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Computer-Interfaced Experiments - Conductivity Measurement Kinetics Alkaline Hydrolysis of Ethyl Acetate - Second Order Reaction - Objectives: Determination of Rate Constants and Activation Parameters Peter Keusch |
Hazards and safety precautions:
Safety glasses and protective gloves required. Theoretical background The reaction rate constant for the alkaline hydrolysis of esters such as ethyl acetate may be determined conductometrically, since the stoichiometry of a typical reaction is  As the reaction proceeds, hydroxide ions are consumed and acetate ions are produced. The conductance of an ion depends on its ionic mobility, which in turn is determined by the size of the ion. Since the conductance of the large acetate ion is less then that of the smaller, more mobile hydroxide ion, the conductivity of the reaction solution decreases as the alkaline ester hydrolysis proceeds. Hence, the progress the reaction may be monitored by following the change in the electrical conductance of the reaction mixture with time. The rate constants will be determined at three temperatures, so that the activation energy for the reaction may be obtained from a plot of lnk versus 1/T. Kinetic equations (Download PDF file) Constructing calibration straight line: The measurements recorded by means of the program Chemex are analyzed using the spread sheet program Microsoft Excel. In order to overlay the individual conductivity curves (different reaction temperatures) correctly, the recorded values need to be corrected by a factor taking into account the temperature dependence of the electrical conductivity (Fig. 5). On the basis of the conductance of 0.005 M H2SO4 the following diagram is yielded for a temperature range of 19 °C to 50 °C. |
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| Value at 25°C: | 5.25 = 1 |
| 40°C | 6.6 = x |
| ---------- | |
| CF = 6.6 ÷ 5.25 = 1.26 |
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In addition to a conductivity measuring cell (1) a temperature sensor is connected to the CHEMBOX via input
Sensor2 (Fig. 2).
A three-necked round bottom flask is fitted with an internal thermometer, a conductivity meassuring cell and a stopper. 100 mL of 0.02 M sodium hydroxide solution (0.002 mol) are pipetted into the flask placed in a water bath. The platinized electrode surfaces of the conductivity measuring cell and the tip of the temperature probe need to be totally submerged in the reaction solution. The position of the temperature probe and the conductivity sensor is adjusted so that they are not struck by the stirring bar (Fig. 2). 100 mL of 0.02 M sodium hydroxide solution (0.002 mol) are pipetted into the flask placed in a water bath. The hydroxide solution is warmed up in the water bath to the desired temperature (40 - 60 °C) using a hotplate stirrer and a contact thermometer. When thermal equilibrium has been reached, using a micropipette 0.39 mL of ethyl acetate (0.004 mol) are added to the sodium hydroxide solution while vigorously stirring. Immediately the sensing software is started. The change in the conductivity and the constancy of the temperature are displayed simultaneously on the measuring screen (Fig. 3). The reaction is studied at three temperatures to determine the activation parameters. |
| Measurement | T [ °C ] | Correction factor (CF) | k¥ [ mS ]
(corrected) 1
| 50.1
| 1.43
| 2.2214
| 2
| 45.5
| 1.35
| 2.1976
| 3
| 39.6
| 1.26
| 2.2042 | |
| 39.6 [ °C ] | 45.5 [ °C ] | 50.1 [ °C ]
| k [ L · mol -1 · s -1 ]
| 0.3487
| 0.4697
| 0.5918 | |
