentration of crystal violet. These equations, having the general form y =
mx + b, suggest making graphs of [CV+], ln[CV+], and 1/[CV+] versus the time, t. The graph for which the
experimental points lie most nearly along a straight line is the one showing the best choice for the value of x.
For example, if a plot of 1/[CV+] versus the time, t, yields a straight line, then the reaction would be said to be
second order in crystal violet, and the value of x would be 2. In addition, the value for kobs can be obtained from the
slope of this same plot; since slope = −kobs for the zeroth and first order plots and slope = kobs for the second
order plot.
One minor problem remains with equations 27-7, -8 and -9, however. You determine values of absorbance
versus time in your experiments, rather than actual concentrations of crystal violet. To see how absorbance would
vary with time, we substitute Equation 27-4 into these three, obtaining
zeroth order:
first order:
second order:
A = −kobsεbt + A0
(27-10)
ln A = −kobst + ln A0
(27-11)
1  k obs 
1

t

A  εb  A 0
(27-12)
Note that the mathematical forms did not change, suggesting plots of A, lnA, and 1/A versus t can be used to
find the best choice for x. Note also, however, that the slopes of the lines for zeroth and second order now include
b. Thus, if 0 or 2 were found to be the best value for x, this would have to be taken into account to find kobs from
the slope. If you need the value of the product b, you need only place a solution of crystal violet with known
concentration in the colorimeter and note the absorbance value displayed. Equation 27-4 can then be used to find
b.
You still need to determine how the rate depends on hydroxide ion concentration, that is, determine the
value of y. You accomplish this by comparing values of kobs determined for reactions with different hydroxide ion
concentrations. In each case the hydroxide ion concentration is much larger than the crystal violet concentration, so
that Equation 27-5 still applies. As an example, suppose doubling the hydroxide ion concentration from 0.10M to
0.20M causes kobs to increase by a factor of 4, from 0.0010 s-1 to 0.0040 s-1. Examination of Equation 27-6 shows
that such a result would indicate that y is equal to 2, indicating that the reaction is second order with respect to
[OH].
kobs (trial 2) k[OH - (trial 2)] y
=
kobs (trial 1) k[OH - (trial 1)] y
0.0040 s -1 k[0.20M] y
=
0.0010 s -1 k[0.10M] y
(27-13)
4 = (2) y
y= 2
Finally, you will use Equation 27-6 to determine the actual rate constant, k, from values of kobs, y, and [OH].
EQUIPMENT NEEDED
colorimeter
beakers
medium test tubes
cuvets for colorimeter (2)
2 mL volumetric pipet
2 mL graduated (Mohr) pipet
5 mL volumetric pipet
10 mL volumetric flask
mercury thermometer
wash bottle
CHEMICALS NEEDED
distilled water
0.20 M sodium hydroxide (NaOH) solution
0.10 M NaOH in 0.1 M sodium chloride (NaCl) solution
3.0  10-5 M crystal violet (CV) solution
Colorimeter
PROCEDURE
You will be performing 6 kinetics trials:
Trial
CV solution used
NaOH solution used
Temperature
1
3.0  10-5 M
0.1 M NaOH in 0.1 M NaCl
Room Temp.
2 (repeat of 1)
3.0  10-5 M
0.1 M NaOH in 0.1 M NaCl
Room Temp.
3
3.0  10-5 M
0.1 M NaOH in 0.1 M NaCl
Warm bath
4 (repeat of 3)
3.0  10-5 M
0.1 M NaOH in 0.1 M NaCl
Warm bath
5
3.0  10-5 M
0.2 M NaOH
Room Temp.
6
1.5  10-5 M
0.1 M NaOH in 0.1 M NaCl
Room Temp.
Setting Up the Colorimeter
1. Connect the colorimeter to one of the 9-pin inputs on the station. (MAKE SURE NO OTHER PROBES ARE
CONNECTED TO THE STATION.) Fill two cuvets with distilled water.
2. Press MAIN MENU on the workstation. A list of measurement types will appear on the screen. (Make a note
of the station number listed at the top of the screen.) Press the function key listed for
COLORIMETRY/FLUOR./TURB., the function key for COLORIMETRY, the function key for the
GREEN LED, and then the function key for KINETICS. Follow the instructions on the screen, which will
prompt you to place the two cuvets containing water in the reference (R)
cell
and sample (S) cell holders in the colorimeter. Be careful to properly
position
align the cells in the holders - almost no force is required to insert them.
cell
See diagram at right. The holder is designed so that the cell is held only
holder
by its corners. This minimizes the likelihood of scratching the sides of
the cell, which would make them unsuitable for further use. Give each
cuvet a light push to make sure they are seated all of the way into the cell holder.
3. When the filled cuvets are in place, slide the colorimeter lid closed and press ENTER. 0%T and 100%T will
now be adjusted, requiring about 10 seconds. Halfway through this time the LED indicator on the top of the
colorimeter will light up green to indicate that the internal green LED has been turned on. When this process is
finished, a new display will list your options. Press DISPLAY, then START/STOP. At this point, the
workstation will display instructions for the first step of the data collection process. Do not start this process
