Kinetics: Decomposition of Hydrogen Peroxide by Dichromate

Computer-Interfaced Experiments - Temperature Measurement
Kinetics
Decomposition of Hydrogen Peroxide catalyzed by Dichromate

Dependance of the Reaction Rate upon the Concentration of the Catalyst

Peter Keusch

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

German version

Chemicals:
0.4 M aqueous potassium dichromate solution
hydrogen peroxide 10 %

0.1, 0.15, 0.2 and 0.3 molar dichromate solutions are prepared by diluting appropriate aliquots of the 0.4 molar stock potassium dichromate solution.

Apparatus and glass wares:

magnetic stirrer
magnetic stirring bar
stirring bar remover
Dewar vessel, 500 mL, with plastic cover, suitable for magnetic stirrer
temperature probe
volumetric pipette 8 mL
volumetric pipette 25 mL
volumetric pipette 50 mL
3 pipette bulbs
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.

Hydrogenperoxide 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. Good ventilation required.

Theoretical background:

Hydrogen peroxide reacts vigorously with dichromate. Energy in the form of heat is released as the reaction proceeds. The reaction mixture warms up. Consequently the decompostion of hydrogenperoxide can be monitored by following the change in the temperature of the reaction mixture with time.

Experimental procedure:

Fig. 1: Experiment set-up The temperature probe is connected to the input Sensor2 of the CHEMBOX.

The Dewar vessel is fitted with a stirring bar. 75 mL of 10 % hydrogen peroxide solution are pipetted into the Dewar vessel and with stirring 8 mL of the 0.1 molar potassium dichromate solution are added. The magnetic stirrer is started.

A holed plastic cover is placed on the top of the Dewar vessel. The shaft of the temperature probe is inserted into the hole so that its tip is immersed in the reaction mixture.

The sensing software is started.

The real-time graph of the temperature rise is displayed on the screen (Fig. 2).

The temperature is recorded until the maximum temperature is reached.

In addition the experiment is carried out using a 0.2 and a 0.32 molar solution of potassium dichromate.

Fig. 2: Realtime graph 0.2 molar dichromate solution

Data analysis using Excel (Download):

A point diagram of the plot of T [°C] against t [s] is created. A best fit straight line is drawn through the highlighted data points of the linear portion of the individual temperature curves. The coefficient in front of x in the straight line equations (y = ax + b) specifies the value for the slope m (Fig. 3), which also can be determined directly on the measuring screen of Chemex (Fig. 2).

Fig. 3: Temperature T 1: 0.1 M 2: 0.15 M 3: 0.2 M 4: 0.3 M dichromate solution)

Result:

Fig. 4: Plot of the slope m versus the molarity of the dichromate solution

The reaction rate is proportinal to the concentration of the catalyst (Fig. 4). This is typically for homogeneous catalyses.

References:
Computer-Interfaced Experiments Kinetics: Catalyzed Decomposition of Hydrogen Peroxide - First Order Reaction
Computer-Interfaced Experiments Kinetics: Decomposition of Hydrogen Peroxide catalyzed by Iodide
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
Kyle Smith und Mark Iannone Improved Data Analysis for an Adiabatic Kinetics Experiment

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