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 |
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.
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.
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).
It neutralises alkalis and carbonates.
5. Hydrogen peroxide is a strong oxidising agent in both acidic and basic media.
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.
7. Hydrogen peroxide acts as a bleaching agent due to the oxidation of colouring matter by nascent oxygen.
Colouring matter + O = Colourless matter
Hydrogen Peroxide Preparation
Merck’s Method
By the action of dilute acids on sodium peroxide.
Laboratory Method
The action of dilute acids on barium peroxide.
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
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:
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.
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.
Depending on its concentration level, hydrogen peroxide has a wide range of applications. Some of the popular ones are listed below.
1. Hydrogen peroxide that is moist cannot be dried over concentrated H2SO4 because
(a) It can catch fire
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
Answer: D
4. 30 volumes of H2O2 means
(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
Answer: D
6. The oxide that gives H2O2 on treatment with dilute acid is
Answer: B
7. In an aqueous solution, hydrogen peroxide oxidises H2S into
(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?
(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
