Hydrogen peroxide is a strange substance. It is somewhat similar to water but has completely different properties. For example, you should never try to put out a fire with hydrogen peroxide. Hydrogen peroxide can react as a powerful oxidizer in high concentrations.
Unlike water, H2O2 is extremely reactive and breaks down easily. In the process, radicals are formed which continue to react particularly violently – sometimes explosively.
The pure substance is liquid at room temperature, colorless and, like the water, slightly bluish in thick layers. Its melting point is almost the same as that of water: -0.43 °C. However, at 150.2 °C, the boiling point is significantly higher than that of water. The reason for this behavior is the tendency to form intermolecular hydrogen bonds.
That’s also why working with hydrogen peroxide solutions can be so dangerous: when you heat them, the more volatile water is first evaporated off. The solution increasingly concentrates on H2O2 – until it explodes.
Chemical reactions of hydrogen peroxide
There are essentially five types of reactions to which hydrogen peroxide can contribute.
1. Redox reactions
Compared to reducing agents (such as MnSO4), H2O2 acts as an oxidizing agent.
Compared to strong oxidizing agents (like KMnO4), H2O2 acts as a reducing agent.
2. Decomposition reaction
Usually, H2O2 is stable, especially in solution. But you should be aware of one thing: Strictly speaking, the solution is metastable – only waiting for a free radical to destroy everything..
With even a small supply of energy, the connection tends to spontaneously break (thermolysis). This is a radical reaction, which can lead to a chain reaction. That is why the decomposition reaction is often explosive. Two hydroxyl radicals are primarily formed.
H2O2 + Energy -> 2 HO•
The dissociation energy is very high: 211 kJ/mol. But the subsequent reactions are strongly exothermic so that the system can increasingly activate itself – a chain reaction. Radicals such as HO2• are also formed.
Hydroxyl radicals are especially important for the chemistry related to our atmosphere. They also play a decisive role in the formation of ozone near the ground and in the formation of the so-called ozone hole. They also form H2O2, which can be detected in acid rain.
Ultimately, however, the decay is an exothermic redox reaction in the sense of a disproportionation.
H2O2 + H2O2 -> 2 H2O + O2 / +196,2 kJ/mol
Here one H2O2 molecule is formally reduced and another oxidized.
This decomposition reaction is generally catalyzed by heavy metal ions. For this reason, complexing agents or phosphates, which bind the heavy metals, are added to the H2O2 for stabilization. Radical scavengers such as certain alcohols (iso-propanol) are also added. Overall, this makes handling 30% H2O2 quite safe.
3. Acid / base reaction
H2O2 is a weak acid, but still about 1000x stronger than water. This relates to the following chemical equilibria.
The pH value (20 °C) of a 30% solution of H2O2 is 4, that of a 3% solution is 6.
The salts of hydrogen peroxide are the peroxides. The peroxides of the alkali and alkaline earth metals tend to hydrolyze immediately in water, with H2O2 and the corresponding alkali being formed.
4. Complex formation reactions
Hydrogen peroxide not only forms simple salts. For some metals such as titanium, chromium, and vanadium, the peroxide ion is a ligand that forms a complex bond with the central metal ion. These peroxo complexes are often colored. Detection reactions such as the blue color of the dichromate or the orange color of the titanyl sulfate complex are based on this.
Cr2O72- + 4 H2O2 —> 2 [CrO5(OH)]– + 3 H2O
TiO2+ + H2O2 —> [TiO2]2+ + H2O
5. Addition- and peroxo compounds
The best known is carbamide peroxide (H2N-CO-NH2 · H2O2), the addition compound with urea, the bleaching agent for teeth, and hair.
But peroxides are also formed with inorganic substances. The best known is probably sodium perborate, which is obtained when boric acid, caustic soda, and H2O2 are combined. A ring-shaped anion is formed, which consists of two borates linked by an O-O bridge.
It is contained in almost all detergents that are not specifically advertised for color protection. The decomposition products of perborate salts are released as H2O2, radicals such as HO• or HO2•, and as oxygen. They all have a destructive effect on dyes – especially in hot, alkaline washing solutions.
The organic peroxides are of the greatest importance. They are used in plastics chemistry, such as peroxybenzoic acid, as a starter substance for radical polymerizations.