Chemical Properties of Hydrogen Peroxide

Hydrogen peroxide has an interesting chemistry because of its ability to function as oxidant as well as a reductant in both acid and alkaline solutions. The oxidation state of oxygen in hydrogen peroxide is -1. It can therefore be oxidized to O₂ (zero oxidation state) or reduced to H₂O or OH^- (-2 oxidation state for oxygen). On the whole, hydrogen peroxide is a very powerful oxidizing agent and a poor reducing agent.

Stability

Thermodynamically, Hydrogen peroxide is unstable as is shown by the following equation:

The reaction from left to right is a spontaneous process and is accompanied by a decrease of free energy. However, the decomposition of Hydrogen peroxide at 25°C in the absence of catalysts is slow. Platinum, silver, copper, cobalt, manganese dioxide, iron, etc. are catalysts, which accelerate its decomposition. There are also a few stabilizers such as, acids, acetanilide, stannates and pyrophosphates.

At concentrations greater than 65 percent, hydrogen peroxide forms highly explosive mixtures with many of the organic compounds. Ignition occurs when it is brought in contact with them.

Acidic nature

Hydrogen peroxide is a weak acid. For the equilibrium,

its dissociation constant is 1.5 x 10^-12 at 20°C.

With alkalies, the corresponding peroxides are given.

Oxidizing properties

Because hydrogen peroxide provides an atom of (nascent) oxygen readily, it acts as a strong oxidizing agent in acidic as well as alkaline solutions.

Some typical oxidation reactions given by H₂O₂ are enunciated.

It oxidises lead sulphide to lead sulphate (in neutral solution)

It oxidizes acidified ferrous sulphate to ferric sulphate (in acidic medium)

It oxidizes acidified potassium iodide to iodine (in acidic medium)

It oxidizes acidified sodium arsenite to sodium arsenate

It oxidizes potassium ferrocyanide to ferrocyanide

It oxidizes acidified potassium dichromate,

It gives a deep blue solution containing CrO₅, which is fairly stable in ethereal solution.

Reducing properties

In the presence of other oxidizing agents, hydrogen peroxide acts as a reducing agent. This is because it can take up an atom of oxygen to give water and oxygen gas.

It reduces silver oxide to silver.

It reduces chlorine to hydrochloric acid.

It decolourises pink colour of acidified solution of potassium permanganate.

It gives a green colour with acidified potassium dichromate solution.

It reduces alkaline potassium ferricyanide to potassium ferrocyanide.

It reduces ozone to oxygen

Addition reactions

Hydrogen peroxide is capable of adding itself to double bond.

As a bleaching agent

It acts as a bleaching agent for delicate materials like wool, silk, ivory. It bleaches hair to golden yellow colour. The bleaching action is due to its oxidizing character of H₂O₂.

Uses of hydrogen peroxide

• As hydrogen peroxide decomposes readily to give oxygen, it is used as a rich source of oxygen required to ignite liquid fuel in jet and rocket engines.

• It is used as a mild bleaching agent for bleaching delicate articles such as hair, silk, wood, etc.

• It is used as an antiseptic in surgery and surgical treatments.

• It restores the colour of the old lead paints blackened by the action of H₂S. The lead sulphide (black) gets oxidized to lead sulphate (white)

• It is used as an oxidizing agent in laboratory.

Hydrogen peroxide v/s ozone

Problems

5. What is the mass of hydrogen peroxide present in 1 litre of a 2M solution? Calculate the volume of oxygen at STP liberated upon the complete decomposition of 100 cm³ of the above solution.

Solution

2 M solution of H₂O₂ means 2 mol of H₂O₂ in one litre of the solution.

Molar mass of H₂O₂ = (2x1 + 2x16) g/mol =(2 + 32) g/mol = 34 g/mol

Thus, there are 68 g (= 2 x 34 g) of H₂O₂ in one litre of the solution. So, Mass of H₂O₂ in 100 mL of solution

Therefore, the volume of oxygen (at STP) liberated from the complete decomposition of 100 mL of 2M H₂O₂ solution is 2.24 L.

6. Calculate the strength in volumes of a solution containing

30.36 g/litre of H₂O₂.

Solution

So if we calculate the volume of O₂, which is liberated by 30.36 g of H₂O₂ at NTP, this will correspond to volume strength of H₂O₂.

As per the equation: 2 x 34 = 68 g of H₂O₂ will liberate 22.4 L of O₂ at NTP

68 g of H₂O₂ will produce O₂ at NTP = 22.4 L

The given solution of H₂O₂ produces 10 L of O₂ at NTP.

Volume strength = 10 volumes.