Hydrogen Peroxide

Hydrogen peroxide is one of the most common chemical substances that we come across in our daily lives. It is found in hair bleaching and medical products. In this lesson, we will dive deeper into learning what hydrogen peroxide actually is including its structure, properties and more.

Hydrogen peroxide is a chemical compound containing molecules of hydrogen and water. Its chemical formula is written as H2O2. When hydrogen peroxide is in its pure form it is usually seen as a clear liquid with a slight pale blue colouration. It has a higher viscosity than water. However, it is a thermodynamically unstable liquid that tends to decompose when subjected to light. This chemical is also found in the human body.

IUPAC Name Hydrogen Peroxide
Chemical Formula H2O2
Molar Mass 34.0147 g/mol
Density 1.45 g/cm³
Melting Point -0.43 °C
Boiling Point 150.2 °C

Structure of H2O2

It has a non-planar open book (skew) structure. The 0-H bond length is 95 pm and O-O bond length is 147•5 pm. The H-O-O bond angle is 94•8 and the dihedral angle (angle between the planes containing the H-O-O group) is 111.5° in the gas phase. The bond lengths and angles are slightly changed in liquid and solid phases due to hydrogen bonding. The bond angle between the two planes reduces to 90•2° in the crystalline state.

The structure of H2O2 can also be explained on the basis of valence bond theory where both the oxygen atoms are sp-hybridized. Two of these hybrid orbitals on each oxygen are occupied by lone pairs of electrons. The third hybrid orbital overlaps with s-orbital of a hydrogen atom to form O-H sigma bond while the fourth one forms a sigma bond with the half-filled hybrid orbital of the second oxygen atom.

Chemical Properties of H2O2

1. It decomposes rapidly on heating in the presence of finely divided metals, such as Cu, Au, Ag, Ca, Fe, Pt, MnO2, carbon, alkali oxides, dust and light.

H2O2 + H2O2 → 2 H2O + O2

∆H = -196 kJ

This constitutes an example of auto-oxidation and auto reduction. One molecule of H2O2 (O.N. of O–1) is oxidised to O2 (0.N.-0) while the other is reduced to H2O (O.N.–2).

2. It is diamagnetic and exhibits dipole moment 2.1 D.

3. It is dense (density 1-4 g/cm³), has a higher b.p. (425 K) and more viscous than water. This is because H202 is more highly associated via hydrogen bonding than water. Oxidation and reduction by H2O2 in the acidic medium are generally slow while it is rapid in alkaline medium.

4. It acts as a weak acid (dissociation constant 1.55 x 1012 at 293 K) and forms two series of salts i.e. hydroperoxides (acidic salts) and peroxides (normal salts).

H2O2 ⇔ H+ + HO2

H2O2 ⇔ H+ + O2²-

It neutralises alkalis and carbonates

5. Hydrogen peroxide is a strong oxidising agent in both acidic and basic media.

H2O2 + H+ +2e- → 2H2O (Acidic medium)

H2O2 + OH +2e- → 3H2O (Basic medium)

6. Hydrogen peroxide as a reducing agent in the presence of a strong oxidising agent in both alkali and acidic media and importantly oxygen is released every time.

H2O2 → 2H+ + 2e- + O2 (Acidic medium)

H2O2 + 2OH → 3H2O + 2e- +O2 (Basic medium)

7. Hydrogen peroxide acts as a bleaching agent due to oxidation of colouring matter by nascent oxygen.

H2O2 → 2 H2O + O

Colouring matter + O = Colourless matter

Hydrogen Peroxide Preparation

Merck’s Method

By the action of dilute acids on sodium peroxide.

Na2O2 + H2SO4 → Na2SO4+ H2O2 (30%)

Laboratory Method

Action of dilute acids on barium peroxide.

BaO28H2O + H2SO4 → BaSO4+ H2O2 + 8 H2O

In this method, anhydrous BaO2 cannot be used since the precipitated BaSO4 forms a protective layer around unreacted BaO2 thereby preventing further reaction.

By Bubbling CO2 Through a Paste of BaO2

BaO2 +H2O+ CO2 → BaCO3+ H2O2

By the Electrolysis of 50% H2SO4 (Manufacture of H202)

Electrolysis of 50 per cent solution of sulphuric acid is done using Pt as anode and graphite as the cathode. The reactions taking place are:

2H2SO4→ 2H++2 HSO4

At cathode:


At anode:

2HSO4 → H2S2O8 + 2e

Peroxodisulphuric acid is distilled with water under reduced pressure when low boiling H2O2 distils over along with water leaving behind high boiling H2SO4.

