kinetics___the oxidation of an alcohol by dichromate ion

Oxidation of Alcohol
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Kinetics: The Oxidation of an Alcohol by Dichromate Ion

INTRODUCTION
In this experiment you will learn to determine how the rate of a reaction is affected by changes in the reactant
concentrations at a constant temperature. You will measure the rate of oxidation of isopropyl alcohol by dichromate ion in
an acid solution. The rate of the oxidation is measured by following the disappearance of Cr2O72-, which has a bright
yellow color, using a spectrophotometer. As the dichromate reacts, the Cr(VI) is reduced to Cr(III) which, although pale
green in color, does not absorb at the same wavelength as the dichromate ion. All other reactants and products are
colorless.

The balanced reaction is shown below.


CH3 CH3

3 CH-OH + Cr2O72- + 8 H+ 3 C O + 2 Cr3+ + 7 H2O

CH3 CH3
isopropyl alcohol acetone

You will determine the order of the reaction with respect to each of the three reactants, and calculate the specific rate
constant. This can be cleverly accomplished using only six experimental runs if you "flood" the system (add large, but
known, excess amounts) with two of the three reactants. The two reagents in large excess remain essentially constant
throughout the course of the oxidation so that an observed rate constant can be determined by monitoring only the
concentrations of the third reactant with time. (The concentration of the third reactant, Cr2 O 7 - , will be followed
2

spectrophotometrically since it is a colored species). In different runs of the experiment, we will use different
concentrations of alcohol and acid (the reagents in excess) in order to observe how these concentrations affect the rate of
the reaction.

The technique of "flooding" the system with all reagents but one is commonly used by kineticists and results in a pseudo-
order reaction with respect to the limiting reagent. "Pseudo" is a prefix from the Greek word that means to deceive. In fact,
you are tricking the system into a critical dependence on only one substance. This can be shown mathematically as follows.
The general rate law for the oxidation of isopropyl alcohol (C3H7OH) with dichromate is:

rate = k[C 3 H 7 OH] x [Cr2 O 7 - ] y [H + ] z
2



If large excesses of C3H7OH and H+ are present (relative to [Cr2O72-]), then the initial and final concentrations of these two
species remain constant. As a rule-of-thumb, the limiting reagent should have a concentration 100 times less than the
others. Since [C3H7OH] and [H+] are relatively constant, they can be considered as part of the observed rate constant.

rate = k obs [Cr2 O 7 - ] y
2



where k obs = k[C 3 H 7 OH] x [H + ] z

Thus, in some experiments you will be seeking data with which to calculate y and kobs. In turn, variations in kobs with
[C3H7OH] and [H+] will give you x, z, and k.
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Spectrophotometry
For a review of the theory and technique of spectrophotometry see Appendix A. Appendix E and a General Chemistry
textbook provide a review of kinetics.

The basic equation for measuring concentrations by spectrophotometry at a set wavelength may be stated as:

I
A = bC = log o
I (1)


where A = absorbance
IO = intensity of light entering sample
I = intensity of light passing through the sample
= molar absorptivity per molar concentration per each centimeter of light path length
b = length of light path in sample (i.e., the diameter of your test tube)
c = molar concentration of the colored substance

This is known as the Beer's Law. You should recognize that using one chromophore, or colored substance, and only one
path length, the absorbance is directly proportional to dichromate concentration. Your readings of A at specific times will
yield the concentration vs time data needed to determine the rate and rate law.

On most spectrophotometers it is more convenient to read percent transmittance, IO/I × 100, than to read A directly. This is
because %T is a linear scale from 0 to 100, while A is a logarithmic scale from 0 to 2. Equation (2) shows the relationship
between absorbance and percent transmission.

100
A = 2 - log(%T) = log (2)
%T


EXPERIMENTAL
The experimental part will be done in a cooperatively. Each person will be assigned into a group. At the end the measured
data will be shared within each group. Data calculation and lab report, however, must be done individually.

Part I. Determination of max for Cr2O72- ion.
1. Turn on the spectrophotometer so that it can warm up for at least 20 minutes.

2. Select and clean a set of cuvettes to serve as blank and sample holders, respectively. Set the wavelength at 400nm.
"zero" the spectrophotometer with the left knob when nothing is in the closed compartment. "100%" the spectrophotometer
with the right knob when the cuvette is filled to the horizontal white line and it is in the closed compartment. Remember,
whenever the wavelength is changed, the instrument must be "zero" and "100%" as described. It is not necessary to make
these adjustments at fixed wavelength though it won't hurt to check them periodically. See Appendix A for more details.

3. In a 100-mL graduated cylinder add 1-mL of 0.0725M K2Cr2O7 stock solution and dilute to the 100-mL mark with 3M
H2SO4. CAUTION: Potassium dichromate (K2Cr2O7) is a strong oxidizer and a poison and is known to cause cancer.
Wear gloves and eye protection as it can be absorbed through the skin. A lab coat or apron is recommended. Sulfuric acid
is both a poison and quite corrosive. The same personal protection requirements apply. Mix thoroughly and rinse a cuvette
several times with small portions of this solution. Then fill the cuvette with the appropriate amount of diluted dichromate
solution. Measure the %T of this solution from 400 nm to 500 nm at 10 nm increments. Examine your data then take more
data on each side of the minimum of %T at 5 nm increments. Remember, each wavelength change must be accompanied
by re-zeroing and "100%T" of the machine before taking the actual sample data. Do not use this solution in Part II.

4. Convert the %T readings to absorbances and plot A vs . Choose max from the spectrum and set the spectrophotometer
dial to this value for all remaining runs.
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Part II. The Kinetic Data
1. Prepare seven clean cuvettes. Pipets should be used to dispense each of the following reagents.

