NEET 2021 Chemistry Question 99 Solution

Sulfur dioxide ($SO_2$) emitted from burning fossil fuels is oxidized in the troposphere to sulfur trioxide ($SO_3$). In the absence of catalysts this is slow, but in polluted air with particulate matter and ozone it occurs rapidly, contributing directly to gaseous tropospheric pollution: $$2\text{SO}_2(g) + \text{O}_2(g) \longrightarrow 2\text{SO}_3(g) \implies \mathbf{\text{(a)-(iv)}}$$

In the Antarctic winter, polar stratospheric clouds form and convert inactive chlorine species ($ClONO_2$ and $HCl$) into hypochlorous acid ($HOCl$). In spring, when sunlight returns, $HOCl$ undergoes photolysis, releasing highly reactive chlorine free radicals ($\dot{C}l$) that initiate catalytic chain reactions destroying stratospheric ozone: $$\text{HOCl}(g) \xrightarrow{h\nu} \dot{\text{O}}\text{H} + \dot{\text{C}}\text{l} \implies \mathbf{\text{(b)-(iii)}}$$

Acid rain contains sulfuric and nitric acids. When sulfuric acid falls on marble monuments ($CaCO_3$, such as the Taj Mahal), it attacks the calcium carbonate, producing calcium sulfate, water, and carbon dioxide. This phenomenon is known as "stone cancer" caused by acid rain: $$\text{CaCO}_3 + \text{H}_2\text{SO}_4 \longrightarrow \text{CaSO}_4 + \text{H}_2\text{O} + \text{CO}_2 \implies \mathbf{\text{(c)-(i)}}$$

Nitrogen dioxide ($NO_2$) absorbs sunlight and photolyzes into nitric oxide ($NO$) and nascent oxygen ($O$). The reactive oxygen atom rapidly combines with atmospheric $O_2$ to form ozone ($O_3$), which subsequently oxidizes unburnt hydrocarbons to acrolein, formaldehyde, and peroxyacetyl nitrate (PAN), forming photochemical smog: $$\text{NO}_2(g) \xrightarrow{h\nu} \text{NO}(g) + \text{O}(g) \implies \mathbf{\text{(d)-(ii)}}$$