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Questions
Give the reactions involved in the formation of smog?
How is photochemical smog formed?
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Solution
(i) Inside engine:
\[\ce{N2 + O2 -> 2NO}\];
Combustion of petrol produces hydrocarbons (HC)
(ii) In atmosphere:
1. Nitric oxide reacts with oxygen to form nitrogen dioxide.
\[\ce{2NO + O2 -> 2NO_2}\]
2. Nitrogen dioxide decomposes in the presence of sunlight (UV radiation).
\[\ce{NO2 ->[UV] NO + O}\]
3. Atomic oxygen combines with molecular oxygen to form ozone.
\[\ce{O + O2-> O3}\]
4. Ozone reacts with nitric oxide to form nitrogen dioxide.
\[\ce{NO + O3 -> NO2 + O2}\]
5. Hydrocarbons undergo photochemical reactions with nitrogen oxides, producing secondary pollutants such as PAN (peroxyacetyl nitrate).
\[\ce{HC + NOx + O2 ->[Sunlight] PAN + other products}\]
Thus, nitrogen dioxide (NO2), ozone (O3) and PAN are the major components of photochemical smog.
