Decomposing hydrogen peroxide
Demonstrations designed to capture the student's imagination, by Colin Baker of Bedford School.
In this issue: decomposing hydrogen peroxide
The chemistry of liquid explosives has attracted intense media interest recently, following the unsuccessful terrorist plot to blow up planes flying between the UK and the US. Intelligence sources named triacetone triperoxide (TATP) as a possible terrorist explosive because it could be made by mixing hydrogen peroxide with other household chemicals.
The reaction between hydrogen peroxide and blood
In this demonstration I mix hydrogen peroxide with blood to produce a foam rather than a liquid explosive.
10 cm3 30 per cent hydrogen peroxide
10 cm3 Blood
Champagne glass Procedure
Pour both liquids simultaneously into the champagne glass.
Wear gloves, hydrogen peroxide is corrosive and can cause skin burns. Contact with the eyes can cause serious, long-term damage. Slow decomposition of the solution in storage may lead to a build-up of pressure in sealed containers. Hydrogen peroxide can form potentially explosive compounds with a wide variety of materials, including ketones, alcohols, esters, glycerine, aniline (phenylamine), triethylamine and sodium carbonate.
Both the hydrogen peroxide and the blood must be fresh. Ask your local butcher for the blood.
Peroxides are compounds in which there is an oxygen-oxygen single bond. Oxygen atoms stick together in pairs to form oxygen gas, O2, and in threes to make ozone, O3, but do not form longer chains, as sulfur does so readily.
This is a simple experiment with which to introduce catalysis. The blood contains the enzyme catalase which on mixing with hydrogen peroxide catalyses its breakdown into water and oxygen via the disproportionation reaction:
H2O2(aq) H2O(l) + ½O2(g) ?H = -98.7 kJ mol-1
The enzyme catalase in blood speeds up the decomposition of hydrogen peroxide
Catalase is very efficient at decomposing hydrogen peroxide; one molecule of the enzyme can catalyse the conversion of over 6000,000 hydrogen peroxide molecules into water and oxygen every second. The enzyme occurs widely in tissues such as the liver and prevents accumulation of, and tissue damage by, hydrogen peroxide that is produced during metabolism. Catalase found in human red blood cells is a complicated chemical consisting of four polypeptide chains with 500 amino acids in each chain. Each peptide chain includes a porphyrin haem group. These are the active components which allow the enzyme to catalyse the decomposition of hydrogen peroxide.
Hydrogen peroxide is a powerful oxidising agent but is unusual in that it can act as reducing agent under certain conditions. The oxidation number of oxygen in hydrogen peroxide is -1, intermediate between 0 in oxygen and -2 in water, and this allows the oxygen to act as both a reductant and oxidant in either acid (H2O2) or alkali (HO2-) solution.
Oxidising agent in acid conditions:
2Fe2+(aq) + H2O2(aq) + 2H+(aq) 2Fe3+(aq) + 2H2O(l)
(H2O2(aq) + 2H+(aq) + 2e- 2H2O(l) E° = +1.77V)
Reducing agent in acid conditions:
2MnO4-(aq) + 5H2O2(aq) + 6H+(aq) 2Mn2+(aq) + 8H2O(l) + 5O2(g)
(O2(g) + 2H+(aq) + 2e- H2O2(aq) E° = +0.68V)
Oxidising agent in alkaline conditions:
Mn2+(aq) + HO2-(aq) + H2O(l) Mn4+(aq) + 3OH-(aq)
(HO2-(aq) + H2O(l) + 2e- 3OH-(aq) E° = +0.87V)
Reducing agent in alkaline conditions:
2Fe3+(aq) + HO2-(aq) + OH-(aq) 2Fe2+(aq) + H2O(l) + O2(g)
(O2(g) + H2O(l) + 2e- HO2-(aq) + OH-(aq) E° = -0.08V)
The electrode potentials for these redox reactions suggest that hydrogen peroxide should be a better oxidising agent in acid than in alkali. However, unless a catalyst is used, acid reactions are slow because of kinetic hindrance. Despite this, acid conditions are often used because hydrogen peroxide may disproportionate in alkali. Conversely, hydrogen peroxide is a better reducing agent in alkaline conditions and therefore pH conditions can be used to determine whether hydrogen peroxide acts as an oxidising agent or a reducing agent. You should point out to students that whenever hydrogen peroxide acts as a reducing agent oxygen is a product of the reaction.