In a modification of the above method, an equimolar mixture of H2SO4 and ammonium sulphate can also be taken as an electrolyte to give a more concentrated solution of hydrogen peroxide.

By Auto Oxidation of 2-Ethyl Anthraquinone (10% solution in benzene and cyclohexane)

Air is bubbled through the solution to get H2O2 and 2-ethyl anthraquinone which is reduced by H2 in the presence of Pd catalyst to give back 2-ethyl anthraquinone.

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 concentrated H2SO4 as a dehydrating agent.
  • Further concentration to 99% is obtained by distillation under reduced pressure. Last traces of moisture in 99% 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 sending a few crystals of solid H2O2.
  • When needle-shaped crystals of 100 per cent H202 separate out these crystals are removed dried and melted to get pure H2O2.

H2O2 Storage

H2O2 is not stored in glass bottles since the alkali oxides present in glass tend to catalyse its decomposition. It is, therefore, stored in Teflon bottles or paraffin wax coated plastic bottles. Small amounts of acid, glycerol, alcohol, acetanilide and H3PO4 are often used as stabilizers to check its decomposition.

Hydrogen Peroxide Uses

Depending on its concentration level, hydrogen peroxide has a wide range of applications. Some of the popular ones are listed below.

  • Hydrogen peroxide is an important chemical that is used in the treatment of domestic and industrial effluents.
  • It is used in pulp and paper bleaching.
  • In detergents.
  • Waste-water treatment.
  • Disinfecting wounds.
  • High concentration H2O2 is used as a propellent.

Solved Questions

1. Hydrogen peroxide that is moist cannot be dried over concentrated H2SO4 because

(a) It can catch fire

(b) It is reduced by H2SO4

(c) It is oxidised by H2SO4

(d)it decomposes H2SO4

Answer: C

2. Hydrogen peroxide molecules are

(a) Monatomic and form X22- ions

(b) Diatomic and form X2, ions

(c) Diatomic and form X ions

(d) Monoatomic and form Xions.

Answer: C

3. Decomposition of H202 is favoured by

(a) Traces of acids

(b) alcohol

(c) Acetanilide


Answer: D

4. 30 volumes of H2O2, means

(d) MnO.

(a) 30% H2O2 solution

(b) 30 cm³ of the solution contains 1 g of H2O2

(c) 1 cm³ of the solution liberates 30 cm³ of O2 at STP

(d) 30 cm³ of the solution contains 1 mole of H2O2

Answer: C

5. The O—O—H bond angle in H2O2 is


(b) 109•28°

(c) 120°

(d) 97°

Answer: D

6. The oxide that gives H2O2 on treatment with dilute acid is

(a) PbO2

(b) Na202

(c) MnO2


Answer: B

7. In aqueous solution hydrogen peroxide oxidises H2S into

(a) Sulphur

(c) Caro’s acid

(b) Sulphuric acid

(d) Marshall’s acid

Answer: A

8. For making H2O2 in the laboratory

(a) MnO2 is added to dilute cold H3PO4

(b) BaO2 is added to CO2 bubbling through cold water

(c) PbO2 is added to an acidified solution of KMnO4

(d) Na2O2 is added to boiling water

Answer: B

9. HCI is added to the following oxides. Which one would give H2O2?

(a) MnO2

(b) PbO2

(c) BaO

(d) None of the above

Answer: D

10. Which one of the following statements, if any, regarding hydrogen peroxide is false?

(a) It is a strong oxidizing as well as a reducing agent in acidic as well as the basic medium

(b) It is decomposed by MnO2

(c) It is more stable in a basic solution

(d) It behaves as a reducing agent towards acidified KMnO

Answer: C

11. The hair dyes available in the market generally contain two bottles, one containing dye and the other hydrogen peroxide. Before applying the dye, the two solutions are mixed. The hydrogen peroxide,

(a) Is added to dilute the solution of the dye

(b) Oxidises the dye to give the desired colour

(c) Reduces the dye to give the desired colour

(d) Acidifies the solution of the dye.

Answer: B