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Hydrogen peroxide – (H₂O₂)

It was discovered by L. J. Thenard in 1818, by the action of sulphuric acid on barium peroxide.

Preparation Methods of H₂O­₂ –

1. By adding calculated amount of sodium peroxide to 20% ice cold H₂SO₄

          Na₂O₂ + H₂SO₄ → Na₂SO₄ + H₂O₂

On further cooling crystals of Na₂SO₄ 10H­_­2O separate and dil solution of H₂O₂ is obtained.

 

2. By adding a paste of hydrated barium peroxide in ice cold water to 20% ice cold H₂SO₄.

          BaO₂ + H₂SO₄ → BaSO₄ + H₂O₂

BaSO₄ is filtered off and the filtrate is treated with calculated amount of BaCO₃ to remove excess of H₂SO_4.

          BaCO₃ + H₂SO₄ → BaSO₄ + H₂O + CO₂

 

3. By passing CO₂ thought a suspension of hydrated barium peroxide in ice – cold water.

          BaO₂ + CO₂ + H₂O → BaCO₃ +H₂O₂

 

Manufacture of H₂O₂ –

1. By the electrolysis of 50% H₂SO₄ at low temperature using Pt anode and a high current density followed by distillation under reduced pressure

          H₂SO₄        H⁺ + HSO₄^–

          At cathode: 2H⁺ + 2e^– → H₂

          At anode: 2HSO₄^– → H₂S₂O₈ + 2e^–

                                      Peroxodisulphuric acid

                   H₂S₂O₈ + H₂O → H_­2SO₅ + H₂SO₄

                                      Caros acid or (peroxo mono sulphuric acid)

                   H₂SO₅ + H₂O → H­₂SO₄ + H₂O₂.

 

2. By the electrolysis of ammonium sulphate and H₂SO₄ solution at low temperature.

          (NH₄)₂SO₄ + H₂SO₄ → 2NH₄ HSO₄

          NH₄HSO^₄ H⁺ + [NH₄SO₄]^–

          At cathode : 2H⁺ + 2e^– → H₂

          At anode : 2[NH₄SO₄]^– → (NH₄)₂ S₂O₈ + 2e^–

          Ammonium peroxo disulphate

Ammonium peroxo disulphate is quickly removed and is distilled with H₂SO₄ under reduced pressure when H₂O₂ is obtained.

(NH₄)₂ S₂O₈ + H₂SO₄ → H₂S₂O₈ + (NH₄)₂ SO₄.

H₂S₂O₈ + 2H₂O → 2H_­2SO₄ + H₂O₂.

 

3. By auto oxidation method – in this method H₂ is passed through 2 ethyl anthraquinone dissolved in an organic solvent in presence of Pd catalyst; when 2 ethyl anthraquinol is formed. Now air is passed when 2 ethyl anthraquinone is reformed and dil solution of H₂O₂ is obtained.

 

Concentration of H₂O₂ solution – dilute solution of H₂O₂ (20 – 30%) is heated on a water bath to 60⁰C when 45% H₂O₂ is obtained. It is then distilled under reduced pressure (15mm and 60⁰C) when 90% H₂O₂ is obtained. This is then evaporated in a vacuum over H₂SO₄ when 99% H₂O₂ is formed after. This it is cooled by means of a freezing mixture of solid CO₂ and then solid H₂O₂ is obtained. Finally solid H₂O₂­ is slightly warmed, when pure anhydrous H₂O₂ is obtained.

 

Physical Properties  –

1. Colorless, odorless, but liquid H₂O₂ in bulk appears slightly blue,
2. Boiling point 85⁰C at 68mm Hg pressure. At 760mm Hg pressure, it boils at 152⁰C, however it explodes at this temperature,
3. Soluble in water alcohol and ether
4. Produces blisters on skin

         

Chemical Properties –

1. Decomposition – On long standing or heating, it decomposes,

          2H₂O₂ → 2H₂O + O₂

Ag, MnO₂, Si, Pt etc, act as positive catalyst for decomposition, while alcohol, H₃PO₄, acetanilide etc act as negative catalyst.

 

2. Acidic Nature – it is feebly acidic, and neutral towards litmus.

          Ba(OH)₂ + H₂O₂ → BaO₂ + 2H₂O

 

3. Oxidizing properties – it oxidizes both in alkaline and acidic media.

          H₂O₂ → H₂O + O

          It oxidizes –

          (A)    PbS + 4(O) → PbSO₄

          (B)    H₂S + O → S + H₂O

          (C)    Na₂SO₃ + O → Na₂SO₄

          (D)    Na₃ASO₃ + O → Na₃ASO₄

          (E)     2FeSO₄ + H₂SO₄ + O → Fe₂(SO₄)₃ + H₂O

          (F)      2KI + H₂O + O → 2KOH + I₂

          (G)     HNO₂ + O → KNO₃

          (H)     2K₄[Fe(N)₆] + H₂SO₄ + O → 2K₃[Fe(CN)₆] + K₂SO₄ + H₂O

           (I)      It oxidizes acidified solution of K₂Cr₂O₇ to blue chromium peroxide.

                   K₂Cr₂O₇ + H₂SO₄ + 4 H₂O₂ → 2 CrO₅ + K₂SO₄ + 5H₂O

                   CrO₅ is CrO(O₂)₂ , Cr oxidation no. is 6.

 

4. Bleaching properties – Due to oxidizing in nature it acts as a bleaching agents. It bleaches wool, silk, hairs etc.

5. Reducing properties –

It reduces

          (A)    Cl₂ + H₂O₂ → 2HCl + O₂

          (B)    O₃ + H₂O₂ → 2O₂ + H₂O

          (C)    Ag₂O + H₂O₂ → 2Ag + H₂O + O₂

          (D)    PbO₂ + H₂O₂ → Pb O + H₂O + O₂

          (E)     MnO₂ + H₂O₂ → Mn O + H₂O + O₂

           (F)     2KMnO₄ + 3H₂SO₄ + 5H_­2­O₂ → K₂SO₄ + 2MnSO₄ + 8H₂O + 5O₂

           (G)     2K₃[Fe(CN)₆] + 2KOH + H₂O₂ → 2 K₄[Fe(CN)₆] + 2H₂O + O₂­

 

Uses –

1. As an antiseptic in washing wounds ears etc.
2. As an antichlor i.e., to remove excess of chlorine from bleached fabrics
3. To regain the whiteness of old oil – paintings since initial lead oil painting react with moisture and produce PbS (black color). If we use H₂O₂ to clean the painting the Pb s converted into PbSO₄ (white)
4. For bleaching wool, silk, ivory etc.
5. As an oxidizing agent in alkaline and acidic media.
6. As a propellant in rockets.

 

Structure of H₂O₂: –

X-rays measurements and dipole moment indicate that H­₂O₂ molecule is non-linear, non-planer.

It is proposed that Hydrogen peroxide is tautomeric mixture of following two structures –

Baeyer’s formula   Kingzett’s formula

 

	R1 (O–O  bond  length)	R2 (O–H bond length)	θ1	θ2
In liquid phase	1.46 Ao	0.97 Ao	97o	94o
In gas phase	1.475 Ao	0.95 Ao	94.8o	111.5o
In crystalline phase	0.988 Ao	1.458 Ao	101.9o	90.20