OZONE O3 = Ozone was first observed through a rotten smell by Van Marum in 1758 which was confirmed by Schonbiln in 1840. Sorret established its formula O3, , and pointed out that the ozone is an allotrope of oxygen.
Ozone prevents the living world from the harmful radiations (UV) coming from sun. Its layer lies 12 to 15 miles above earth’s surface. Near earth’s surface it is decomposed by dust particles.
Preparation of ozone
Ozone is prepared in laboratory by passing silent electric discharge through dry oxygen. By passing the electric current some of the oxygen molecules dissociate and then atomic oxygen combines with oxygen molecules to form ozone.
The mixture thus obtained contains 5 – 10% ozone by volume and the mixture is called as ozonised oxygen.
The apparatus used for this purpose is known as ozoniser.
Recovery of pure ozone from ozonised oxygen
The ozonised oxygen is passed through a spiral cooled by liquid air. Ozone condenses as its condensation temperature is 112.4°C. The liquid ozone thus obtained may contain some dissolved oxygen. This liquid is fractionally distilled to get pure ozone.
Some other ozone forming reactions
(i) Fluorine reacts with water at a very low temperature when mixture of ozonised oxygen is formed.
2F2 + 2H2O → 4HF + O2
2F2 + 3H2O → 6HF + O3
(ii) By action of ultraviolet rays, oxygen can be converted into ozone.
(iii) The electrolysis of acidified water with high current density and platinum anode yields ozone on the anode. Nascent oxygen discharged at anode combines with molecular oxygen to form ozone.
(i) It is having characteristic rotten smell with pale blue colour.
(ii) It can be liquefied into pale blue liquid at 112.4°C. If we reach at 249.7°C, it converts into violet black crystals.
(iii) It is heavier than air.
(iv) Solubility – It is slightly soluble in water but more soluble in turpentine oil, glacial acetic acid or carbon tetrachloride.
(v) It causes headache and nausea when inhaled in small amount.
(vi) Moist iodine is oxidised to iodic acid
(d) Formation of ozonides
Structure of ozone
Tests of ozone
(i) It has a strong rotten smell.
(ii) Metallic mercury loses its fluidity in contact with O3 .
(iii) It turns an alcoholic solution of benzidine brown.
(iv) It turns an alcoholic solution of tetramethyl base violet.
(v) It turns starch – iodide paper blue.
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