CHEMBOX Experiment: Iodination of Hydroquinone

Computer-aided Experiments - Measurement: Absorbance

Kinetics
Iodination of Hydroquinone - a Reaction pseudo-first Order
Determination of rate constants and activation parameters

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

German version

Chemicals:

0.4 M hydroquinone solution

0.05 M iodine solution

Apparatus and glass wares:

magnetic stirrer hotplate

magnetic stirring bar

stirring bar remover
crystallizing dish d = 190 mm, h = 90 mm (for water bath)

contact thermometer

thermometer 0 - 50°C (resolution: 0.1°C)

2 beakers 50 mL

2 volumetric pipettes 20 mL

2 volumetric pipettes 2 mL
4 pipette bulbs
photometer fitted with a recorder output: Spectronic 20 Bausch & Lomb
test tube cuvettes (Spectronic)

disposal container

Theoretical background:

Fig. 1: Iodination of hydroquinone

On the given conditions the reaction is pseudo-first order (see procedure).

Kinetic equations (Download)

Calibration of the photometer and matching of the program Chemex is carried out analog to the procedure described in experiment Bromination of reactive Aromatics (Download).

Spectronic 20 (Download) features an analog output on the bottom of the instrument. The analog output of the photometer is connected to the input Sensor1 of the CHEMBOX.

The photometer has been designed so that when it displays 100 % transmittance, the analog signal at its output connector is 1 VDC; when the instrument displays 0 % transmittance, the output voltage is 0 VDC.

The absorbance wavelength is set to 490 nm.

The calibration of the photometer is performed using a decolorized mixture of 2 ml of 0.05 M iodine solution and 2 ml of 0.4 M hydroquinone solution.

Procedure:

Fig. 2: Experiment set-up 20 mL of a 0.4 molar aqueous solution of hydroquinone is pipetted into a beaker. 20 mL of 0.05 M iodine solution are transferred into a further beaker, which is covered with a parafilm foil. The two beakers are placed in a water bath, in which a contact thermometer and a thermometer with a resolution of 0.1 °C are immersed (Fig. 2). A reaction temperature below the room temperature is obtained and maintained by careful addition of ice or cold water to the water bath.

During the thermal equilibration period the calibration of the photometer and the matching of the programm are carried out.

Using the wavelength control knob the wavelength is set to 490 nm. After the photometer has been zeroed, a cuvette is filled with 2 mL of iodine solution mixed with 2 mL of hydroquinone solution. The cuvette, covered mith a bit of Parafilm, is inverted 2-3 times and placed into the sample compartment. When the transmittance maximum has been reached, the instrument is adjusted so that the meter needle reads 100% transmittance (zero absorbance).

After thermal equilibrium has been reached the reaction temperature is read to nearest 0.1 °C. The hydroquionone solution is poured smoothly and rapidly to the iodine solution. The solutions are thoroughly mixed together by shaking the beaker. A cuvette filled about 2/3 with the reaction solution is covered with a Parafilm foil and placed into the sample compartment of the photometer. Simultaneously the sensing software is started. The measuring interval is 4 seconds.

The change in transmittance is displayed on the measuring screen (Fig. 3).

The in-situ determination of the reaction rate on the basis of a continuous logging of photometrical data is allowed in rapid reactions (small change in temperature during te reaction).

Fig. 3: Evaluation screen (1: 16°C 2: 21°C 3: 25°C)

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

Excel functions are used to convert the transmittances to absorbances and -lnA values(Tab. 1). A plot of A versus t (Fig. 4) and a plot of -lnA versus t are generated (Fig. 5).

Excel

Tab. 1: Measured values T(t), calculation of A and of -lnA

Fig. 4: Absorbance (1: 16°C 2: 21°C 3: 25°C)

Fig. 5: Determination of the pseudo-first order rate constant k' (1: 16°C 2: 21°C 3: 25°C)

Measurement	T [°C]	k' [s^-1]	k [L · mol^-1 · s^-1]
1	16	0.01499	0.07495
2	21	0.0254	0.132
3	25	0.03894	0.1947

Tab. 2: Calculation of the true rate constants k
(According to the reaction conditions: k = k' / 0.2)

If the reaction temperatures and the corresponding rate constants are entered into the table of them Excel file Activation parameter (Download), then all activation parameters (Tab. 3) will be calculated and the ARRHENIUS and the EYRING plot will be generated (Fig. 6)

Tab. 3: Calculation of the activation parameters

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