expt 8-ethanol-english.pdf

7/28/2019 Expt 8-ethanol-english.pdf

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Expt.8-1

Experiment 8

Solution Kinetics: Oxidation of Ethanol by Dichromate

Introduction

The oxidation of ethanol (E) by dichromate (D) is represented by the reaction:

3 CH3CH2OH Cr2O72-

8 H+

3 CH3CHO 2 Cr3+

7 H2O+ + + +

E D P

The product ethanal (P) can be further oxidized to ethanoic acid. We will run this

experiment with a large excess of ethanol so that dichromate will preferentially oxidize

the ethanol and not the product ethanal.

The rate law can be written as:

]][[][][

DEkdt

Dd

dt

Pd== ------ (1)

The reaction will be first order in E, first order inD, and second order overall. We will

use a large excess of ethanol. Consequently, the ethanol concentration will remain

almost constant throughout and the reaction will be pseudo 1

st

-order.

]['][

Dkdt

Dd= ------ (2) where k=k[E]

On integration this becomes:

tkD

D o '][

][ln = ------(3)

The dichromate ion has a yellow-orange color and is readily monitored by its

absorption in the blue region of the four-channel photometer. To study the reaction

kinetics we monitor the concentration of dichromate in an excess of ethanol, by

measuring the absorbance A. A is directly proportional to the concentration of

dichromate. At time zero the absorbance is Ao while the initial concentration of

dichromate is [D]o. The concentration of dichromate at time tis [D] and the absorbance

isAt. At long times, t, some absorption will remain and A0 and this background

absorption must be corrected for all our measurements.


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Expt.8-2

The equation becomes:

tkAA

AA

t

o ')ln( =

or tkAAAA to ')ln()ln( = ------(4)

Plot )ln(

AAt versus twill be a straight line whose slope equals -k.

The temperature dependence of the rate constant is described by the Arrhenius equation:

RT

Ea

aek

= ------(5)

where a is the preexponential factor andEa is the activation energy.

In our case we measure a pseudo first order rate constant k, that can be used to

determine the second order rate constant k.

where k = kx (concentration of ethanol)

ethanolofionconcentrat

kk

'=

In logarithmic form:

RT

Eak a= lnln

If rate constants are available at several temperatures, a plot of ln kversusT

1will

have a slopeR

Ea and an intercept ln a. T should be in absolute temperature.

Procedure

Preparation of reagents

1. Prepare 250 mL of a stock solution of 4 Msulfuric acid.

2. Prepare 100 mL of 0.02 M potassiumdichromate solution.

3. Obtain 10 mL of absolute ethanol.4. Pipet 5 mL of dichromate solution and 50 mL

of the stock sulfuric acid in a conical flask.

Shake the solution well.


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Expt.8-3

Running the experiment at room temperature

1. Measure the blank potentialEo by using the 4 M sulfuric acid.2. Place the acidic dichromate solution in a water bath at room temperature for a few

minutes (record the temperature). (Fig. 1)

3. Add 0.15 mL pure ethanol into the acidified dichromate solution and start the timerimmediately.

4. Stir the solution.5. When the time is getting close to 1-minute interval, use a dropper to transfer a few

mL of solution to the measuring cell.

6. Measure the potential by the 4-channel photometer. (Fig. 2)7. Repeat the measurements at 2, 3, 4 and 5 minute intervals.8. Allow the reaction solution to stand undisturbed for 30 minutes. (Fig. 3)9. Determine the absorbance corresponding to the infinite time.

Fig. 1 Fig. 2 Fig. 3

Repeat the experiment at different temperatures

1. Prepare an ice-water bath and repeat the experiment. (Fig. 4)2. Use a heater to maintain the temperature of water bath at about 30C. Repeat the

experiment. (Fig. 5)

Fig. 4 Fig. 5


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Expt.8-4

Measuring the absorbance

1. Turn on the photometer and select the blue LED at least 5 minutes beforemeasurement.

2. Measure the blank TP2 potentialEo of the blank solution (4 M sulfuric acid).3. Measure the TP2 potentialEof the reaction solution.4. The absorbance can be calculated:

E

EA olog=

Results

Blank potentialEo =

Time

(min)

Room temp. ( C) Low temp. ( C) High temp. ( C)

E(V) A E(V) A E(V) A

1

2

3

4

5

Data Treatment

Assume the addition of ethanol does not change the volume of acidified dichromate

solution.

1. Calculate the initial concentrations of dichromate ion and ethanol in the solution.Can we assume the concentration of the ethanol constant throughout the reaction?

2. Plot )ln(

AAt versus t for each run and determine the pseudo first order rate

constant k.3. Calculate the second order rate constant kfor each run.4. Plot ln k versus

T

1and determine the activation energy and the preexponential

factor.

Reference

1. P. W. Atkins, Physical Chemistry, 7th Ed., Oxford University Press, 2002.

expt 8-ethanol-english.pdf

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