Lab Report

by My Name

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Inversion of Sucrose

Teaching Assistant: My TA

Chem 375-003

Group #5

2/13/97

Performed: 1/30/97



Lab Partners:

Name #1

Name #2



Abstract

The rate of reaction of glucose to form sucrose and fructose was measured by observation of angle of rotation. The reaction was catalyzed with hydrochloric acid in one case, and chloroactec acid in another. A digital polarimeter was used to determine the angle of rotation as the reaction proceeded.

Introduction & Theory

A chemical enzyme is a chemical that promotes a chemical reaction, but does not itself react to form products. Enzymes are not consumed in a chemical reaction, but are simply used in a step of a larger sequence of reactions. The enzyme reacts with a molecule briefly, then the catalyst allows another molecule in the reaction mixture to react with it, cleaving the enzyme and allowing it to react again with a molecule of reactant.

In the simplest case, the reaction is as follows if the enzyme is denoted E, the reactant S (substrate), the enzyme-reactant complex ES, and the product P with reaction rates kx.

The rate of reaction for eq. (2) is relatively slow, and as the concentration of products, at least initially, is low, the back reaction from product to complex may be ignored. Therefore, the rate of product formation is given by

Assuming the net rate of change in ES is small compared with the rates of creation and destruction of ES, ES may be determined by

Where Km is a convenient way of expressing the constant

(k-1 + k2)/k1. Substitution of this result into eq. (3) yields

Noting that the enzyme is conserved, the following relation is apparent:

Combining this with eqs. (3) and (5) yields

Which defines the rate of reaction in terms of rate constants, initial enzyme concentration, and product concentrations.

The increased rate of reaction in the presence of an enzyme is a result of an enzyme's ability to lower activation energy barriers within the reaction. An enzyme cannot cause a reaction that would otherwise be thermodynamically impossible, but it can speed reactions that are normally so slow that they do not appear to be occurring to reasonable rates. This is a result the fact that enzymes are not permanently changed over the course of a reaction, only used; hence they cannot donate energy to propel a reaction.

Experimental

The procedure in the handout(1) was followed, with the only exception being a failure to collect the last data point for the Sucrose/Chloroacetic acid reaction due to a lack of time.

Results

Table 1-Angles of Rotation for Sucrose and HCl

Number Time (mins) () - () log10[(0-)/(-)]
1 4.75 +7.944 11.724 0.103
2 5.75 +7.440 11.220 0.122
3 6.75 +6.448 10.228 0.162
4 7.75 +5.552 9.332 0.202
5 8.75 +4.743 8.523 0.242
6 9.75 +3.987 7.767 0.282
7 10.75 +3.262 7.042 0.324
8 11.75 +2.614 6.394 0.366
9 12.75 +2.025 5.805 0.408
10 13.75 +1.498 5.278 0.450
11 14.75 +1.016 4.796 0.491
12 16.75 +0.167 3.947 0.576
Number Time (mins) () - () log10[(0-)/(-)]
13 18.75 -0.530 3.250 0.660
14 20.75 -1.100 2.680 0.744
15 22.75 -1.583 2.197 0.830
16 24.75 -1.983 1.797 0.918
17 26.75 -2.310 1.470 1.005
8 28.75 -2.572 1.208 1.090
19 30.75 -2.788 0.992 1.176
20 32.75 -2.970 0.810 1.264
21 34.75 -3.122 0.658 1.354
22 38.75 -3.358 0.422 1.547
23 42.75 -3.507 0.273 1.736
24 46.75 -3.610 0.170 1.942
25 50.75 -3.678 0.102 2.164
26 54.75 -3.724 0.056 2.424
27 64.75 -3.780 0.000 Undefined
28 -3.780 0.000 Undefined



The angle of rotation at time = 0 seconds 0 is obtained by linearly extrapolating vs time for the first 10 readings and is equal to +11.084.



Graph 1 - Sucrose and HCl









log10[(0-)/(-)]















Time (minutes)



Table 2 - Angles of Rotation for Sucrose and Chloroacetic Acid

Number Time (mins) () - () log10[(0-)/(-)]
1 3.00 +3.154 6.934 3.542e-01
2 5.00 +11.800 15.580 2.612e-03
3 7.00 +11.754 15.534 3.897e-03
4 9.00 +11.710 15.490 5.128e-03
5 11.00 +11.670 15.450 6.251e-03
6 13.00 +11.623 15.403 7.575e-03
7 23.00 +11.434 15.214 1.294e-02
8 33.00 +11.258 15.038 1.799e-02
9 43.00 +11.061 14.841 2.372e-02
10 53.00 +10.863 14.643 2.955e-02



The value for reading number one clearly may be disregarded, it is a result of latency in instrument readings, see the discussion for details.





Graph 2 - Sucrose and Chloroacetic Acid











log10[(0-)/(-)]











Time (minutes)

By linear regression, the slope of Graph 1 is 0.0455 with a coefficient of correlation of 0.998. Graph 2 has a slope of 5.52x10-4 and a coefficient of correlation of 0.9998.

Discussion

This experiment shows a strong linear relationship between log10[(0-)/(-)] and time. The small hump in Graph 1 near t=45 mins has no apparent explantion - the unit was checked carefully for air bubbles, the sample had plenty of time to come to thermal equilibrium, and no known external influences affected the reading. It is possible that a temporary change in water pressure caused the apparatus to change temperature briefly and return. This is purely a guess however, and no evidence exists to support this hypothesis. Regardless, the high coefficient of correlation to a linear fit for both solutions suggests that random error was negligible.

The first reading for sucrose and chloroacetic acid is clearly invalid. The digital polarimeter apparently used a mechanical mechanism to determine the angle of rotation as new samples always started the sound of motors from withing the apparatus. In this case, it seems that the data was recorded before the mechanism had been able to orient itself to the correct position. A delay of 30 seconds between placement of the sample in the chamber and the taking of the first reading would have eliminated the error.

Some error may have been introduced by changes in temperature throughout the experiment. The water bath used to equilibrate chemicals was at 23.8C, however the polarimeter was set to 29C. The chemicals were also outside of a constant temperature bath for several minutes while the solutions were being mixed and the cell was being rinsed. These temperature changes would tend to impart some non-linear features to the results, especially during the first few minutes of each run.

The slope of log10[(0-)/(-)] vs time is equal to the velocity constant for each reaction. The good linear fit of the reactions suggests the required constant slope.

References

1. Physical Chemistry Laboratory CHEM 375 Winter 1997, The Rate of Inversion of Sucrose By Acid Catalyzed Hydrolysis.