KINETICS: THE OXIDATION OF ISOPROPYL ALCOHOL (IPA)
Introduction: Understanding the theory of kinetics and the relationship of kinetics data to the
mechanism of a reaction are challenging problems. The measurement of kinetic data in the
laboratory can also be challenging and not always straightforward. Very few organic reactions
are simple one step reactions that offer simple methods of measuring their kinetics. Usually sets
of approximations or mathematical tricks have to be used to simplify the kinetic description and
make the experimental approach a little easier. Whatever is done, ultimately the concentration,
or a function of the concentration (i.e. absorbance), of reactant is measured vs. time. The goal of
this lab is to determine the rate expression for the reaction below:
OH
3 H3C CHCH3
O
+
K 2Cr2O7
+ 8 H+
3 H3C CCH3
+
2 Cr +3 + 2 K+
+
7 H2O
rxn. rate = k[IPA]a[Cr2O72-]b[H+]c where a, b, and c are unknown values. These values may
be zero,  integers, or  fractions, but typically they are small, positive whole numbers.
The values of a, b, and c have no relation to the balanced chemical equation.
Pseudo Order Kinetics Approach: The easiest concentration to follow is that of Cr2O72- (note
1) which is colored and the concentration is directly proportional to the intensity (measured by
recording absorbance values) (note 2) of the color. Absorbance is measured experimentally
using a Spectronic 20 UV-Visible spectrometer tuned to the exact wavelength absorbed by the
Cr2O72- (350nm).
1. Pseudo Order Kinetics in Cr2O72- It is necessary to simplify the rate expression or the
mathematics become too complex. One way this can be done is to use large excesses of
all but one reactant. Thus, [IPA] & [H+] >>> [Cr2O72-]
Although both [IPA] and [H+] disappear as the reaction progresses, the amount is
stoichometrically governed by [Cr2O72-], which is assumed to be small. If you follow the
reaction for a short period of time, it will appear that the [IPA] and [H+] are constant
because the amount that reacts is negligible compared to the initial concentrations.
Simplified rxn. rate = k′(1)[Cr2O72-]b
Kinetics
where k′(1) = k[IPA]a[H+]c
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The k′ (1) is not a true specific rate constant and the assumption works only in the initial
stages of the reaction (no more than 2 or 3 half-lives). Now that the full rate expression
has been simplified, the order of the reaction with respect to [Cr2O72-] can be determined.
2. Graphical Analysis. The order of the reaction can be determined graphically by process
of elimination, looking for a linear plot. Remember, what you are actually plotting is the
absorbance values of [Cr2O72-].
If b = 0, a plot of [Cr2O72-] vs. time will be linear
If b = 1, a plot of ln[Cr2O72-] vs. time will be linear
If b = 2, a plot of 1/[Cr2O72-] vs. time will be linear
To save time, first try plotting ln[Cr2O72-] vs. time. The slope of the straight line will
equal –k′ if ln functions were used. Or the slope will equal –k′/2.303 if log functions
were used.
3. Computer Analysis. Instead of plotting your points of ln(absorbance) vs. time, visually
determining the best straight line, and manually calculating the slope (which is k′) one
may use a computer instead. The computer mathematically determines the best straight
line through your points using the principle of least squares and calculates the slope and
intercept.
4. Determining the Order With Respect to the Other Reactants. This is done by running a
second reaction with everything held constant except say the [IPA], which is doubled.
The same type of graphical or computer analysis is done and a new k′ is calculated. The
second k′, k′(2), will be larger than the k′(1) because of the assumptions you made when
simplifying the reaction rate equation (top of page). The exact increase allows the
exponent of [IPA], a, to be calculated as follows from the simplified equation at the top
of the page by dividing the equation for k′(2) by that of k′(1).
k′(2)  k[IPA]a(2)[H+]c(2)
k′(1)
k[IPA]a(1)[H+]c(1) but [H+](1) = [H+](2) and [IPA](2) = 2[IPA]
therefore 
k′(2) 
k′(1)
(2[IPA](1))a [H+]c(1)
[IPA]a(1)[H+]c(1)
thus  k′(2) = 2a
k′(1)
From the magnitude of the ratio of k′(2) /k′(1) it may be obvious that a is
log(k′(2)/ k′(1)) = log 2a = a log 2
Kinetics
so that a = log(k′(2)/ k′(1)) / log2
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To obtain c, a third experiment is conducted keeping [Cr2O72-] and [IPA] constant and
increasing [H+]. From this you can determine k′(3) based on the slope and comparing k′(3)
to either k′(2) or k′(1) in a manner similar to the mathematics explained above.
5. Calculation of the Specific Rate Constant. Now that the exponents of a & c have been
determined, the real specific rate constant, k, can be calculated from the simplified rate
equation:
Simplified rxn. rate = k′[Cr2O72-]b where k′(1) = k[IPA]a[H+]c
k′(1) = k[IPA]1a[H+]1c
therefore, k =
k′(1)
[IPA]1a[H+]1c
The k′(1) has already been determined from the slope and the [IPA]1 and [H+]1 can be
calculated from the volumes and concentration initially mixed together. For example, in
experiment #1, suppose that 2.00 mL of 1.00M IPA was mixed with Cr2O72-, HClO4, and
distilled water so that the final volume was 9.00 mL. Then the [IPA] has been diluted
according to the dilution equation:
[M]1V1 = [M]2V2
(2.00 mL)(1.00 M) = (9.00 mL)[IPA]
therefore, [IPA]1 = 0.222 M
[H+]1 can be determined in a similar fashion. Calculate the true k for as many trials as
you ran and report the average value.
