Hydrogen Peroxide (H2O2)

Hydrogen Peroxide (H₂O₂)

Hydrogen Peroxide is an important chemical used in pollution control treatment of domestic and industrial effluents.

Preparation

It can be prepared by the following methods.

(i) Acidifying barium peroxide and removing excess water by evaporation under reduced pressure gives hydrogen peroxide.

BaO₂.8H₂O (s)  +  H₂SO₄ (aq)  →   BaSO₄ (s)  +  H₂O₂ (aq)  8H₂O (I)

(ii) Peroxodisulphate, obtained by electrolytic oxidation of acidified sulphate solutions at high current density, on hydrolysis yields hydrogen peroxide.

This method is now used for the laboratory preparation of D₂O₂.

K₂S₂O₈ (s)  +  2D₂O (I)   -   2KDSO₄ (aq)  +  D₂O₂ (I)

(iii) Industrially it is prepared by the autooxidation of 2-alklylanthraquinols.

In this case 1%  H₂O₂ is formed. It is extracted with water and concentrated to ~30% (by mass) by distillation under reduced pressure. It can be further concentrated to ~85% by careful distillation under low pressure. The remaining water can be frozen out to obtain pure H₂O₂.

Physical Properties

In the pure state H₂O₂ is an almost colorless (very pale blue) liquid. Its important physical properties are given in below table.

H₂O₂ is miscible with water in all proportions and forms a hydrate H₂O₂.H₂O₂ (mp221K). A 30% solution of H₂O₂ is marketed as ‘100 volume’ hydrogen peroxide. It means that one milliliter of 30% H₂O₂ solution will give 100V of oxygen at STP. Commercially, it is marketed as 10V, which means it contains 3% H₂O₂.

Structure

Hydrogen peroxide has a non-planar structure. The molecular dimensions in the gas phase and solid phase are shown below.

Chemical Properties

It acts as an oxidizing as well as reducing agent in both acidic and alkaline media. Simple reactions are described below.

Oxidizing action in acidic medium

2Fe²⁺ (aq)  +  2H⁺ (aq)  +  H₂O₂ (aq)  →   2Fe³⁺ (aq)  +  2H₂O (I)

PbS (s)  +  4H₂O₂ (aq)  →  PbSO₄ (s)  +  4H₂O (I)

Reducing action in acidic medium

2MnO₄^-  +  6H⁺  +  5H₂O₂   →  2Mn²⁺  +  8H₂O  +  5O₂

HOCl  +  H₂O₂   →  H₃O+  +  Cl^-  +  O₂

Oxidizing action in basic medium

2Fe²⁺  +  H₂O₂   →   2Fe³⁺  +  2OH^-

Mn²⁺  +  H₂O₂   →   Mn⁴⁺  +  2OH^-

Reducing action in basic medium

I₂  +  H₂O₂  +  2OH^-   →   2I^-  +  2H₂O  +  O₂

2MnO₄^-  +  3H₂O₂   →   2MnO₂  +  3O₂  +  2H₂O  +  2OH^-

Storage

H₂O₂ decomposes slowly on exposure to light.

2H₂O₂ (I)   →   2H₂O (I)  +  O₂ (g)

In the presence of metal surfaces or traces of alkali (present in glass containers), the above reaction is catalysed. It is, therefore, stored in wax-lined glass or plastic vessels in dark. Urea can be added as a stabilizer. It is kept away from dust because dust can induce explosive decomposition of the compound.

Uses

Its wide scale use has led to tremendous increase in the industrial production of H₂O₂. Some of the uses are listed below:

• In daily life it is used as hair bleach and as a mild disinfectant. As an antiseptic it is sold in the market as perhydrol.
• It is used to manufacture chemicals like sodium perborate and per-carbonate, which are used in high quality detergents.
• It is used in the synthesis of hydroquinone, tartaric acid and certain food products and pharmaceuticals (cephalosporin) etc.
• It is employed in the industries as a bleaching agent for textiles, paper pulp, leather, oils fats, etc.
• Nowadays it is also used in Environmental (Green) Chemistry. For example, in pollution control treatment of domestic and industrial effluents, oxidation of cyanides, restoration of aerobic conditions to sewage wastes, etc.