Hydrogen peroxide

 Key point: Hydrogen peroxide is susceptible to decomposition by disproportionation at elevated temperatures or in the presence of catalysts. Hydrogen peroxide is a very pale blue, viscous liquid. It has a higher boiling point than water (150C) and a greater density (1.445 g cm3 at 25C). It is miscible in water and is usually handled in aqueous solution. The Frost diagram for oxygen (Fig. 16.2) shows that H₂O₂ is a good oxidizing agent, but it is unstable with respect to disproportionation:

This reaction is slow but is explosive when catalysed by a metal surface or alkali dissolved from glass. For this reason, hydrogen peroxide and its solutions are stored in plastic bottles and a stabilizer is added. This reaction can be considered in terms of the reduction half-reactions

Any substance with a standard potential in the range 0.70 1.68 V that has suitable binding sites will catalyse this reaction. As can be inferred from these standard potentials, hydrogen peroxide is a very powerful oxidizing agent in acid solution:

However, in basic solution hydrogen peroxide can act as a reducing agent:

The underlying reason for the oxidizing nature of hydrogen peroxide lies in the weakness of the O-O single bond (146 kJ mol 1). Hydrogen peroxide reacts with d-metal ions such as Fe²⁺ to form the hydroxyl radical in the Fenton reaction:

The Fe³⁺ product can react with a second H₂O₂ to regenerate Fe²⁺ , so that the production of hydroxyl radical is catalytic. The O hydroxyl radical is one of the strongest oxidizing agents known (E ْ=+2.85 V) and the reaction is used to oxidize organic mat ter. In living cells, its reaction with DNA has potentially lethal consequences.

Hydrogen peroxide is a slightly stronger acid than water:

Deprotonation occurs in other basic solvents such as liquid ammonia, and NH4OOH has been isolated and found to consist of NH4 and HO₂ ions. When solid NH₄OOH melts (at 25C), the melt contains hydrogen-bonded NH₃ and H₂O₂ molecules. The oxidizing ability of hydrogen peroxide and the harmless nature of its byproducts lead to its many applications. It is used in water treatment to oxidize pollutants, as a mild antiseptic, and as a bleach in the textile, paper, and hair-care industries (Box 16.2).

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مواضيع ذات صلة

Water

Selenium, tellurium, and polonium

Sulfur

Reactivity of oxygen

Oxygen

Condensed phosphates

Oxoanions of phosphorus, arsenic, antimony, and bismuth

Oxides of phosphorus, arsenic, antimony, and bismuth

Nitrogen(V) oxides and oxoanions

Oxides and oxoanions of nitrogen

Oxohalides

Halides of the heavy elements