Hydrogen Peroxide – preparation, reactions, structure, and use

Hydrogen peroxide’s discovery was done by a French chemist J.L.Thenard. We write molecular formula of hydrogen peroxide as H_2 O_2.

Preparation of Hydrogen peroxide: Hydrogen peroxide can be prepared in the laboratory by the action of cold, dilute sulphuric acid on sodium or barium peroxide. Let’s have a look at each of them one by one:

  • From sodium peroxide ( Merck’s process):
    It is prepared by adding a calculated amount of sodium peroxide into cold, dilute solution of sulphuric acid. This addition is carried out slowly in a small amount with constant stirring. $Na_{2}O_{2 }+ H_{2}SO_{4} \to Na_{2}SO_{4}+ H_{2}O_{2}$
    Upon cooling, we get crystal of $Na_{2}SO_{4}$.10$H_{2}O$. The crystals of $Na_{2}SO_{4}$.10$H_{2}O$ are decanted leaving behind the solution of hydrogen peroxide. By this method, 30 \% of hydrogen peroxide is obtained.
  • From barium peroxide: J.L.Thenard first prepared hydrogen peroxide in 1818 by acidifying barium peroxide and removing the excess of water by evaporation under reduced pressure. By this method, a paste of hydrogen peroxide is obtained in ice-cold water and is treated with a cold solution of sulphuric acid.
    $BaO_{2}.8H_{2}O_{(s)}+ H_{2}SO_{4(aq)} \to BaSO_{4(s)}+ H_{2}O_{2(aq)}+ 8H_{2}O_{(l)}$
    The white ppt. of barium sulphate is removed by filtration leaving behind 5% solution of hydrogen peroxide.

Structure of Hydrogen peroxide:

Hydrogen peroxide has a non-planar structure in which two hydrogen atoms are arranged in two directions almost perpendicular to each other and to the axis joining the two oxygen atoms.

The O-O linkage is called the peroxide linkage. The structure of $H_{2}O_{2}$ in the solid and gas phase are shown below:

Image result for structure of hydrogen peroxide in solid and gas phase

Chemical reactions of Hydrogen peroxide:

Decomposition reaction:

Pure Hydrogen peroxide is an unstable liquid, hence decomposes into water and oxygen upon heating.

$2H_{2}O_{2(l)} \to 2H_{2}O_{(l)}+ O_{2(g)}$

In hydrogen peroxide oxidation state of oxygen is -1 which is intermediate between the values for $ O_{2}$ (zero) and $H_{2}O$ (-2). Therefore, an aqueous solution of hydrogen peroxide readily decomposes.

Reactions as an oxidizing agent:

  • In acidic medium: In presence of an acid, $H_{2}O_{2} $ can accept electrons and act as an oxidizing agent. See the reaction here: $H_{2}O_{2}+ 2H^{+}+ 2e^{-} \to 2H_{2}O$
  • In alkaline medium: Hydrogen peroxide can also accept electrons in the alkaline medium and act as an oxidizing agent. $H_{2}O_{2}+ OH^{-}+ 2e^{-} \to 3OH^{-}$

Reactions as a reducing agent:

  • In acidic medium: Hydrogen peroxide has a tendency to take up oxygen from strong oxidizing agents and thus, act as a reducing agent.$H_{2}O_{2} \to 2H^{+}+ O_{2}+ 2e^{-}$
  • In alkaline medium: $H_{2}O_{2}+ 2OH^{-} \to 2H_{2}O+ O_{2}+ 2e^{-}$

Bleaching action:

Hydrogen peroxide acts as a bleaching agent due to the release of nascent oxygen. It oxidizes the coloring matter to a colorless product. It is used to bleach materials like silk, wool, ivory, feathers, etc.

Addition reactions:

Hydrogen peroxide on adding with alkenes form glycols.

 

Uses of Hydrogen Peroxide:

  • Hydrogen peroxide is domestically used as hair bleach and as a mild disinfectant.
  • It is used as a bleaching agent in industries like textiles, papers, straw, leather, fats, etc.
  • Hydrogen peroxide is also used as an antiseptic for washing wounds, ears and teeth under the chemical name perhydrol.
  • It is used for the production of propylene oxide, epoxides and polyurethanes.
  • It is used in the manufacture of many inorganic compounds such as percarbonates, sodium perborates which are important constituents of high-quality detergents.

 

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