Question

Why does sulphur dioxide gas evolved when $\mathrm{HCl}$ gas is added to Sodium thiosulfate?

Answer 1

Reaction of Hydrochloric acid and Sodium thiosulfate:-

• When Hydrochloric acid is treated with Sodium thiosulfate$\left({\mathrm{Na}}_2{\mathrm{S}}_2{\mathrm{O}}_3\right)$, Sodium chloride, Sulphur dioxide, water, and sulphur are formed as its product.
• The complete reaction is:

$\underset{\mathrm{Sodium}\mathrm{thiosulphate}}{\mathrm{N}{\mathrm{a}}_2{\mathrm{S}}_2{\mathrm{O}}_3\left(\mathrm{aq}\right)}+\underset{\mathrm{Hydrochloric}\mathrm{acid}}{2\mathrm{HCl}\left(\mathrm{aq}\right)}\to \underset{\mathrm{Sodium}\mathrm{chloride}}{2\mathrm{NaCl}\left(\mathrm{aq}\right)}+\underset{\mathrm{Water}}{{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}+\underset{\mathrm{Sulphur}\mathrm{dioxide}}{\mathrm{S}{\mathrm{O}}_2\left(\mathrm{g}\right)}+\underset{\mathrm{Sulphur}}{\mathrm{S}\left(\mathrm{s}\right)}$

• Here we can see that Sulphur dioxide gas is being evolved.
• This is because Sulphur is a non-metal and it does not react with acids.
• Also sulphur is less reactive when compared to hydrogen.
• As a result, the hydrogen is not displaced by the sulphur atom from Hydrochloric acid.
• This is the reason why Sulphur dioxide gas evolved when $\mathrm{HCl}$ gas is added to Sodium thiosulfate$\left({\mathrm{Na}}_2{\mathrm{S}}_2{\mathrm{O}}_3\right)$.