Why does water covered with excessive algae growth become polluted?
Presence of excessive algae growth shows that water contains a lot of phosphate due to inflow of fertilizers, etc., from the surroundings. The decomposition of algae growth produces bad smell and unattractive appearance making it unfit for recreational use like swimming, boating etc. Further, decrease in dissolved oxygen may be harmful for aquatic species like fishes etc.
A factory was started near a village. Suddenly villagers started feeling the presence of irritating vapours in the village and cases of headache, chest pain, cough, dryness of throat and breathing problems increased. Villagers blamed the emissions from the chimney of the factory for such problems. Explain what could have happened. Give chemical reactions for the support of your explanation.
The symptoms observed in a village indicate that nitrogen oxide and sulphur oxide are released from the chimney of the factory. These are produced by the burning of fossil fuels such as gasoline, coal, natural gas etc. In an automobile engine, at high temperature when fossil fuel is burnt, dinitrogen and dioxygen combine to yield NO is i.e., nitric oxide.
$\begin{aligned} & \mathrm{N}_2+\mathrm{O}_2 \xrightarrow{1200-15001 \Upsilon\mathrm{C}} 2 \mathrm{NO} \\ & 2 \mathrm{NO}+\mathrm{O}_2 \xrightarrow{1100 \Upsilon\mathrm{C}} \mathrm{NNO}_2\end{aligned}$
$\mathrm{SO}_2$ is produced by burning of sulphur containing fossil fuel or by roasting of sulphide ores such a iron pyrites, copper pyrites etc.
$$\mathrm{Cu}_2 \mathrm{~S}+\mathrm{O}_2 \longrightarrow 2 \mathrm{Cu}+\mathrm{SO}_2 $$
0xidation of sulphur dioxide into sulphur trioxide in the absence of a catalyst is a slow process but this oxidation occurs easily in the atmosphere. Explain how does this happen? Give chemical reactions for the conversion of $\mathrm{SO}_2$ into $\mathrm{SO}_3$.
The oxidation of sulphur dioxide into sulphur trioxide can occur both photochemically or non-photochemically. In the near ultraviolet region, the $\mathrm{SO}_2$ molecules react with ozone photochemically.
$\mathrm{SO}_2+\mathrm{O}_3 \xrightarrow{h v} \mathrm{SO}_3+\mathrm{O}_2$
$2 \mathrm{SO}_2+\mathrm{O}_2 \xrightarrow{h v} 2 \mathrm{SO}_3$
Non-photochemically, $\mathrm{SO}_2$ may be oxidised by molecular oxygen in presence of dust and soot particles.
$$2 \mathrm{SO}_2+\mathrm{O}_2 \xrightarrow{\text { Particulates }} 2 \mathrm{SO}_3$$
From where does ozone come in the photochemical smog?
Sunlight cause photochemical decomposition of $\mathrm{NO}_2$ into NO and O.
$$\mathrm{NO}_2 \xrightarrow{h v} \mathrm{NO}+[\mathrm{O}]$$
Atomic oxygen is a highly reactive species. It combines with diatomic oxygen and forms ozone.
$$\mathrm{O}_2+\mathrm{O}+\mathrm{M} \longrightarrow \mathrm{O}_3+M$$
where, $M$ is inert gas such as nitrogen. This, $\mathrm{O}_3$ is formed during the formation of smog.
How is ozone produced in stratosphere?
Ozone in stratosphere is a product of action of UV-radiations on dioxygen $\left(\mathrm{O}_2\right)$ molecules. The UV radiation split apart molecular oxygen into free oxygen atoms. These oxygen atoms combine with the molecular oxygen to form ozone.
$\mathrm{O}_2(\mathrm{~g}) \xrightarrow[\text { UV-radiation }]{h v} \mathrm{O}+\mathrm{O}$
$\mathrm{O}_2(g)+\mathrm{O}(g) \underset{\text { radiations }}{\stackrel{\mathrm{UV}}{\rightleftharpoons}} \mathrm{O}_3$