Hydrogen Peroxide:

Hydrogen peroxide (H₂O₂) was discovered by French chemist Thenard.

(1) Preparation : It is prepared by

i) Laboratory method : In laboratory, H₂O₂ is prepared by Merck’s process. It is prepared by adding calculated amounts of sodium peroxide to ice cold dilute (20%) solution of H₂SO₄.

Na₂O₂ + H₂SO₄ → BaSO₄ ↓+ H₂O₂ + 8H₂O

It must be noted that anhydrous barium peroxide does not react readily with sulphuric acid (because a coating of insoluble barium sulphate is formed on its surface which stops further action of the acid). Therefore, hydrated barium peroxide, BaO₂.8H₂O must be used.

(b) 3BaO₂ + 2H₃PO₄ → Ba₃(PO₄)₂ + 3H₂O₂

Ba₃(PO₄)₂ + 3H₂SO₄ → 3BaSO₄ + 2H₃PO₄

Phosphoric acid is preferred to H₂SO₄ because soluble impurities like barium persulphate (from BaO₂.8H₂O + H₂SO₄ ) tends to decompose H₂O₂ while H₃PO₄ acts as preservative (negative catalyst) for H₂O₂ .

(iii) Industrial method: On a commercial scale, H₂O₂ can be prepared by the electrolysis of 50% H₂SO₄ solution. In a cell, peroxy disulphuric acid is formed at the anode.

This is drawn off from the cell and hydrolysed with water to give H₂O₂.

H₂S₂O₈ + 2H₂O ——→ 2H₂SO₄ + H₂O₂ The resulting solution is distilled under reduced pressure when H₂O₂ gets distilled while H₂SO₄ with high boiling point, remains undistilled.

(iv) By redox process: Industrially H₂O₂ is prepared by the auto-oxidation of 2-alkylanthraquinols. The process involves a cycle of reactions. The net reaction is the catalytic union of and to give .

The H₂O₂ formed (about 1%) is extracted with water and concentrated.

(2) Physical properties

(i) Pure hydrogen peroxide is a pale blue syrupy liquid.

(ii) It freezes at – 0.5°C and has a density of 1.4 in pure state.

(iii) Hydrogen peroxide is diamagnetic.

(iv) It is more highly associated via hydrogen bonding than water.

(v) Although it is a better polar solvent than H₂O. However, it can’t be used as such because of strong autooxidation ability.

(vi) Dipole moment of H₂O₂ is 2.1 D.

(3) Chemical properties

(i) Decomposition : Pure H₂O₂ is an unstable liquid and decomposes into water and O₂ either upon standing or upon heating, 2H₂O₂ → 2H₂O + O₂; ?H = –196.0 kJ

(ii) Oxidising nature : It is a powerful oxidising agent. It acts as an oxidising agent in neutral, acidic or in alkaline medium. e.g.

2KI + H₂O₂ → 2KOH + I₂ [In neutral medium]

2FeSO₄ + H₂SO₄ + H₂O₂ → Fe₂(SO₄)₃ + 2H₂O [In acidic medium]

MnSO₄ + H₂O₂ + 2NaOH → MnO₂ + Na₂SO₄ + 2H₂O [In alkaline medium]

(iii) Reducing nature: H₂O₂ has tendency to take up oxygen from strong oxidising agents and thus, acts as a reducing agent,

It can act as a reducing agent in acidic, basic or even neutral medium.

In acidic medium, H₂O₂ → 2H⁺ + O₂ + 2e^–

In alkaline medium, H₂O₂ + 2OH^– → 2H₂O + O₂ + 2e^–

(iv) Bleaching action: H₂O₂ acts as a bleaching agent due to the release of nascent oxygen.

H₂O₂ → H₂O + O

Thus, the bleaching action of H₂O₂ is due to oxidation. It oxidises the colouring matter to a colourless product, Colouring matter +O → Colour less matter.

H₂O₂ is used to bleach delicate materials like ivory, silk, wool, leather etc.

(v) Acidic nature :Anhydrous hydrogen peroxide is acidic in character (K_a = 1.55 × 10^–12 at 298 K). its dissociation in aqueous solution may be given as

H₂O₂ + H₂O + H₃O⁺ + HO^–₂

It forms two types of salts

NaOH + H₂O₂ → NaHO₂ + H₂O
                                    ^{Sod. hydroperox ide
                                           (Acidic salt)}

2NaOH + H₂O₂ → Na₂O₂ + 2H₂O
                                       ^{Sod. peroxide
                                      (Normal salt)}
(vi) Addition reactions :Hydrogen peroxide is capable of adding itself to ethylenic linkage.

(4) Structure of H₂O₂ : Hydrogen peroxide is non-linear, non-planar molecule. It has a open book structure. The –O–O– linkage is called peroxy linkage. The structure is shown below.

(5) Concentration of H₂O₂: Dilute H₂O₂ is concentrated to about 50% by slow evaporation on a water bath. It is further concentrated to 90% in a vacuum desiccator using conc. H₂SO₄ as dehydrating agent. Further concentration to 99% is obtained by distillation under reduced pressure. Last traces of moisture in 99% of H₂O₂ are removed or anhydrous H₂O₂ is obtained by cooling it to 263 K in a cold bath of ether and dry ice followed by seeding with a few crystals of solid H₂O₂ when needle-shaped crystals of 100% H₂O₂ separate out. These crystals are removed, dried and melted to get 100% H₂O₂.

(6) Storage of H₂O₂: H₂O₂ is not stored in glass bottles since the alkali metal oxides present in glass catalyse its decomposition. It is, therefore, stored in paraffin wax coated glass, plastic or teflon bottles. Small amounts of acid, glycerol, alcohol, acetanilide and H₃PO₄ are often used as stablizers to check its decomposition.

Uses of hydrogen peroxide

(i) For bleaching delicate articles like wool, hair, feather, ivory, etc.

(ii) For restoring colour of old lead paintings whose white lead has blackened due to formation of PbS by H₂S of atmosphere. Hydrogen peroxide converts the black lead sulphide to white lead sulphate

(iii) As an aerating agent in production of spong rubber.

(iv) As an antiseptic and germicide for washing wounds, teeth and ears, under the name of perhydrol.

(v) In the manufacture of sodium perborate, sodium percarbonate. These are used in high quality detergents.

(vi) As an antichlor.

(vii) As an oxidant for rocket fuel.

(viii) In the detection of Ti, V and Cr ions with which it forms peroxides of characteristics colours.

(ix) In the production of epoxides, propylene oxide and polyurethanes.

(x) In the synthesis of hydroquinone, pharmaceuticals (cephalosoporin) and food products like tartaric acid.

(xi) For pollution control of domestic effluents where it restores the aerobic conditions of sewage wastes. For pollution control of industrial effluents containing CN^– ions. H₂O₂ oxidises CN^– ions to harmless products.