Experiment 20

Experiment 20: Kinetics of Acid- Catalyzed Hydrolysis of Sucrose Andrew Fleming, Amanda Petty, Jordan Haskins, Gino Moore

Introduction In this experiment we are trying to measure the change in the optical rotation of light during the course of the acid-catalyzed hydrolysis of sucrose. From the data, the order of the reaction with respect to hydronium ion concentration can be calculated [1].

Materials and Methods For this experiment we used: six 100 mL volumetric flasks for the six different solutions one 500 mL volumetric flask for mixing the sucrose solution one 250 mL volumetric flask for mixing the 4.0MHCl solutions a HAAKE A81 and Model 800 Fisher Isotemp Constant Temperature Circulator in order to drive the reaction to completion a Model 51 Polarimeter (Rudolph Instruments, Inc.) was used to measure optical rotation in each sample a Metler Toledo AB204 weighing scale to weigh the desired amount of sucrose

Materials and Methods A stock solution of sucrose was made to have a constant concentration of 20.0 g/100 mL The sucrose solution was made using 100.0560 g placed in a 500 mL volumetric flask diluted to the line with D.I. water. excess was made for potential retrial

Table of Dilutions Trial #

Sucrose solution (mL)

HCl (M)

HCl (mL)

H2O (mL)

1

25.0

2.0

25.0

0.0

2

25.0

1.6

20.0

5.0

3

25.0

1.2

15.0

10.0

4

25.0

0.8

10.0

15.0

5

25.0

0.4

5.0

20.0

6

25.0

0.0

0.0

25.0

Materials and Methods The solutions were mixed one at a time just prior to their trial on the polarimeter. The glass, polarimeter tube was cleaned prior to each trial. Each of the six solutions were used to fill the glass tube. Air bubbles present in the tube were guided to a raised nodule in the glass to prevent obstruction of light. Data was recorded by measuring the angle of rotation of the polarized light passing through the solutions.

Results Data was entered in Microsoft Excel for analysis. Ln((α0-α∞)/(αt-α∞)) the values for this equation for each trial were graphed versus the time in seconds the slope of the best fit line yielded our rate constants

Results

Results

Results

Results

Results Concentration

Slope (rate constant) ± Standard Deviation

2.0 MHCl

8.97 x 10-4 ± 0.16 x 10-4

1.6 MHCl

4.95 x 10-4 ± 0.08 x 10-4

1.2 MHCl

3.45 x 10-4 ± 0.12 x 10-4

0.8 MHCl

2.68 x 10-4 ± 0.07 x 10-4

0.4 MHCl

6.61 x 10-4 ± 0.17 x 10-4

Results Concentrati on of HCl (M) 2.0

log of [HCl]

log of slope [Ln((a0-af)/(at-af)) vs. time]

0.3010

-3.0473

1.6

0.2041

-3.3053

1.2

0.0792

-3.4617

0.8

-0.0969

-3.5720

0.4

-0.3979

-3.1801

0.0

Undefined

---------

Discussion The slope of our “log (k) vs. log [H+]” plot gives us the order with respect to the hydronium ion. The order with respect to the hydronium ion is 0.1379 ± 0.4334 when the data for the 0.4 MHCl is included in the plot. However, the correlation coefficient for this plot was very low. When the data for 0.4 MHCl is excluded from the plot, the order with respect to the hydronium ion is found to be 1.2564 ± 0.2966.

Conclusion The HCl acted as a catalyst for the dissociation of sucrose into fructose and glucose. Since the two products are also chiral molecules, they have different optical rotations than the starting product. Observing a change in the optical rotation of each sample gives evidence of the reaction taking place.

Conclusion Our trial with the 0.0 MHCl solution (uncatalyzed) yeilded no appreciable change in the sucrose concentration. We know that water breaks down sucrose by hydrolysis, however the process is so gradual that there was no change in the concentration of sucrose detected in our experiment for the 0.0 MHCl solution. Thus, we can see that HCl is an appreciable catalyst.

Citations Clark, Roy W.; Howard, James C. Physical Chemistry Lab Manual, 4th ed., RonJon Publishing, Denton, TX, 1996. Fleming, A.J.; Physical Chemistry Laboratory Notebook, 2007 p. 10-11. Fleming, A.J.; Physical Chemistry Laboratory CD, 2007. Haskins, J.R.; Physical Chemistry Laboratory Notebook, 2007 p. 1-3. Haskins, J.R.; Physical Chemistry Laboratory CD, 2007. Moore, L.G.; Physical Chemistry Laboratory Notebook, 2007 p. 1-3. Moore, L.G.; Physical Chemistry Laboratory CD, 2007. Petty, A..; Physical Chemistry Laboratory Notebook, 2007 p. 4-5. Petty, A..; Physical Chemistry Laboratory CD, 2007.