Question

What is acid rain? Write balanced reactions to explain its formation. How does acid rain affect a marble statue?

Answer 1

Part 1: Acid rain

When an excess amount of oxides of Sulfur($\mathrm{S}$) and Nitrogen($\mathrm{N}$) which are released by cars and industrial processes react with rainwater and results in precipitation that is highly acidic, with a pH less than 5.6 is called acid rain.

• When oxides of Sulfur and Nitrogen react with water vapours present in the atmosphere Sulphuric acid and Nitric acid are formed respectively.
• Also, some of the organic acids are formed like Formic and Acetic acids.

Part 2: Chemical reactions

• The formation of Sulfuric acid $\left({\mathrm{H}}_2{\mathrm{SO}}_4\right)$ and Nitric acid $\left({\mathrm{HNO}}_3\right)$ takes place when oxidation of Sulphur dioxide$\left({\mathrm{SO}}_2\right)$ and Nitrogen dioxide$\left({\mathrm{NO}}_2\right)$.

$$\begin{gathered} \underset{(\mathrm{Sulphur}\mathrm{dioxide})}{2{\mathrm{SO}}_2\left(\mathrm{g}\right)}+\underset{\left(\mathrm{Water}\right)}{2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}+\underset{(\mathrm{Oxygen}\mathrm{gas})}{{\mathrm{O}}_2\left(\mathrm{g}\right)}\to \underset{(\mathrm{Sulphuric}\mathrm{acid})}{{\mathrm{H}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)} \\ \underset{(\mathrm{NItrogen}\mathrm{dioxide})}{4{\mathrm{NO}}_2\left(\mathrm{g}\right)}+\underset{\left(\mathrm{Water}\right)}{2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}+\underset{(\mathrm{Oxygen}\mathrm{gas})}{{\mathrm{O}}_2\left(\mathrm{g}\right)}\to \underset{(\mathrm{NItric}\mathrm{acid})}{4{\mathrm{HNO}}_3\left(\mathrm{aq}\right)} \end{gathered}$$.

• Also, water and Carbon dioxide react to form Carbonic acid.

$\underset{(\mathrm{Carbon}\mathrm{dioxide})}{{\mathrm{CO}}_2\left(\mathrm{s}\right)}+\underset{\left(\mathrm{Water}\right)}{{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}\to \underset{(\mathrm{Carbonic}\mathrm{acid})}{{\mathrm{H}}_2{\mathrm{CO}}_3\left(\mathrm{aq}\right)}$.

Part 3: The harmful effect of acid rain on marble statues

• Marble statues are largely affected by acid rains. When acid rain falls on the marble, the colour of the statue fades away, surfaces get roughened up and also due to corrosion, damage could also happen and loss of carved details could be seen.
• Marble which is made up of calcium carbonate reacts with sulphuric acid to form calcium sulphate.

$\underset{(\mathrm{Calcium}\mathrm{carbonate})}{{\mathrm{CaCO}}_3\left(\mathrm{s}\right)}+\underset{(\mathrm{Sulphuric}\mathrm{acid})}{{\mathrm{H}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)}\to \underset{(\mathrm{Calcium}\mathrm{sulphate})}{{\mathrm{CaSO}}_4\left(\mathrm{s}\right)}+\underset{\left(\mathrm{Water}\right)}{{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}+\underset{(\mathrm{Cabon}\mathrm{dioxide})}{{\mathrm{CO}}_2\left(\mathrm{g}\right)}$.