Question

Equivalent weight of \[S{{O}_{2}}\]​ is found to be 32 in a reaction taking place in an acidic medium. This reaction is represented as:
\[S{{O}_{2}}+2{{H}_{2}}O\to S{{O}_{4}}^{2-}+4{{H}^{+}}+2{{e}^{-}}\]
State whether the given reaction takes place in backward direction or forward direction?

Answer 1

Hint: A redox reaction is balanced in an acidic medium by adding positive hydrogen ions to the product side. Also, in acidic medium water gets ionised to form ${{H}^{+}}\text{ and O}{{\text{H}}^{\text{-}}}$ ions.

Complete step by step answer:
Let us first analyse \[S{{O}_{2}}\] as a compound before moving onto the specifics of this reaction and determining its direction.
Sulphur dioxide is the chemical compound with the formula \[S{{O}_{2}}\]. It is a toxic gas responsible for the smell of burnt matches. It is released naturally by volcanic activity and is produced as a by-product of copper extraction and the burning of fossil fuels contaminated with sulphur compounds. It is also sometimes used as a preservative for dried apricots, dried figs, and other dried fruits, owing to its antimicrobial properties and ability to prevent oxidation.
Sulphur dioxide is an intermediate in the production of sulfuric acid, being converted to sulphur trioxide, and then to oleum, which is made into sulfuric acid.
\[\begin{align}
 & 2S{{O}_{2}}+{{O}_{2}}\xrightarrow[{{450}^{\circ }}C]{{{V}_{2}}{{O}_{5}}}2S{{O}_{3}} \\
 & S{{O}_{3}}+{{H}_{2}}S{{O}_{4}}(conc.)\to {{H}_{2}}{{S}_{2}}{{O}_{7}} \\
 & {{H}_{2}}{{S}_{2}}{{O}_{7}}+{{H}_{2}}O\to 2{{H}_{2}}S{{O}_{4}} \\
\end{align}\]

The reaction given to us is: \[S{{O}_{2}}+2{{H}_{2}}O\to S{{O}_{4}}^{2-}+4{{H}^{+}}+2{{e}^{-}}\]
Let us now observe the specifics of this reaction and the mechanism involved.
In the acidic medium, the water molecules get oxidised into the formation of positively charged Hydrogen ions and the oxidation of Sulphur Dioxide into the sulphate ion, with its oxidation state increasing from +4 to +6, as we can see in the above reaction.
Since, this is very much what happens in the given reaction, therefore we can conclude that the given reaction occurs in forward direction.

Note: Acidic conditions usually imply a solution with an excess of H+ concentration, hence making the solution acidic. The balancing starts by separating the reaction into half-reactions. However, instead of immediately balancing the electrons, balance all the elements in the half-reactions that are not hydrogen and oxygen.