3.00 M H2SO4
7.50 M isopropyl alcohol in water
deionized water
7.25 × 10-2 M K2Cr2O7

For each run, the indicated volumes of sulfuric acid, isopropyl alcohol, and water should be added into one beaker while the
dichromate in another.

Experimental H2SO4 C3H7OH H2O K2Cr2O7
Run mL mL mL mL
1 25.0 10.0 30.0 10.0
2 25.0 20.0 20.0 10.0
3 35.0 10.0 20.0 10.0
4 35.0 20.0 10.0 10.0
5 50.0 10.0 5.0 10.0
6 50.0 5.0 10.0 10.0

2. When ready, mix the two solutions, that will be time zero. Mix the solutions well, then fill one of the cuvettes with the
mixture and begin recording %T as a function of time. Consider the following: if the run is proceeding very quickly (i.e.,
%T is changing rapidly), read %T and time quickly so sufficient data points are obtained before the dichromate is all
consumed. If the run is slow, however, the time interval should be more widely spaced. As a start, take data every ten
seconds for five minutes.

3. At the end of each run, do not discard the solution, save the sample by simply setting it aside for a later measurement of
A which is the absorbance of the solution after the reaction has gone to completion.

4. Repeat procedure steps 2 and 3 for runs 2-6. You may find that one or more behaved unexpectedly, i.e., reacting much
too fast or not at all. If so, repeat that run.

5. When all six runs are finished, carefully measure %T for all the runs.

6. Place all of your discarded solutions into the properly labeled hazardous waste container in the fume hood.

DATA ANALYSIS
1. Your data table for each run should contain the following columns: time, %T, A, (A - A), ln(A - A), and 1/(A - A)
(relate this to Experiment 0, the tutorial exercise).

2. Make plots to check for zero, first, and second order with respect to the Cr2O72- concentration (see Appendix E). This
means plot A - A vs. time, ln(A - A) vs. time, and 1/(A - A) vs. time. The linearity of the plot should be based on the
R2 value - the Pearson product moment correlation coefficient. Do this for ALL six runs. Question: If run 1 data, (A - A)
vs. time, yields a linear plot, should all the other runs also give a straight line when (A - A) is plotted against time and
why?

3. From the six linear graphs, use the least square method to find the values of kobs. Formulate a table of kobs values in
conjunction with [H+] and [C3H7OH]. Calculate x and z. Using x, z, and kobs, determine the value of the specific rate
constant, k.

4. Finally, write the complete rate law for this reaction. That is one based on the actual k, x, y, and z and another, based on
the rounded x and z.

Have you
1. wipe down all benchtops, including the sink, hood, and balance areas?
2. return all stock chemicals to their proper locations?
3. return all equipment to your drawer or general supply area?
Oxidation of Alcohol
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4. lock your desk drawer?
Oxidation of Alcohol
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1)

Page2>part1>3>line6

“Measure the %T of this solution from 400 to 500nm at 10nm
increments……….at5nm increments�

Why do we measure %T? Why from 400 to 500nm? Why 10nm increments? Why
measure again at 5nm increments?

2)

Page2>part 1>4

Why conver %T to A? why plot A vs wavelength? Why set the
spectrophotometer dial to the “max. wavelength� for all remaining
runs?

3)

Page3>part2�mix the two solutions, that will be time zero. Mix the
solutions well, then fill one of the cuvettes with the mixture and
begine recording%T as a function of time�

Why do I need to measure %T as function of time?

4) data analysis>2

Why making plot of A-Ainfinit vs time, ………

Also, the answer for the question in that paragraph

5) data analysis>3

why plot the graph

6) data analysis>4

How to find the rate law

1a. Use half-reactions to balance the redox equation

1b. What are the oxidizing and reducing agents in this reaction?

2. The dimerization of butadiene was studied at 500 K.

2 C4H6(g) → C8H12(g)

The following data were obtained where

Time (s) [C4H6] (mol/L)

195 1.6 × 10−2

604 1.5 × 10−2

1246 1.3 × 10−2

2180 1.1 × 10−2

6210 0.68 × 10−2



Determine the form of the integrated rate law and the rate constant for
this reactionWrite the rate law for the reaction. (These are actual
experimental data. Use the least square method to obtain the best
straight line.)

3. Why are x, y, and z not equal to the coefficients of [H+], [C3H7OH],
and [Cr2O72−] in the balanced reaction?

4. Explain what is meant by the term "pseudo-order reaction" and how it
is experimentally accomplished?

5. The reaction, 2NO → N2 + O2 , has the following rate law:

After a period of 2000 s, the concentration of NO falls from an initial
value of 2.8 × 10−3 mol/L to 2.0 × 10−3 mol/L. What is the rate
constant, k?





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B

D

H

. The following data were obtained at 25 °C:

[A]0 [B]0 [C]0 Rate

(M) (M) (M) (M/s)

0.1 0.2 0.3 0.063

0.3 0.4 0.2 0.084

0.6 0.4 0.2 0.168

0.3 0.4 0.1 0.021

0.6 0.2 0.2 0.168



What is the correct rate law? Oxidation of Alcohol 3

7. The following questions refer to the gas-phase decomposition of
ethylene chloride

C2H5Cl → products

Experiment shows that the decomposition is first order. The following
data show kinetics information for this reaction:

Time (s) ln [C2H5Cl] (M)

1.0 –1.625

2.0 –1.735

a. What is the rate constant for this decomposition?

b. What was the initial concentration of the ethylene chloride?

c. What would the concentration be after 5.0 seconds?

d. What is the time to half-life?