Kinetics: Alkaline Hydrolysis of Ethyl Acetate

Computer-Interfaced Experiments - Conductivity Measurement
Kinetics
Alkaline Hydrolysis of Ethyl Acetate
- Second Order Reaction -

Objectives: Determination of Rate Constants and Activation Parameters

Peter Keusch

Datalogging using the Program CHEMEX and the Analog-Digital-Converter CHEMBOX
IBK electronic + informatic

German version

Chemicals:
ethyl acetate (m.w. = 88.1 g / mol, d = 0.9 g / mL)
0.02 M sodium hydroxide solution
0.005 M sulfuric acid

Apparatus and glasswares:
magnetic stirrer hotplate
2 magnetic stirring bars
stirring bar remover
crystallizing dish 190 × 90 (for water bath)
100 mL flask 3 necks (NS 14 / NS 29)
contact thermometer
conductivity measuring cell
temperature sensor
micropipette
volumetric pipette 50 mL
pipette bulb

Hazards and safety precautions:

Ethyl acetate is highly flammable. Harmful if swallowed in quantity. Vapours may cause drowsiness.

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, hydroxyl ions are consumed and acetate ions are produced. Hydroxyl ion has a very much larger specific conductance than acetate ion. Hence, the alkaline hydrolysis of ethyl acetate may be monitored by following the change in the conductance of the reaction mixture with time. By determination of the conductivity as a function of time, the rate constant is found. If the reaction is carried out at different temperatures, the activation energy can also be found.

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.

Fig. 1: Calibration straight line

The reference point is the temperature, at which the conductivity measuring cell was calibrated (25 °C). The conductance value of 5.25 mS/cm corresponds to the factor "1". For any values of the available temperature range the appropriate correction factors can be determined from the above linear correlation, e.g.:

Value at 25°C:	5.25 = 1
40°C	6.6 = x
	----------
	CF = 6.6 ÷ 5.25 = 1.26

Experimental procedure:

Fig. 2: Experiment set-up

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.

Using a hotplate stirrer and a contact thermometer the hydroxide solution is warmed up in the water bath to the desired temperature (40 - 60 °C).

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.

Fig. 3: Real-time plot

Data analysis using Excel - determination of the rate constants and the activation parameters:

As aforementioned the k-values are converted by dividing them by the appropriate correction factor (Tab. 2).

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

Tab. 1: Correction of the measured values

With longer reaction time - as in the case under consideration - the time values are logged in "ks" in the program of Chemex. These values need to be multiplied by 1000 in order to create graphs with the time in seconds along the ordinate.

Tab. 2: Data sheet - k(t), conversion according to y = ln (0.5 · (k_o- k_¥) / (k- k_¥) + 0.5) · 50

After creating a plot of k against t (Fig. 4) according to equation (15) Kinetic equations (Download PDF file), the conductivity values are converted. In doing so, a plot of
ln(0.5 · ( k_o- k_¥) / (k- k_¥) + 0.5) · 50

against t is enabled (Fig. 6). The multiplication factor 50 considers x_o in equation (15) Kinetic equations (Download PDF file), i.e. the molarity of the used sodium hydoxide solution.

Fig. 4: Conductivity curves 1: 39.6 °C 2: 45.5 °C 3: 50.1 °C

The measuring interval is 1 sec. For the reason of clearness, every fourth data point is shown in the diagrams of Fig. 4 and Fig. 5. A dialog module programmed in Visual basic allows the hiding of excess data pairs in the data sheet.

Fig. 5: Second order kinetics plot - determination of the rate constants k
y = ln (0.5 · (k_o- k_¥) / (k - k_¥) + 0.5) · 50

	39.6 [ °C ]	45.5 [ °C ]	50.1 [ °C ]
k [ L · mol^-1 · s^-1 ]	0.3487	0.4697	0.5918

Tab. 3: Rate constants

If the reaction temperatures and the corresponding rate constants are entered into the table of the Excel file Activation parameters (Download), then all activation parameters (Tab. 4) will be calculated and the ARRHENIUS and the EYRING plot will be generated (Fig. 6).

Tab. 4: Calculation of the activation parameters

Arrhenius und Eyring

Fig. 6: ARRHENIUS (1) and EYRING plot (2)

References:
Georg Schmeer Alkaline Hydrolysis of Trifluoro acetic acid ethyl ester with LiOH (+ 1 water) - Animation Computer-Interfaced Experiments Kinetics: Alkaline Hydrolysis of Esters - Second Order Reaction
Computer-Interfaced Experiments Kinetics: Hydrolysis of Methyl Formate using an acidic Ion Exchanger - First Order Reactio
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