Ozone Gas : Van Marum experienced a peculiar smell near electric discharge machines in 1785. Schonbien in 1840 attributed this characteristic odor to the presence of a new gas and named it ozone. Soret 1866 determined its molecular formula. Its molecular formula is O3.
Where is ozone present?
Ozone is found in small amounts in ordinary air. Its concentration is higher in the upper atmosphere at about 25 km above sea level. This part of the atmosphere is called the ozone layer.
The Ozone layer absorbs the harmful ultraviolet rays emanating from the sun and thus protects the human body from many diseases. Due to the continuous air pollution of the atmosphere, the concentration of gases like NO in the atmosphere keeps increasing.
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By reaching the upper atmosphere, these gases decompose ozone and due to this the Ozone layer is getting thinner or in other words it is getting holes. In the future, if the atmosphere is not prevented from becoming more polluted, then the chances of developing diseases like skin cancer etc. will increase.
Preparation of Ozone
Ozone gas is obtained by conducting silent electric discharge in dry oxygen gas. The device used to make ozone is called Ozonizer.
Laboratory Methods of Ozone Preparation:
It consists of two concentric glass tubes connected at one end as shown in the figure. A thin sheet of tin is covered on both sides of these drains. The tin sheets are joined by an induction coil.
The induction coil acts as a high voltage source. Dry oxygen gas is slowly passed through these drains and on passing electric charge, oxygen turns into ozone. Ozonised oxygen is obtained by the above method, in which the percentage of ozone is about 10%.
It is a U shaped tube, one part of which is thin and the other part is wide. A test tube is attached in the widest part. Keep it in a wide vessel. Dilute sulfuric acid is filled in test tube and vessel.
A platinum wire is inserted into the vessel and test tube through which a silent electric discharge is passed through the induction coil. When dry oxygen is passed through it, it gets converted into ozone due to silent electric discharge in the device. About 12-14% ozone is obtained by this method.
Siemans and Halskes Ozoniser
Industrial production of ozone is done with the help of this method. As shown in the figure, it consists of an iron box which is held by two glass cylinders containing aluminum rods. Dry air is passed between the rod and the cylinder.
Oxygen gets converted into ozone when an electric discharge is passed through a potential difference of about 8000 to 10000 volts. Water is passed through the center of the device to keep it cool. The iron box is earthed.
Another method of obtaining a higher proportion of ozone is the electrolysis of a concentrated solution of parchloric acid at -50°C. Lead is used as cathode and platinum as anode.
Water is oxidized at the anode and ozonised oxygen is obtained, in which the proportion of ozone is high.
Method to separate ozone from ozonised oxygen:
The gas obtained from ozoniser is a mixture of ozone and oxygen. This mixture is passed through the drains cooled by liquid air. Ozone converts to liquid state at -120°C and oxygen at -183°C. On the heating of liquid air, both these gases change into liquid state.
Therefore, a blue liquid is obtained in which liquid oxygen and liquid ozone are in the form of solution. After fractional distillation of this blue liquid at low pressure, oxygen is obtained in the receiver as a colorless liquid and ozone remains as a blue liquid in the distillation flask.
It is a rotten fish-like gas. Its color is dark blue in the liquid state. Ozone is heavier than air, soluble in small amounts in water and soluble in large amounts in turpentine oil. It is purple in color in the solid state.
In the presence of silver platinum or peladium it decomposes to give oxygen.
2O3 → 3O2
The decomposition of ozone also occurs in the same way under the influence of heat.
2O3 → 3O2
The decomposition of ozonised oxygen occurs slowly while the decomposition of pure ozone is accompanied by rapid explosion.
As an oxidising agent:
It is a strong oxidising agent as it is easily decomposed to produce nascent oxygen.
O3 → O2 + O
Oxidizer of Sulfur:
It oxidises sulfur to sulfuric acid in the presence of water.
S + H2O + 3O3 → 3O2 + H2SO4
Oxidation of phosphorus:
In the presence of water it oxidises phosphorus to phosphoric acid.
2P + 3H2O + 5O3 → 5O2 + 2H3PO4
Oxidation of Arsenic: In the presence of water it oxidises arsenic to arsenic acid.
2As + 3H2O + 5O3 → 5O2 + 2H3AsO4
Oxidation of iodine: It converts moist iodine into iodic acid.
I2 + H2O + 5O3 → 5O2 + 2HIO3
Oxidation of silver: It oxidises silver to Ag2O.
2Ag + O3 → Ag2O + O2