6. Calculation of the Rate of the Reaction. Finally, using the data from the first experiment,
calculate the rate of the reaction at the time when [Cr2O72-] has gone through 2 half-lives.
To do this you need to know [IPA]1, [Cr2O72-], and [H+]1 (you have already calculated
two of these values). The balanced equation must be considered, since [H+] decreases 8
times faster than [Cr2O72-] and 3 times faster than [IPA]. For example, if the [Cr2O72-]
has decreased from 0.0004 M initially to 0.0001 M, a change of 0.0003 M, the
corresponding amount of [IPA] that has reacted is 3(0.0003 M) or 0.0009 M [IPA]. This
change in [IPA] is then subtracted from the initial [IPA] to obtain the [IPA] at the time
when we wish to determine the rate.
Rate = k[IPA]a[Cr2O72-]b[H+]c
= k(0.222-0.0009)a(0.0001)b[H+]c
A similar calculation would be done to find [H+]. Now k, all the concentrations, and the
exponents are known and the rate can be calculated.
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Procedure:
Obtain a clean sample tube (note 3) and add 2.0 mL of 5.00 M HClO4 solution, 4.0 mL of
distilled water, and 2.00 mL of 1.00 M isopropyl alcohol solution (note 4). Mix the contents of
the tube thoroughly; this is your first ‘blank solution’. Use the blank solution and the directions
for the Spectronic 20 included with this experiment to prepare the instrument (note 5). Remove
the cell from the spectrometer and add 1.00 mL of the 0.0035 M K2Cr2O7 solution. Mix
thoroughly and rapidly and insert the tube into the cell compartment, close the lid, and begin
recording the initial absorbance at time = 0 (note 6). Record the time and % transmittance at
regular intervals between 10 % transmittance and 70 % transmittance. The start of a typical trial
is shown below.
Time (seconds)
0
22
40
60
81
106
% Transmittance
12.0
14.5
16.3
18.8
20.1
23.0
The above data is then analyzed either graphically or with the computer to determine k′(1) from
the slope.
A similar procedure is used to determine k′(2) in a second trial; in this instance doubling the [IPA]
by mixing 4.00 mL of 1.00 M IPA, 2.00 mL of 5.00 M HClO4 solution, and 2.00 mL of distilled
water; this is your second ‘blank solution’. The instrument is then zeroed as before prior to the
addition of 1.00 mL of 0.0035 M K2Cr2O7 solution. Readings are taken as before. More care
should be taken since the absorbance will drop faster with time than in trial 1.
Finally, k′(3) and c can be determined in a similar fashion in a third trial. If the [H+] is doubled,
the reaction may proceed too quickly to be accurately measured. Therefore, either of the two
choices below is recommended.
a. Increase the amount of HClO4 from 2 to 3 mL and calculate c by comparing k′(3) to k′(1)
(i.e. 2 mL IPA, 3 mL HClO4, 3 mL distilled water, and 1 mL K2Cr2O7 solution).
b. Decrease the amount of HClO4 from 2 to 1 mL and calculate by comparing k′(3) to k′(2)
(i.e. 4 mL IPA, 1 mL HClO4, 3 mL distilled water, and 1 mL K2Cr2O7 solution).
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It may be necessary to repeat any of the 3 trials to verify that consistent data are being collected.
Accordingly, it is recommended that k′(1), k′(2), k′(3), a, b, and c are calculated prior to leaving lab
so that duplicate trials may be run if questionable data is obtained.
---NOTES--1. The exact species responsible for the color as well as the color itself changes depending
on the conditions, in this case, the pH. In this reaction at the concentrations of K2Cr2O7
and HClO4 used the chromium is nearly all present as HCrO4-. This does not affect our
calculations and we will pretend the chromium is present as Cr2O72-.
2. Beer′s Law, which relates concentration and absorbance, is given as:
Absorbance = abc
(where a and b are constants and c is the concentration)
3. Use only the special Spectronic-20 cuvettes. Ordinary test tubes will not work correctly.
4. These volumes are most accurately measured using appropriately sized syringes. Each
group should have a syringe for each solution to prevent contamination. At the end of lab
make sure to place the used syringes into the biohazard sharps container located in the
hood.
5. If the initial absorbance is much greater than 1.0 at 350 nm, a slightly longer wavelength
may be used. This can be tested in advance by mixing 2.0 mL of 5.0 M HClO4, 6.0 mL
distilled water, and 1.0 mL of 0.0035 M K2Cr2O7.
6. The most common source of error in this experiment is poor mixing at this point.
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Bausch and Lomb Spectronic 20 Instructions
-Kinetics Lab-
1. Turn on the instrument using the left hand knob. Allow machine to
warm up for 15 minutes.
2. Check to make sure the instrument is set to the proper wavelength (350
nm).
3. Be sure the sample compartment is empty and the lid is closed.
4. Use the left-hand knob to set the needle on the meter to read infinite
absorbance (0% transmittance).
5. Insert the blank sample (H2O, IPA, and acid) and close the lid of the
sample compartment. The blank should be in a special Spec 20
cuvette. The tube should be approximately half full and should be
inserted so that the white mark aligns with the front of the instrument.
6. Use the right-hand knob to set the needle on the meter to read 0
absorbance (100% transmittance).
7. Remove the blank and insert the sample. The sample should also be in
a special Spec 20 cuvette. The tube should be almost full, well mixed,
and dry. Insert the cuvette so that the white mark aligns with the front
of the instrument. Close the lid on sample compartment.
8. Read the absorbance of the sample from the meter and record.
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