yet; you must first prepare the solutions.
Preparation of Solutions for Trials 1-4
4. Fill a 400 mL beaker ~1/2 full with distilled water. This will serve as a water bath to control the temperature of
each kinetics trial to be run at room temperature. Place a mercury thermometer in the beaker. Fill a wash
bottle with distilled water.
5. Fill another 400 mL beaker ~1/2 full with hot tap water. This will serve as a water bath to control the
temperature of each kinetics trial to be run above room temperature.
6. Obtain ~20 mL each of the crystal violet and 0.10 M NaOH in 0.1 M NaCl solutions (the sodium chloride is
added to ensure constant ionic strength for each trial to be run). Obtain ~10 mL of the 0.20 M NaOH solution.
Do not take more solution than needed, and do not return any to the stock bottles.
7. Using a 2 mL volumetric pipet, transfer 2.00 mL of the crystal violet solution into each of four clean, dry
medium test tubes. Place two of the test tubes in the room temperature water bath, and the other two in the
warm water bath.
8. Using a graduated pipet, transfer 2.00 mL of the 0.10 M NaOH in 0.1 M NaCl solution into each of four
different medium test tubes. Place the two of the test tubes in the room temperature water bath, and the other
two in the warm water bath.
Carrying Out the Reactions
Special Note 1: Two different starting times are distinguished when using the colorimeter. The first
is the time the reaction actually begins. This is when you mix the solutions to begin the reaction. The
second starting time is when you begin actually collecting data. This will probably be 15-30 seconds
after the start of the reaction, since it will take that long to mix the reactants, fill the cuvet with the
reacting mixture, place the cuvet in the colorimeter, and begin the measurements. The experiments
will proceed most smoothly if one person mixes and transfers the solutions while his/her partner
handles pressing the START/STOP button.
Special Note 2: The reactant concentrations you need to know in the experiments you carry out are
those in the reaction mixture, not in the stock solutions. When you follow the procedure detailed
below, you will combine equal volumes of two solutions, one containing crystal violet and the other
containing hydroxide ion. The result of combining the two solutions to start the reaction is, in every
case, to dilute both the crystal violet and the hydroxide ion concentrations to half their original
values. Therefore, be sure the concentrations you enter into the results table are the diluted
values, the actual concentrations in the reaction mixture in each case.
For each of the six trials, carry out steps 9-13, using the solutions described below.
9. Remove the cuvet from the SAMPLE cell holder in the colorimeter (labeled S), pour out the contents of the
cuvet, and shake the cuvet to remove as much of the remaining liquid as possible.
10. For each kinetics trial, record the temperature of the water bath to the nearest 0.1ºC. Remove from the water
bath the two test tubes containing the solutions you are going to mix for the trial. Work together with your
partner to simultaneously pour the contents of one test tube into the other and press START/STOP on the
station to begin the time counter. (The absorbance is measured and displayed twice per second, but no data are
stored at this point.) Quickly, but carefully, pour the reaction solution back and forth between the test tubes 2-3
times to ensure complete mixing, then transfer some of the reacting mixture into the empty sample cuvet. Place
the cuvet in the sample holder in the colorimeter, close the lid, and note the absorbance reading displayed on
the workstation screen. Press START/STOP again—this causes the system to begin actually storing the
absorbance values and plotting them in the station display. Leave the rest of the solution in the test tube so that
you can monitor the reaction progress visually.
11. Monitor the reaction for about two half-lives. That is, continue collecting data until the absorbance value falls
to somewhere around one-fourth of the value you observed when you began collecting data (note: do not allow
the absorbance value to drop below 0.05; at this point, the relative error of the data will increase significantly).
At this point press START/STOP (again!) to stop the data collection.
12. Print your results. Press FILE OPTIONS, read the menu that appears, and press the function key listed for
PRINT custom. Enter the number of copies to be printed, and press ENTER. You will now be asked for a 3digit print code. Enter 300 and press ENTER. The page printed will contain three plots corresponding to
different treatments of the data: absorbance vs. time, ln(absorbance) vs. time, and 1/absorbance vs. time. Show
the plot to your TA to make sure the data is acceptable. Note: if the R2 value (which is an indicator of how well
the data fits the straight line drawn through the points) for plot the most linear plot is less than 0.995, you
