Question

The compound which is responsible for the green colour formed when \[S{O_2}\] is bubbled through acidified potassium dichromate solution is:
A.Nitroso iron [II] sulphate
B.Iron [III] chloride
C.Chromium sulphate
D.Lead [II] chloride

Answer 1

Hint: When sulphur dioxide gas is passed through an acidified potassium dichromate solution, potassium dichromate gets reduced to its sulphate by redox reaction. This is a green coloured solution formed from orange dichromate solution.

Complete step by step answer:
The chemical reaction between acidified potassium dichromate and sulphur dioxide gas is characterised by colour change, from orange of dichromate to green of chromium sulphate. Sulphur dioxide gas reduces acidified potassium dichromate from \[C{r_2}O_7^{2 - }\](orange) to \[C{r^{3 + }}\](green) ions without leaving a residue and itself oxidized from \[SO_3^{2 - }\]ions in sulphuric acid to \[SO_4^{2 - }\]ions in the same acid.
\[{K_2}C{r_2}{O_7} + {H_2}S{O_4} + 3S{O_2} \to {K_2}S{O_4} + C{r_2}{(S{O_4})_3} + {H_2}O\]
Here, the oxidising agent is potassium dichromate while the reducing agent is sulphur dioxide. This results in a redox reaction. The orange-coloured dichromate solution will turn green due to the formation of chromium sulphate, \[C{r_2}{(S{O_4})_3}\]. Sulphuric acid is used as a dehydrating agent here.

Hence, the correct option is (C).

Additional information:
Potassium dichromate is an inorganic chemical reagent used as an oxidizing agent in chemical laboratories. It can oxidize alcohol into aldehydes and further carboxylic acid.

Note:
This reaction is a specific reaction used as a test for sulphur dioxide gas. If a filter paper is impregnated with potassium dichromate solution and it turns green, sulphur dioxide may be present (reduction of chromium(VI) to chromium(III)).