Introduction to Ozone

Ozone is an important allotropes of oxygen. It is a triatomic molecule in which three oxygen atoms are covalently linked to each other. It is in gaseous state at room temperature.
The ozone layer lies in stratosphere which lies 10-50 kilometre from the surface of the earth. Naturally, ozone is formed from the oxygen molecules. When oxygen molecule splits into atom, it absorbs ultraviolet radiation from the sun. The oxygen then combine with O₂ molecule to give ozone. The reaction involved are shown below:
$$\ce{O2->[hv]O + O}$$
$$\ce{O2 + O->O3}$$
In lower level of atmosphere, it is formed due to the decomposition of NO₂ gas by sunlight. NO₂ gas is formed at the atmosphere due to the oxidation of pollutant NO gas.
$$\ce{NO2->[hv]NO + O}$$
Structure of ozone:
Ozone is a V-shaped molecule having bond angle of 116.8^o and bond length o

Preparation and Properties of ozone

Ozone in lab is prepared by passing silent electric discharge through pure and dry oxygen in a special apparatus called ozonizer.
$$\ce{3O2->[\text{Electric discharge}]2O3 + 284KJ/mole}$$
This reaction is exothermic in nature. Two types of ozonizer is used in preparation of ozone in lab.
a. Simen’s Ozonizer
b. Brodie’s Ozonizer
a. Simen’s Ozonizer: The apparatus consists of two co-axial glass tube coated with tin foil in the inner side of the inner tube and outer side of outer tube. The inner and outer tin coating is connected to the terminal of the induction coil. A slow current is passed through the annular space which causes the silent electric discharge. This ozone is 10-15 percent pure. The yield of oxygen can be increased by passing pure and dry oxygen at low pressure and high temperature.The reaction involved in Simen’s ozonizer are:
$$\ce{O2->[\text{UV radiation}]O + O}$$
$$\ce{O2 + O->O3}$$

Simen’s Ozonizer

Brodie’s Ozonizer: This ozonizer works on the same principle as that of Simen’s ozonizer. It uses dilute H₂SO₄ instead of tin foil. In this ozonizer, two carbon or platinum electrodes are dipped in sulphuric acid solution. A slow current is passed through the annular space which causes the silent electric discharge. This ozone thus obtained is 15-20 percent pure.

Brodie’s Ozonizer

Physical properties of ozone:
– Ozone in gas is a pale light blue color and liquid in deeper blue.
– It is 1.5 times heavier than air.
– It has characteristics fishy smell.
– It is slightly soluble in water and highly soluble in turpentine oil and acetic acids.
– It is highly poisonous.
– It is more powerful oxidizing agent than oxygen.

Depletion of Ozone Layer

The ozone layer in stratosphere absorbs the harmful ultraviolet rays coming from the sun. This layer does not allow the uv rays to reach the earth surface. So, it acts as a protective layer for the earth. It has been found that the layer of ozone layer is thinning due to the excessive use of green house gases like carbon monoxide, nitric acid, Chloro Fluoro Carbon etc. This is called ozone layer depletion. Chloro Fluoro Carbon is major responsible factor for the depletion of ozone layer.
How does Chloro Fluro Carbon deplets ozone layer?
CFC absorbs uv rays from the sun and gets decomposed to give chlorine atom. The chlorine atom decomposes ozone. The chlorine atom is regenerated from ClO and combines with another ozone molecule. The oxygen atom generated at the decomposition of ClO combines with ozone to decompose it into oxygen molecule. Thousands of ozone molecules are being destroyed through this chain reaction. This creates a depletion of ozone layer. In this way ozone layer is depleted by CFC. The reaction involved is shown below:

CHClF2−→hvCHF2+Cl

Cl+O3⟶ClO+O2

ClO⟶Cl+O

O+O3⟶2O2

Note: Researchers have estimated that a single molecule of CFC can destroy one lakhs of ozone molecules.

Effect of ozone layer depletion:
– It causes hole in ozone layer which is called ozone hole.
– The harmful UV rays can reach the earth surface directly and causes skin cancer, blood cancer etc.
– It causes reduction in crops productivity.
– It can cause rise in temperature of the earth surface consequently leading to global warming.
– It can cause many life threatening diseases and the existence of living beings will be in vain.
So, researcher has urged all the people to reduce the use of harmful green house gases and focus on the alternative source of energy. Awareness campaign should be launched in order to aware all the living people in the earth.