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.

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 oxygen atoms are sp-hybridised. Two of these hybrid orbitals on each oxygen are occupied by lone pairs of electrons. The third hybrid orbital overlaps with the s-orbital of a hydrogen atom to form an 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 is more viscous than water. This is because H₂0₂ is more highly associated with hydrogen bonding than water. Oxidation and reduction by H₂O₂ in the acidic medium are generally slow, while it is rapid in an 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⁺ + O₂^²-

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 is 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 the 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

The 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 BaO_2, 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 a 50 per cent solution of sulphuric acid is done using Pt as the anode and graphite as the cathode. The reactions taking place are as follows:

2H₂SO₄→ 2H⁺+2 HSO₄^–

At cathode:

2H⁺+2e→H2

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 a 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, a concentration of 99% is obtained by distillation under reduced pressure. The 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 stabilisers 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.
• It is used in detergents.
• It is used for waste-water treatment.
• It is used for disinfecting wounds.
• High-concentration H₂O₂ is used as a propellant.

Hydrogen Peroxide – Video Lesson

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 X₂^2- ions

(b) Diatomic and form X₂^–, ions

(c) Diatomic and form X^– ions

(d) Monoatomic and form X^– ions.

Answer: C

3. Decomposition of H₂0₂ is favoured by

(a) Traces of acids

(b) alcohol

(c) Acetanilide

(d) MnO

Answer: D

4. 30 volumes of H₂O₂ means

(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

(a)106°

(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

(d) TiO₂

Answer: B

7. In an 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 will give H₂O₂?

(a) MnO₂

(b) PbO₂

(c) BaO

(d) None of the above

Answer: D

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

(a) It is a strong oxidising 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