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Computer-Interfaced Experiments - Conductivity Measurement

Kinetics
Hydrolysis of Benzoyl Chloride - First Order Reaction

Objectives: Determination of Rate Constants and Activation Parameters

Peter Keusch



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


German version




Chemicals:
benzoyl choride  (m.w. = 140.57 g / mol, d = 1.215 g / mL)
ethanol 99 %

Apparatus and glass wares:
magnetic stirrer hotplate
2 magnetic stirring bars
stirring bar remover
crystallizing dish d = 190 mm, h = 90 mm  (for water bath)
6 beakers 250 mL
contact thermometer
temperature probe
conductivity measuring cell
micropipette
2 pipettes 10 mL
volumetric pipette 50 mL
3 pipette bulbs


Hazards and safety precautions:

Benzoyl chloride is a poison. May be fatal if inhaled. Possible carcinogen. Harmful if swallowed. Corrosive - causes severe burns. Vapour is very irritating. Experimental tumorigen by skin contact. Skin, eye and respiratory irritant.
Ethanol is highly flammable.

Safety goggles and gloves must be worn. The experiment should be performed in a fume hood!


Theoretical background:

Benzoyl chloride is hydrolyzed by an aqueous solution of ethanol.


equation


Hence the hydrolysis of benzoyl chloride is monitored by following the change in conductance of the reaction mixture with time. By determining the conductivity as a function of time, the rate constant can be found. If the reaction is carried out at different temperatures, the activation energy can also be found.


Experiment 1: Test for a first order reaction

Experimental procedure:






experimental set-up
Fig. 1: Experiment set-up		 In addition to a conductivity measuring cell  (1)  a temperature probe  (2)  is connected to the CHEMBOX via input Sensor2  (Fig. 1).



100 mL of 99 % ethanol are pipetted into a beaker placed in a water bath. Using a hotplate stirrer and a contact thermometer the water of the water bath is warmed to the desired temperature of approx. 50°C. Ethanol is allowed to equilibrate in the constant-temperature water bath.

After thermal equilibrium has been established, 92.6 mL of benzoyl chloride (0.8 mmol) are pipetted into the ethanol solution while vigorously stirring. Immediately the measuring process is started.

The measuring interval is 1 second.

The change in conductivity and the constancy of the temperature are displayed simultaneously on the measuring screen (Fig. 2)

In addition, the experiment is conducted using 68.5 mL of benzoyl chloride (0.6 mmol) and 46.3 mL of benzoyl chloride (0.4 mmol). All the experiments are carried out under the same temperature conditions.



screen
Fig. 2: Multigraph screen: Hydrolysis of benzoyl chloride    concentration effect    T = 50.4 °C
1:  0.4 mmol     2:  0.6 mmol     3:  0.8 mmol)     in 99 % ethanol



Data analysis:

Determination of the half-life t1/2 and calculation of the rate constant

When dealing with first-order reactions, the use of a half-life rather than a rate constant is often convenient. The half-life t½ of a reacting substance is simply the time required for half of the amount originally present to react. t1/2 is displayed on the measuring screen by appropriate positioning of the cursor (Fig. 2). The rate constant k is computed according to equation (8)  Kinetic equations (Download PDF file).


0.4 mmol 0.6 mmol 0.8 mmol
k¥ [ mS ] 394 554 720
k¥ / 2 [ mS ] 197 277 360
t½ [ s ] 64.4 59.6 61.8
k [ s -1 ] 0.0108 0.0116 0.0112
Tab. 1: Half-life t1/2 and rate constant k

Data analysis using Excel (Download) - determination of the rate constants


According to  equation (6)  Kinetic equations (Download PDF file)   the conductivity values are converted(Tab. 2). In so doing, a plot of
-ln(x · (1 - k / k ¥))

x  =  0.004  (0.006,   0.008
against t is allowed  (Fig. 3).


measured values
Tab. 2: Measured values k(t)     conversion according to y = -ln(x · (1 - k / k¥))
x  =  0.004  (0.006,   0.008)


rate constant
Fig. 3: First order kinetics plot - determination of rate constant k
1:  y = -ln(0.004 · (1 - k / k¥))     2: y = -ln(0.006 · (1 - k / k¥))     3:  y = -ln(0.008 · (1 - k / k¥))


0.4 mmol 0.6 mmol 0.8 mmol
k [ s -1 ] 0.0110 0.0111 0.0113
Tab. 3: Rate constant k

Discussion:

The reaction is first order. The half-life does not depend on the initial concentration of benzoyl chloride.



Experiment 2: Determination of activation parameters

Experimental procedure:

Benzoyl choride (0.8 mmol) is converted with 100 mL of 99 % ethanol at three different reaction temperatures.

Data analysis:

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


conductivity curves
Fig. 4: Conductivity curves
1:  45.7 °C     2:  51.2 °C     3: 54.4 °C)


rate constants k
Fig. 5: First order kinetics plot - determination of the rate constants k
y = -ln(0.008 · (1 - k / k¥))


45.7 [ °C ] 51.2 [ °C ] 54.4 [ °C ]>
k [ s -1 ] 0.0079 0.0122 0.0156
Tab. 4: Rate constants k

If the reaction temperatures and the corresponding rate constants are entered into the table of the Excel file  Activation parameters (Download), all activation parameters (Tab. 5) will be calculated and the plots according to the ARRHENIUSand EYRING equation (Fig. 5) will be generated.


activation parameters
Tab. 5: Calculation of the activation parameters


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



Note that SN1 reactions in which the nucleophile is also the solvent are commonly called solvolysis reactions. Solvent as the nucleophile makes kinetic order indeterminate (pseudo-first-order because [solvent] is ~ constant).


Reference:
Hydrolysis of Benzoyl Clkoride - First-Order Reaction

Index of Chembox Experiments




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