Kinetics: Catalyzed Decomposition of Hydrogen Peroxide

Computer-Interfaced Experiments - Volume Measurement

Kinetics
Catalyzed Decomposition of Hydrogen Peroxide
- First Order Reaction

Objectives: Determination of Rate constants and Activation Parameters

Peter Keusch

Datalogging using the Program "Measuring and Evaluating"
and the Analog-Digital-Converter CASSY-E - LEYBOLD DIDACTIC

German version

Chemicals:
potassium chromate
potassium dichromate
hydrogen peroxide 30 %

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)
100 mL round bottom flask with center neck NS 29/32 and 2 angled side necks 14/23
pressure equalizing dropping funnel, 14
adapter connecting, 29
contact thermometer
thermometer 0 - 50 °C (resolution: 0.1 °C)
gas syringe 100 mL
volumetric pipette 3 mL
volumetric pipette 10 mL
volumetric pipette 30 mL
3 pipette bulbs
motion transducer
silicone tubing
silicone cord
weight
path cords
disposal container

Hazards and safety precautions:

Potassium dichromate: Hexavalent chromium compounds are generally more toxic than trivalent chromium compounds. May be fatal if absorbed through the skin, if swallowed or inhaled. Contains chromium (VI), a known cancer hazard. Allergen. Skin eye and respiratory irritant. May act as a sensitizer.
Potassium chromate: Poison - may be fatal if swallowed. Very harmful if inhaled or absorbed through the skin. Chromium (VI) is a known cancer hazard.

Hydrogenperoxide 30 % is toxic, corrosive - can cause serious burns. Eye contact can cause serious injury, possibly blindness. Harmful by inhalation, ingestion and skin contact.

Safety goggles and gloves must be worn when handling dichromate, chromate and hydrogen peroxide. The preparation of a solution of potassium dichromate and potassium chromate in water is carried out in a fume hood!

Measurement of the volume using a transducer

Whenever a gas is produced in a chemical reaction, the best way of determining the reaction rate is to measure the volume of gas evolved at different times. Gas syringes are commonly used to measure gas volumes. The plunger of the syringe moves outward as gas fills it. The reaction is complete when syringe no longer moves. The plunger is moistened with graphite lubricant. In order to compensate the frictional resistance of the plunger, a weight acting contra the frictional force is fastened on its end. The motion of the plunger is transfered by means of a silicone cord to the potentiometer shaft. In this manner, the linear movement is translated into a rotary movement and converted into a resistance change. The change in resistance is converted to voltage. The measured output voltage is proportional to the distance moved by the plunger (Fig. 1).

Fig. 1: Equipment for volume measurement

Fig. 2: Wiring diagram of the transducer

Assembling of the transducer

Parts list:
plastic box 10 · 4.9 · 5 cm
ten-turn helipot (P1), 10 kW +/- 5 %, lin +/- 0.25 %
pontentiometer (P2), 22 kW lin, 0.25 Watt
control knob, suitable for P2
pre-set potentiometer (P3), 150 kW lin, 0.25 Watt
resistor (R1), 250 kW, 0.25 Watt
resistor (R2), 1 kW, 0.25 Watt
red light emitting diode D
miniature switch, ON/Out
2 telephone sockets black, 2 telephone sockets red
1 metal bar (l = 13 cm)
a short silicone tubing suitable as covering for the helipot shaft (P1)
30 cm jumper wire

Literature: Brand, B. H.: Eine Möglichkeit der weithin sichtbaren Demonstration des Gasinhaltes eines Kolbenprobers in Praxis der Naturwisssenschaften (Chemie) 10/79 (1979).

Fig. 3: Holes in the plastic housing
Fig 4: Motion transducer

The plastic box is provided with holes of different diameters (Fig. 3) and the components are bolted with the box. Finally, the wires and resistors are soldered according to the wiring diagram (Fig. 2). The metal bar with two drill holes is fastened at the bottom of the plastic box with screws.

Experimental procedure:

Fig. 5: Experiment set-up

The motion transducer connected to the Input B of CASSY INTERFACE is supplied with 5 volts direct current.

Before the voltage supply is switched on, it should be checked whether the value of ±30 V is not exceeded, since otherwise the INTERFACE could be damaged. Starting with 0 V one increases the voltage slowly to the value of 0.5 V. After a constant voltage of 5.00 V is reached, the voltage supply is attached to the motion transducer.

Calibration of the transducer:

In the program 'Measuring and Evaluating' the subprogram 'Multimeter' is activated. The plunger of the the syringe is set to 0 mL . By pressing the function key < F1> the sensing software is started. The displayed voltage value is listed and the measurement is terminated.
By means of the registered voltage value the transducer can be calibrated. By switching to <F3> 'Select measur. quantities'®'Reselect channel B'®'Calibrate DC'
a subprogramm 'B calibrate' opens, in which one enters the quantity, the physical unit, the factor and offset. The factor is set to 1. Under the menu item Offset the listed voltage value is entered. The input is to be confirmed (Fig. 6).