should strongly consider redoing the trial. Label each printed page to identify the trial and show the
temperature value.
13. Press DISPLAY, then START/STOP to clear out the data in preparation for the next trial.
Trial 1. Use the crystal violet and 0.1 M NaOH solutions from the room temperature bath.
Trial 2. Repeat trial 1.
Trial 3. Transfer the thermometer to the warm temperature bath. When the mercury no longer rises, record the
temperature. Use the crystal violet and 0.1 M NaOH solutions from the warm water bath.
Trial 4. Repeat Trial 3, recording the new temperature reading.
Preparation for Trial 5
14. Transfer 2.00 mL of the crystal violet solution into a medium test tube. Place the test tube in the room
temperature water bath.
15. Rinse the volumetric pipet you used to transfer the 0.1 M NaOH solution with a small amount of the 0.2 M
NaOH solution. Transfer 2.00 mL of the 0.2 M NaOH solution into a medium test tube. Place the test tube in
the room temperature water bath.
Trial 5. Use the crystal violet and 0.2 M NaOH solutions from the room temperature bath.
Preparation for Trial 6
16. Using a 5 mL volumetric pipet, transfer 5.00 mL of the crystal violet solution into a 10 mL volumetric flask.
Dilute to the line with distilled water from the wash bottle.
17. Rinse the volumetric pipet you used to transfer the crystal violet solution with a small amount of the diluted
crystal violet solution. Transfer 2.00 mL of the diluted crystal violet solution into a medium test tube. Place the
test tube in the room temperature water bath.
18. Rinse the volumetric pipet you used to transfer the 0.2 M NaOH solution with a small amount of the 0.1 M
NaOH solution. Transfer 2.00 mL of the 0.1M NaOH solution into a medium test tube. Place the test tube in the
room temperature water bath.
Trial 6. Use the diluted crystal violet and 0.1 M NaOH solutions from the room temperature bath.
19. Waste disposal: All solutions can be poured down the sink. Rinse the glassware and cuvets with distilled water
before returning them.
LAB REPORT
1. For each trial, fill in the temperature data on the Report Sheet on Chem21. Submit and then confirm the data
submission.
2. After you fill in the Introduction and Procedure sections (you can skip over these for now if you wish),
determine and enter the order with respect to crystal violet concentration, [CV+]. Use Equations 27-10, 27-11 or
27-12 to make this determination.
3. For each trial, you will enter, order, the initial concentrations of crystal violet and hydroxide ion, the slope of
line from the appropriate plot (as determined in step 2), and the pseudo rate constant kobs, A few notes on each
entry:
When entering data using scientific notation, use ‘e’ in place of ‘x 10’. For example, if the concentration is
3.0 x 10-5, you would enter 3.0e-5.
Remember that since you mixed equal volumes of the two reactants, the initial concentration for each will
be half of the original concentration.
The plot that gives the best straight line should be the same for each trial. In other words, if you determine
that the second order plot gives the best straight line for Trial 1, then you should use the slope of the second
order plot for all trials.
Be careful with the sign of the slope! If you enter the data incorrectly, you will need to contact your TA to
get it modified, which will cost you points off of your lab report score.
4. Determine and enter the order with respect to hydroxide ion concentration, [OH-]. Use Equation 27-13 as a
guide to make this determination.
5. For each trial, use Equation 27-6 to calculate the rate constant k. Calculate and record the average rate constant
for Trials 1 and 2.
6. For Trials 3 and 4, use Equation 27-3 to calculate the activation energy. In each calculation, the average k
value for Trials 1 and 2 will represent k1, and the temperature for Trial 1 will represent T1. Record the
answer in kJ/mol.
EXPERIMENT 27
REPORT SHEET
Name: ____________________________________
Date:__________
Partner: __________________________________
Reaction order with respect to [CV+]
Trial #
Temp.
[CV+]
[OH−]
slope
kobs
1
2
---
---
---
3
4
5
6
Reaction order with respect to [OH−]
---
---
Average
k for
Trials 1
and 2
Trial #
k
Activation
Energy
1
---
2
---
Average
k for
Trials 1
and 2
---
3
4
5
---
6
---
Notes—Exp. 27
Kinetics By Colorimetry
Procedure Considerations

Make sure the temperature probe is not connected to workstation!

For each trial, you will press Start/Stop 4 times:
1) To prepare the workstation for data acquisition
2) To begin the timer when the solutions are first mixed together.
3) To start the acquisition of absorbance data when the cuvet is placed into the colorimeter.
4) To stop the acquisition of data.

While you are waiting for one of your first four trials to reach completion, start preparing the solutions
for Trial 5.

If you place the test tubes for Trials 3 and 4 in the water bath at the same time would be a good idea to
label them to make sure you don’t mix them up.

Be sure to rinse out all borrowed glassware before returning!
4/11