Question

When the KMnO4 solution is added to the peroxide- acid solution, explain why it is important that the total volume of the reaction mixture (Volume of KMnO4 + Volume of H2O2 +Volume of Acid) is kept constant for every experiment.

Answer 1

The catalytic decomposition of hydrogen peroxide provides a range of project opportunities of varying length and complexity. This is because of the variety of catalysts that will increase the rate of decomposition and the methods that can be used to monitor the reaction:

2H

2

O

2

→ 2H

2

O + O

2

Using an enzyme catalyst

In many living organisms hydrogen peroxide is a product of metabolism that must be broken down, since in appreciable concentrations it is toxic. The rate of decomposition is increased by the intra-cellular enzyme catalase.

As a fairly simple project you could compare the effectiveness of the enzyme from different sources such as potato, celery and liver. This might lead you on to a more thorough investigation of other factors that affect the activity of the catalase, including, perhaps, temperature, pH, concentrations of substrate and enzyme and the presence of enzyme inhibitors.Such experiments may at first sight seem quite straightforward, but as you try to interpret your results you will be led into quite complex, but very interesting, aspects of chemical kinetics and reaction mechanisms.

Using an inorganic catalyst

As an interesting contrast, a similar increase in the rate of decomposition of hydrogen peroxide can be achieved using an inorganic catalyst such as manganese(IV) oxide or lead(lV) oxide.

You could set out to look at how factors such as concentration of the peroxide, amount of the catalyst and temperature affect the rate of the decomposition. Experiments of this kind could lead you towards a possible reaction mechanism and towards the calculation of the activation energy involved.

Contact lens chemistryThe catalytic decomposition of hydrogen peroxide will be very familiar to some students, as it is an integral part of one system for cleaning contact lenses. In this method the l8 December, 2008 clean it and then the remaining solution is decomposed with the help of a platinum-coated catalyst. You might like to investigate how the effectiveness of this catalyst changes under different conditions and compare it with the other catalysts.

Monitoring the decomposition

There are several ways in which you can monitor the decomposition reaction. You might decide to follow the fall in concentration of the hydrogen peroxide. You can do this by taking samples of the reaction mixture at different times and running them into an acidified solution of potassium iodide. The iodine produced can be titrated with a solution of thiosulphate using starch indicator:

2H

+ H

2

O

2

+ 2I

→ I

+ 2H

O

2

2Platinum-coated discs are part of the ‘Septicon’ system for cleaning contact lenses. They are available from Boots chemist shops.

A glass syringe sometimes sticks. You can minimise this problem by tapping it continually with a pencil.

sodium thiosulphate solution using 2 cm3 of 1% starch solution as indicator.

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