Fig. 6: Menu screen calibration
Matching of the program:
In order to create a plot of V versus t, the voltage values are to be converted into volume values. Again the plunger the syringe is set to 0 mL and the appropriate voltage value (e.g. 0.003 V) is listed. Now the plunger is pushed to the 60 mL mark on the syringe barrel. The resulting voltage value (e.g. 0.146 V) is listed. After the program is switched to <F4> 'Automatic/Param./Select formula'®'Enter formula' one enters the physical symbol, the physical unit, the number of decimal places and a formula. The formula converts the voltage levels into volume values (Fig. 7). Considering the aforementioned values (Fig. 6) yields:

V = (U - 0.003) · 60 / 0.146

Fig. 7: Conversion of the voltage values into volume values

A three-necked round bottom flask is fitted with an internal thermometer, a dropping funnel and an adapter connecting reducing. The adapter is connected via a silicone tubing to the gas syringe (Fig. 5). 64 g (0.33 mol) of potassium chromate are dissolved with 16 g (0.054 mol) of potassium dichromate in a liter of distilled water. Per experiment 30 mL of the orange solution are pipetted into the flask placed in a water bath. The solution is allowed to equilibrate in the constant-temperature bath. A reaction temperature below the room temperature is obtained and maintained by careful addition of ice or cold water to the water bath. After thermostating for about 15 minutes, the solution will come to the temperature of the waterbath.

The dropping funnel is filled with 4 mL 3% hydrogen peroxide solution. A stopper is placed on the dropping funnel. The stirrer is started. The stopcock of dropping funnel is opened and hydrogen peroxide solution is allowed to drain from the funnel. When the addition of hydrogen peroxide is complete, the stopcock of the dropping funnel is closed and the stopcock of the syringe is opened. Immediately the sensing software is started and the reaction temperature is read to the nearest 0.1 °C.

In a couple of seconds gassing begins and the syringe plunger is moved forward. During the reaction the solution is colored deep-red (formation of potassium peroxochromates). Finally the deep-red color disappears and the typical orange of the potassium chromate solution is formed again.

Three experiments are carried out at different reaction temperatures in the range from 7 °C to 21 °C. A reaction temperature below the room temperature is obtained and maintained by careful addition of ice or cold water to the water bath.

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

After generating a plot of V versus t (Fig. 8), the measured values are converted by entering a formula (Tab. 1). Thus, a plot of
ln(0.115 · (1- (V_t / V_¥))

versus t can be generated. The final volume V_¥ is the volume when reaction is finished.
Thus, according to equation (7) Kinetic equations (Download PDF file) the rate constant can be determined (Fig. 9). The factor 0.115 considers the concentration of hydrogen peroxide in the reaction solution.

spread sheet

Tab. 1: Excel data sheet measuring value V(t) conversion according to y = ln(0.115 · (1 - (V_t / V_¥))

volumes

Fig. 8: Temperature effect 7 °C (1) 14°C (2) 21°C (3)

4 mL 30 % hydrogen peroxide solution contain 3.92 mmol hydrogenperoxide. 0.5 mol oxygen is formed from 1 mol hydrogen peroxide. With a pressure of
1020 hPa and a temperature of 20°C (293 K) the oxygen volume calculates to:

Since the reaction runs exothermically and accordingly the reaction solution is somewhat warmed up, the values for the measured final volumes V_¥ are always higher, than the theoretical values (Tab. 2).

Measurement	T [ °C ]	V _¥ [ mL ] calculated	V _¥ [ mL ] measured
1	7	45.0	47.2
2	14	46.1	49.2
3	21	47.3	52.1

Tab. 2: Calculated and measured oxygen volumes

rate constante

Fig. 9: First order kinetics plot determination of the rate constants k
y = ln(0.115 · (1 - (V_t / V_¥))

	7 [°C]	14 [°C]	21 [°C]
k [s^-1]	0.007	0.012	0.0205

Tab. 3: 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 (Table 3) will be calculated and the ARRHENIUS and EYRING plot (Fig. 10) will be generated.

Tab 4: Calculation of the activation parameters

Arrhenius and Eyring

Fig. 10: ARRHENIUS (1) and EYRING plot (2)

References:
Computer-Interfaced Experiments Kinetics: Decomposition of Hydrogen Peroxide catalyzed by Potassium Iodide
Computer-Interfaced Experiments Kinetics: Decomposition of Hydrogen Peroxide catalyzed by Dichromate
Computer-Interfaced Experiments Enzyme Kinetics: Enzymatic Decomposition of Hydrogen Peroxide
Demonstration Experiment on Video: Decomposition of Hydrogen Peroxide with Catalase
Kyle Smith und Mark Iannone Improved Data Analysis for an Adiabatic Kinetics Experiment

Index of CASSY Experiments