TeacherExpt:Catalysts for the decomposition of hydrogen peroxide

This procedure is designed as a demonstration to be performed by a teacher or other qualified person.

Several measuring cylinders are set up each containing a little washing up liquid and a small amount of a catalyst for the decomposition ofhydrogen peroxide. Hydrogen peroxide is poured into the cylinders and a foam rises up the cylinders at a rate that depends on the effectiveness of the catalyst.

Contents [hide]

Apparatus and chemicals

The chemical quantities given are for one demonstration.

Safety goggles and nitrile gloves must be worn by the demonstrator
Several 250 cm3 measuring cylinders – one for each catalyst to be used.
A large tray to catch any foam that spills over the top of the cylinders.
Stopwatch or clock with second hand.
75 cm3 of 100 volume hydrogen peroxide solution (Harmful at this concentration)
About 0.5 g of powdered manganese(IV) oxide (manganese dioxide, MnO2) (Harmful)
About 0.5 g of lead(IV) oxide (lead dioxide, PbO2) (Toxic and dangerous for the environment)
About 0.5 g of iron(III) oxide (red iron oxide, Fe2O3) (Low hazard)
A small piece (about 1 cm3) of potato.
A small piece (about 1 cm3) of liver.

Technical notes

75 cm3 of 100 volume hydrogen peroxide solution (Harmful at this concentration) Refer to CLEAPSS® Hazcard 50

About 0.5 g of powdered manganese(IV) oxide (manganese dioxide, MnO2) (Harmful at this concentration) Refer to CLEAPSS® Hazcard 60

About 0.5 g of lead(IV) oxide (lead dioxide, PbO2) (Toxic and dangerous for the environment) Refer to CLEAPSS® Hazcard 56

About 0.5 g of iron(III) oxide (red iron oxide, Fe2O3) (Low hazard). Refer to CLEAPSS® Hazcard 55A

Used liver should be wrapped up in paper and placed in the dustbin.
The use of the washing up liquid helps prevent the catalyst from going into the atmosphere but there may be some hydrogen peroxide aerosol.
To dispose of the lead(IV) oxide, filter it off from the solution, wash it and let it dry. It could be reused.

Procedure Health & Safety

The demonstrator should wear goggles and nitrile gloves (see CLEAPSS® Hazcard 50)

Procedure Before the demonstration

A. Line up five 250 cm3 measuring cylinders in a tray. Add 75 cm3 of water to the 75 cm3 of 100 volume hydrogen peroxide solution to make 150 cm3 of 50 volume solution.

The demonstration

A. Place about 1 cm3 of washing up liquid into each of the measuring cylinders. To each one add the amount of catalyst specified above. Then add 25 cm3 of 50 volume hydrogen peroxide solution to each cylinder. The addition of the catalyst to each cylinder should be done as nearly simultaneously as possible – using two assistants will help. Start timing. Foam will rise up the cylinders. The lead dioxide will probably be fastest, followed by manganese dioxide and liver. Potato will be much slower and the iron oxide will barely produce any foam. This order could be affected by the surface areas of the powders. Time how long each foam takes to rise to the top (or other marked point) of the cylinder. The foam from the first three cylinders will probably overflow considerably.

B. Place a glowing spill in the foam; it will re-light confirming that the gas produced is oxygen.

Teaching notes

Some students may believe that the catalysts – especially the oxides – are reactants because hydrogen peroxide is not noticeably decomposing at room temperature. The teacher could point out the venting cap on the peroxide bottle as an indication of continuous slow decomposition. Alternatively s/he could heat a little hydrogen peroxide in a conical flask with a bung and delivery tube, collect the gas over water in a test-tube and test it with a glowing spill to confirm that it is oxygen. This shows that no other reactant is needed to decompose hydrogen peroxide.

NB: Simply heating 50 volume hydrogen peroxide in a test-tube will not succeed in demonstrating that oxygen is produced. The steam produced will tend to put out a glowing spill. Collecting the gas over water has the effect of condensing the steam. It is also possible to ‘cheat’ by dusting a beaker with a tiny, almost imperceptible, amount of manganese dioxide prior to the demonstration and pouring hydrogen peroxide into it. Bubbles of oxygen will be formed in the beaker.

Theory

The reaction is : 2H2O2(aq) → 2H2O(l) + O2(g)

This is catalysed by a variety of transition metal compounds and also by peroxidase enzymes found in many living things.

Extensions

Repeat the experiment but heat the liver and the potato pieces for about five minutes in boiling water before use. There will be almost no catalytic effect, confirming that the catalyst in these cases is an enzyme that is denatured by heat.

Investigate the effect of using lumpy or powdered manganese dioxide. The powdered oxide will be more effective because of its greater surface area.

Try using other metal oxides or iron filings as catalysts.

Animal blood may be used instead of liver if local regulations allow this.

One teacher suggested measuring the height of the foam over suitable time intervals and plotting a graph.

Reference This experiment has been adapted from Classic Chemistry Demonstrations, Royal Society of Chemistry, London, p.145-146

http://www.rsc.org/learn-chemistry/wiki/TeacherExpt:Catalysts_for_the_decomposition_of_hydrogen_pero...

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