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

Last updated date: 13th Jun 2024
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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.

> 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.

- 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.