Question

What happens when sulphur dioxide gas is passed through an aqueous solution of a Fe(III) salt?

Answer 1

Hint: As we know that Sulphur dioxide $S{{O}_{2}}$ is a colourless, heavy gas. It is produced in large amounts in intermediate steps of sulphuric acid production. Sulphur dioxide acts as a reducing agent in chemical reaction.

Complete answer:
> When sulphur dioxide reacts with water the following reaction takes place:
\[S{{O}_{2}}+2{{H}_{2}}O\to SO_{4}^{2-}+4{{H}^{+}}+2{{e}^{-}}\]
Consider the above reaction as 1. Here, we can see that sulphur is donating electrons and hence acts as electron donor (that is reducing agent).
- Now, further iron which is present in $F{{e}^{3+}}$ form will accept the electron and will form $F{{e}^{2+}}$. Here, we can see that change in oxidation state takes place from +3 to +2. We can see the following reaction that takes place, and consider it as reaction 2:
\[F{{e}^{3+}}+{{e}^{-}}\to F{{e}^{2+}}\]
- Now, from reaction 1 and 2, we can write the complete reaction as:
\[\begin{align}
& S{{O}_{2}}+2{{H}_{2}}O\to SO_{4}^{2-}+4{{H}^{+}}+2{{e}^{-}} \\
& \left[ F{{e}^{3+}}+{{e}^{-}}\to F{{e}^{2+}} \right]\times 2 \\
& S{{O}_{2}}+2{{H}_{2}}O+2F{{e}^{3+}}\to S{{O}_{4}}^{2-}+4{{H}^{+}}+2F{{e}^{2+}} \\
\end{align}\]

> Hence, we can conclude that when sulphur dioxide gas is passed through an aqueous solution of a Fe(III) salt, then $S{{O}_{2}}$ acts as a reducing agent and it reduces Fe(III) to Fe(II) that is ferric ions to ferrous ions.

Additional information:
- Sulphur dioxide is found to be mild toxic and can be harmful in high concentration. It is also found that $S{{O}_{2}}$ is a major air pollutant and also has a dangerous effect on human health. In the atmosphere it also can influence the plant and animal habitat.

Note: 

> We should not get confused in terms of oxidising and reducing agents. As an oxidising agent it gains electrons and is itself reduced in a reaction. It is also called an electron acceptor.
> Whereas, reducing agent loses electrons and itself gets oxidized. It is also called an electron donor.