Question

A student added concentrated Sulphuric acid to a substance and noted that a colourless gas was given off, which decolourised bromine water. He concluded that Sulphur dioxide had been produced by the reduction of Sulphuric acid.

(a) He may have been wrong in his first conclusion. Why?

(b) Name one substance which would produce Sulphur dioxide when acted upon by concentrated Sulphuric acid without involving the reduction of Sulphuric acid. Write a balanced equation for the reaction and use it to show that reduction of acid has not occurred.

Answer 1

(a)

1. When carbon $\left(\mathrm{C}\right)$ reacts with concentrated Sulphuric acid $\left({\mathrm{H}}_2{\mathrm{SO}}_4\right)$, it produces water $\left({\mathrm{H}}_2\mathrm{O}\right)$ and two colourless gases: Carbon dioxide $\left({\mathrm{CO}}_2\right)$ and Sulphur dioxide $\left({\mathrm{SO}}_2\right)$, through the reduction process.
2. The reaction takes place according to the following balanced chemical equation:
   $\mathrm{C}\left(\mathrm{s}\right)+\mathrm{conc}. 2{\mathrm{H}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)\to 2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)+2{\mathrm{SO}}_2\left(\mathrm{g}\right)+{\mathrm{CO}}_2\left(\mathrm{g}\right)$
3. Therefore, he may be wrong in his first conclusion.

(b)

1. The name of a substance that would produce Sulphur dioxide when acted upon by concentrated Sulphuric acid without involving the reduction of Sulphuric acid is Sodium sulphite $\left({\mathrm{Na}}_2{\mathrm{SO}}_3\right)$.
2. The balanced equation for the reaction between Sulphuric acid and Sodium sulphite is given below:
   ${\mathrm{Na}}_2{\mathrm{SO}}_3\left(\mathrm{s}\right)+\mathrm{dil}. {\mathrm{H}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)\to {\mathrm{Na}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)+{\mathrm{SO}}_2\left(\mathrm{g}\right)$
3. Sodium sulphite $\left({\mathrm{Na}}_2{\mathrm{SO}}_3\right)$ reacts with dilute Sulphuric acid $\left({\mathrm{H}}_2{\mathrm{SO}}_4\right)$ to give Sodium sulphate $\left({\mathrm{Na}}_2{\mathrm{SO}}_4\right)$ salt, water $\left({\mathrm{H}}_2\mathrm{O}\right)$ and Sulphur dioxide $\left({\mathrm{SO}}_2\right)$ gas, without involving the reduction of Sulphuric acid. Hence, the reduction of Sulphuric acid has not occurred.