Hydrogen Peroxide

(i) Preparation

(a) Na₂O₂ + dil. H₂SO₄ H₂O₂ + Na₂SO₄

(b) BaO₂ + H₂SO₄ H₂O₂ + BaSO₄↓

(c) 3BaO₂ + 2H₃PO₄ 3H₂O₂ + Ba₃(PO₄)₂↓

(d) BaO₂ + H₂O + CO₂ H₂O₂ + BaCO₃

(ii) By electrolysis of Ammonium hydrogen sulphate (NH₄HSO₄) in presence of excess of H₂SO₄

(a) NH₄HSO₄ →  NH₄SO^-₄ + H+

at anode

2NH₄SO^-₄  →  (NH₄)₂S₂O₈ + 2e−

Ammonium perdisulphate

at cathode:−

2H+ + 2e− → H₂↑

On heating Ammonium perdisulphate in presence of water hydrolysis occurs and H2O2 is produced.

(NH₄)₂S₂O₈ + 2H₂O H₂O₂ + NH₄HSO₄

(iii) Auto oxidation of 2−ethyl or Butyl Anthraquinone

The aqueous product contains 20% H₂O₂.

PHYSICAL PROPERTIES

(1) Pale blue colour, diamagnetic, surupy liquid.

(2) Unstable in presence of sunlight.

(3) Have high dielectric constant and therefore its aqueous solution is very good solvent for ionic compounds.

CHEMICAL PROPERTIES

(1) Unstable in presence of sunlight

2H₂O₂(l) 2H₂O (l) + O₂↑

(2) Acidic nature: Weak acid

H₂O₂ → H⁺ +

It turns blue litmus red, decomposes carbnonates to CO2 and forms salts with bases.

(3) Oxidising and reducing behaviour:

Acts as both oxidizing agent and reducing agent because here oxygen has intermediate oxidation state of '−1' in H2O2.

(a) H₂O₂ as an oxidising agent:

In acidic and basic both medium H₂O₂ behaves as a very good oxidizing agent.

O.A.

It oxidising power can be understood better by

H₂O₂ + 2H⁺ + 2e- →  2H₂O E° = 1.77 V

+ H₂O + 2e- →  3OH- E° = 0.87 V

e.g. (i) PbS↓ + 4H₂O₂ →  PbSO₄↓ + 4H₂O

Black white

[H₂O₂ →  H₂O + [O] ] × 4

PbS + 4[O] →  PbSO₄

—————————————————

PbS + 4H₂O₂  →  PbSO₄ + 4H₂O

—————————————————

Therefore old oil painting rendered black by atmospheric H₂S can be whitened with H₂O₂.

(ii) Na₂SO₃ + H₂O₂ →  Na₂SO₄ + H₂O

(iii) KNO₂ + H₂O₂ →  KNO₃ + H₂O

(iv) Fe²⁺+ 2H⁺+ H₂O₂ →  Fe³⁺ + 2H₂O

Light green yellow

2FeSO₄ + H₂SO₄ + H₂O₂ →  Fe₂(SO₄)₃ + 2H₂O

(v) 2K₄[Fe(CN)₆] + H₂O₂ →  K3[Fe(CN)₆] + KOH

Ferrocyanide Ferriccyanide

(vi) 2I− + 2H⁺ + H₂O₂  →  I2↑ + 2H₂O

(vii) With acidic K₂Cr₂O7, deep blue colour of CrO₅ is obtained which have greater stability in ether.

 Cr₂O^-2₄+ H2O2 CrO₅ + H₂O

(b) H₂O₂ as Reducing agent:

H₂O₂ →  O₂ + 2[H]

e.g. (i) O₃ + H₂O₂ →  2O₂ + H₂O

(ii) Cl2 + H₂O₂  → 2HCl↑ + O₂↑

(iii) Decolourises violet colour of acidic KMnO₄.

2KMnO₄ + 3H₂SO₄ + H₂O₂ → 2MnSO4 + K₂SO₄ + 8H₂O + 5O₂↑

Violet almost colourless

In alkaline solution MnO₂ is formed.

2KMnO₄ + 3H₂O₂ → 2MnO₂↓ + 3O₂ + 2H₂O + 2KOH

Brown

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

(v) Ag₂O + H₂O₂ → 2Ag + H₂O + O2↑

(iv) Bleaching Action

Causes permanent bleaching as it occurs due to oxidising behaviour of H2O2.

H₂O2 → H₂O + [O]

Colour + [O] → Colourless

Test of H₂O₂

(i) It gives blue ethereal layer with acidic K₂Cr₂O₇ having some ether.

K₂Cr₂O₇ + H₂SO₄ + H₂O₂ → CrO₅ + K₂SO₄ + H₂O

forms blue

ethereal layer.

(ii) It gives Iodine with KI which turns starch paper blue.

2KI + H₂O₂ → 2KOH + I₂

Starch + I₂ → Blue Starch Iodine Complex

(iii) Forms orangish yellow solution with acidic TiO₂

TiO₂ + H₂SO₄ + H₂O₂ → orangish red or yellow (when dilute).

TiO₂.nH₂O or H₂[TiO₂(SO₄)₂]

pertitanic acid. peroxo−disulphate titanic acid

Note: The Concentration of H₂O₂ solution is measured in terms of volume strength, which you have learnt already in stoichiometry. 30% solution of H₂O₂ is sold in market under the trade name perhydrol.