Chapter 14: Environmental Chemistry

Ans: Due to the excessive growth of algae water contains a lot of phosphate due to the inflow of the fertilizers from the surroundings. The decomposition of algae leads to the bad smell and taste and even makes it unfit for  drinking , bathing, washing, swimming, boating etc. and concentration of dissolved oxygen also decreases which may be harmful for aquatic life. 

Ans: The symptoms which were observed in  the village indicate that oxides of sulphur and nitrogen are released from the chimneys of the factory. These gases are obtained as the product of combustion of fossil fuels such as coal,  gasoline etc. In an automobile engine, at high temperature, when fossil fuel is burnt, dinitrogen and dioxygen get combined to give significant quantities of nitric oxide and nitrogen dioxide. The chemical reactions can be given as-

\[{{\text{N}}_{\text{2}}}\text{+}{{\text{O}}_{\text{2}}}\text{(g)}\xrightarrow{\text{1483K}}\text{2NO(g)}\]

\[\text{2NO(g)+}{{\text{O}}_{\text{2}}}\text{(g)}\to \text{2N}{{\text{O}}_{\text{2}}}\text{(g)}\]

\[\text{S(s)+}{{\text{O}}_{\text{2}}}\text{(g)}\to \text{S}{{\text{O}}_{\text{2}}}\text{(g)}\]

Ans: The uncatalyzed oxidation of \[\text{S}{{\text{O}}_{\text{2}}}\]is a slow process. However, the presence of particulate matter in the polluted air catalyses the oxidation of \[\text{S}{{\text{O}}_{\text{2}}}\] to \[\text{S}{{\text{O}}_{\text{3}}}\]. The reaction for the conversion can be given as-

\[\text{2S}{{\text{O}}_{2}}\text{(g)+}{{\text{O}}_{2}}\text{(g)}\to 2\text{S}{{\text{O}}_{\text{3}}}\text{(g)}\]

The reaction can be promoted by \[{{\text{O}}_{3}}\] or by \[{{\text{H}}_{\text{2}}}{{\text{O}}_{\text{2}}}\].

\[\text{S}{{\text{O}}_{\text{2}}}\text{(g)+}{{\text{O}}_{\text{3}}}\text{(g)}\to \text{2S}{{\text{O}}_{\text{3}}}\text{(g)+}{{\text{O}}_{\text{2}}}\text{(g)}\]

\[\text{S}{{\text{O}}_{\text{2}}}\text{(g)+}{{\text{H}}_{\text{2}}}{{\text{O}}_{\text{2}}}\text{(l)}\to {{\text{H}}_{\text{2}}}\text{S}{{\text{O}}_{\text{4}}}\text{(aq)}\]

Ans: \[\text{N}{{\text{O}}_{\text{2}}}\]absorbs energy from sunlight and breaks up into nitric oxide and free oxygen atom and this can be shown as- 

\[\text{N}{{\text{O}}_{\text{2}}}\text{(g)}\xrightarrow{\text{hv}}\text{NO(g)+O(g)}\]

Here the oxygen atom produced is very reactive and combines with oxygen molecule in the air to give rise to ozone. 

\[\text{O(g)+}{{\text{O}}_{\text{2}}}\text{(g)+M}\to {{\text{O}}_{\text{3}}}\text{(g)+M}\]

Here ‘M’ is an inert gas like Nitrogen gas and ozone gets produced during the formation of smog.

Ans: Ozone in the stratosphere is formed when UV radiation falls on the dioxygen (\[{{\text{O}}_{\text{2}}}\] ) molecules. The UV radiations split apart molecular oxygen into free oxygen (O) atoms, these oxygen atoms combine with molecular oxygen to form ozone. The reactions involved can be given as-

\[{{\text{O}}_{\text{2}}}\text{(g)}\xrightarrow{\text{UV}}\text{O(g)+O(g)}\]

\[O\left( g \right)+{{O}_{2}}\left( g \right)\underset{{}}{\overset{UV}{\longleftrightarrow}}{{O}_{3}}\left( g \right)\]

Ans: Ozone is found to be thermodynamically unstable and undergoes decomposition into molecular oxygen and a dynamic equilibrium exists between the production and decomposition of ozone. The reactions involved can be shown as- 

\[{{\text{O}}_{\text{2}}}\text{(g)}\xrightarrow{\text{UV}}\text{O(g)+O(g)}\]

\[O\left( g \right)+{{O}_{2}}\left( g \right)\underset{{}}{\overset{UV}{\longleftrightarrow}}{{O}_{3}}\left( g \right)\]

Ans: Scientists working in the Antarctica region reported that depletion of the ozone layer  is commonly termed as an ozone hole over the South Pole in the Antarctic region. It was observed that a unique set of conditions were responsible for the ozone hole. In summer, nitrogen dioxide and methane react with chlorine monoxide and chlorine atoms forming chlorine sinks, preventing much ozone depletion, whereas in winter, special types of clouds called polar stratospheric clouds are formed over Antarctica. These polar stratospheric clouds provide a surface on which chlorine nitrate gets hydrolysed to form hypochlorous acid. It also reacts with hydrogen chloride produced to give molecular chlorine.

\[\text{Cl}\overset{\bullet }{\mathop{\text{O}}}\,(g)\text{+N}{{\text{O}}_{\text{2}}}\text{(g)}\to \text{ClON}{{\text{O}}_{\text{2}}}\text{(g)}\]

\[\overset{\bullet }{\mathop{\text{Cl}}}\,\text{(g)+C}{{\text{H}}_{\text{4}}}\text{(g)}\to \overset{\bullet }{\mathop{\text{C}{{\text{H}}_{\text{3}}}}}\,\text{(g)+HCl(g)}\]

\[\text{ClON}{{\text{O}}_{\text{2}}}\text{(g)+}{{\text{H}}_{\text{2}}}\text{O(g)}\to \text{HOCl(g)+HN}{{\text{O}}_{\text{3}}}\text{(g)}\]

\[\text{ClON}{{\text{O}}_{\text{2}}}\text{(g)+HCl(g)}\to \text{C}{{\text{l}}_{\text{2}}}\text{(g)+HN}{{\text{O}}_{\text{3}}}\text{(g)}\]

When sunlight returns to the Antarctica in the spring, then the sun’s warmth breaks up the clouds and HOCl and \[\text{C}{{\text{l}}_{\text{2}}}\]are photolyzed in the presence of sunlight. The chlorine radicals thus formed, initiate the chain reaction for ozone depletion. and the reactions can be given as-

\[\text{HOCl(g)}\xrightarrow{\text{hv}}\overset{\text{}}{\mathop{\text{OH}}}\,\text{(g)+}\overset{\text{}}{\mathop{\text{Cl}}}\,\text{(g)}\]

\[\text{C}{{\text{l}}_{\text{2}}}\text{(g)}\xrightarrow{\text{hv}}\text{2}\overset{\text{}}{\mathop{\text{Cl}}}\,\text{(g)}\]

Ans: Excessive amount of sulphate (i.e., greater than 500 ppm) in drinking water causes laxative effect, otherwise at moderate levels it is harmless.

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