Introduction,structure and preparation of Ozone

Introduction to Ozone

Ozone is an allotrope of oxygen. Oxygen (O₂) and ozone (O₃) are the two allotropes of the same element oxygen. Allotropy is the property by virtue of which an element may exist in different forms in same physical state differing in physical properties and also to some extent in chemical properties.

Occurrence of Ozone

The ozone layer lies in the stratosphere (upper atmosphere, 10-50 km from earth's surface) between 12-25 kilometers from the earth surface where its concentration is about 10 ppm. Naturally, ozone is formed in the stratosphere from O₂ molecules. O₂ molecules split into atoms as they absorb ultraviolet radiation from the sun. The oxygen atom then combines with the O₂ molecule to give ozone molecule.

$$O_2(g)\longrightarrow{O+O}$$

$$O_2(g)+O\longrightarrow{O_3}$$

In the lower level of atmosphere, it also gets formed due to decomposition of NO₂ gas by sunlight. NO₂ gas is formed at atmosphere due to oxidation of pollutant NO gas.

$$NO_2(g)\xrightarrow{uv}{NO+O}$$

$$O(g)+O_2\longrightarrow{O_3(g)}$$

Structure of Ozone

The microwave studies have shown that O₃ molecule is V-shaped with bond angle 116.8° and bond length 1.278A°. The bond length is intermediate between that for O-O single bond (1.48A°) and for O = O double bond (1.21A°). Therefore, O₃ is considered to be a resonance hybrid of the following two canonical forms. Each O-atom may be thought as roughly sp²-hybridised.

Preparation of Ozone

Ozone is best prepared by silent electric discharges through pure and dry oxygen.

$$3O_2\rightleftharpoons{2O_3-96\;cal}$$

Different types of ozonizers can be used for this purpose. The ozonizer contains an annular space between two tubes through which oxygen passes. In Siemen'sozoniser, the inner tube is of iron or glass and it's concentric with outer one of glass, partly coated with tin foil. Both these are connected to the two secondary terminals of an induction coil and the ozone produced is continuously swept out by the fresh current of oxygen.

Properties of ozone

Pure ozone is a pale blue gas and liquid form is deep blue. It's heavier than air, slightly soluble in water and readily soluble in turpentine oil. It has the characteristic unpleasant fishy smell. In large quantities, it has poisonous nature. It's more powerful oxidizing agent than oxygen.

Uses of Ozone

Ozone can be used as a strong germicide and disinfectant, sterilizing agent, to purify atmosphere in underground railways, tunnels, mine, kitchens, slaughter houses, theaters etc. and in ozonolysis of organic compounds to get carbonyl compounds. Ozone is used for bleaching ivory, wax, and delicate fabrics. It's also used in the production of synthetic camphor and artificial silk.

Depletion of Ozone

The ozone layer in the stratosphere absorbs ultraviolet rays from the sun. The layer doesn't allow the UV-rays to reach the earth surface and protects the earth from harmful effects caused by UV-rays. Thus ozone layer acts as a protective layer for the earth. NASA scientist discovered a hole in the ozone layer over America in 1958. It has been found that the ozone layer depletion is being increased day by day.

Substances responsible for ozone layer depletion:Different pollutants like carbon monoxide, nitric oxide, methane are responsible for the depletion of ozone layer. But more important than these, different chlorofluoro carbons (CFC's) are the majorly responsible substances for such depletion. CFC is an extremely stable, non-corrosive and inert compound containing chlorine fluorine and carbon. CFCl₃ (Freon-11), CHClF₂ (Freon-22), CF₄ are some of the CFC's. CFC's are also called freons or Teflon. CFC's are used as coolants in refrigerators, air-conditioners, and propellants in the aerosol spray. These are used in scents, plastic foams heat pump and as a solvent for DDT. CFC remains in the atmosphere for a long time and finally migrates to the stratosphere and depletes the O₃ layer. R₁₁ and R₁₂ are the major CFC's applied in Nepal.

How does CFC deplete ozone layer?

CFC absorbs UV-rays from the sun and decomposes to give chlorine atom. The Cl-atom decomposes ozone. The Cl atom is regenerated from ClO in the presence of UV-light as shown in the reaction below. Thus generated Cl atom combines with another O₃ molecule. Furthermore, oxygen atom generated along with Cl atom combines with an ozone molecule. This means thousands of ozone molecules are destroyed through the chain reaction. This creates a depletion of ozone layer. It is estimated that one molecule of CFC can destroy 1,00,000 ozone molecules.

$$CHClF_2(g)\xrightarrow{uv}{CHF_2\;(g)+Cl\;(g)}$$

$$Cl+O_3\longrightarrow{ClO+O_2}$$

$$ClO\xrightarrow{uv}{Cl+O}$$

$$O+O_3\longrightarrow{2O_2}$$

Harmful effects of ozone layer depletion

As the ozone layer depletes, the UV-rays can directly reach the earth surface. The UV-radiation causes the skin cancer. It's calculated that for every 1% decrease in the concentration of O₃ in stratosphere, skin cancer would increase by 2%. The other effects caused by UV-rays are eye diseases, the decrease in productivity of crops, deforestation, loss of immunity etc.

Attempts for ozone protection

A convention in Montreal was held for the protection of ozone layer where Nepal had signed. Nepal has been celebrating September 16 as Ozone Day from 1997. Many countries have banned for the use of CFC and many have reduced the use.

Bibliography:

Gewali, Dr. Mohan Bikram and Dr. Pradyumna Wagley. Principles of Chemistry. Kathmandu: Buddha Academic Publishers and Distributors Pvt. Ltd., 2009.