ü  Ozone: Ozone is an allotropic form of oxygen.

Ø  Preparation: It is prepared by passing silent electric discharge through pure and dry oxygen 10 – 15 % oxygen is converted to ozone.

3 O2(g)→ 2 O3(g); ∆H= +142 kJ/mol

 

Since the formation of ozone from oxygen is an endothermic process, a silent electric discharge should be used, unless the ozone formed undergoes decomposition.

 

Ø  Properties:

·       Pure ozone is a pale blue gas.

·       Dark blue liquid and violet-black solid.

·       Ozone has a characteristic smell.

·      It is slightly soluble in water but more in turpentine oil, glacial acetic acid or CCl4.

·       O3 molecule is diamagnetic but O3– is paramagnetic.

·  Ozone is thermodynamically unstable with respect to oxygen since its decomposition into oxygen results in the liberation of heat (∆H is negative) and an increase in entropy (∆S is positive). So the Gibbs energy change (∆G) for this process is always negative (∆G = ∆H – T∆S).

·       Due to the ease with which it liberates nascent oxygen (O3 → O2 + O), it acts as a powerful oxidising agent. For e.g.,

It oxidises lead sulphide to lead sulphate   

PbS(s) + 4O3(g) → PbSO4(s) + 4O2(g)

It oxidises H2S to S

H2S + O3 ¾® H2O + S ¯ (yellow)

Oxides of nitrogen (particularly nitric oxide) combine very rapidly with ozone and deplete it. Thus, nitrogen oxides emitted from the exhaust systems of supersonic jet aeroplanes, slowly depleting the concentration of the ozone layer in the upper atmosphere.

NO(g) + O3(g) → NO2(g) + O2(g)

 

Ø  Bleaching Action: O3 also bleaches coloured substances through oxidation.

O3 ¾® O2 + [O]

                     Nacent Oxygen

Colouring Matter +[O] ¾® Decolourised Matter

 

Ø  Tests for Ozone:

i)                A filter paper soaked in a alcoholic benzidine becomes brown when brought in contact with O3 (this is not shown by H2O2)

ii)              Tailing of mercury Pure mercury is a mobile liquid but when brought in contact with O3 its mobility decreases and it starts sticking to glass surface forming a type of tail due to the dissolution of Hg2O (mercury sub-oxide) in Hg.

2Hg + O3 ¾® Hg2O + O2

Ø  Estimation of Ozone: When ozone reacts with an excess of potassium iodide solution buffered with a borate buffer, iodine is liberated. The liberated iodine can be titrated against a standard solution of sodium thiosulphate. This is a quantitative method for estimating O3 gas.


Ø  Uses:

·       It is used as a germicide, disinfectant and for sterilising water.

·       It is also used for bleaching oils, ivory, flour, starch, etc.

·       It acts as an oxidising agent in the manufacture of potassium permanganate.

·       For detecting the position of double bond in the unsaturated organic compounds.