Question

Some bacteria obtained their energy by oxidising sulphur, producing sulphuric acid as a by-product. In the laboratory or industrially, the first step in the conversion of sulphur to sulphuric acid is to produce sulphur dioxide. The sulphur dioxide is converted to sulphur trioxide, which reacts with water producing sulphuric acid.

(a) Name one catalyst used industrially, which speeds up the conversion of sulphur dioxide to sulphur trioxide.

(b) Write the equation for the conversion of sulphur dioxide to sulphur trioxide. Why does this reaction supply energy?

(c) What is the name of the compound formed between sulphuric acid and sulphur trioxide?

Answer 1

(a)

• A catalyst is used to speed up the chemical reaction.
• Vanadium pentoxide $\left({\mathrm{V}}_2{\mathrm{O}}_5\right)$ catalyst is used industrially to speed up the conversion of Sulphur dioxide $\left({\mathrm{SO}}_2\right)$ to Sulphur trioxide $\left({\mathrm{SO}}_3\right)$.

(b)

• The balanced chemical equation for the transformation of Sulphur dioxide $\left({\mathrm{SO}}_2\right)$ to Sulphur trioxide $\left({\mathrm{SO}}_3\right)$ by using oxygen $\left({\mathrm{O}}_2\right)$ gas is written below:
• $2{\mathrm{SO}}_2\left(\mathrm{g}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\stackrel{{\mathrm{V}}_2{\mathrm{O}}_5}{\to }2{\mathrm{SO}}_3\left(\mathrm{g}\right)+\mathrm{Heat}$
• This reaction releases heat energy to the surroundings because it is an exothermic reaction.

(c)

• Oleum or pyrosulphuric acid $\left({\mathrm{H}}_2{\mathrm{S}}_2{\mathrm{O}}_7\right)$ is formed as the product when concentrated Sulphuric acid is reacted with Sulphur trioxide.
• ${\mathrm{SO}}_3\left(\mathrm{g}\right)+\mathrm{conc}. {\mathrm{H}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)\to {\mathrm{H}}_2{\mathrm{S}}_2{\mathrm{O}}_7\left(\mathrm{aq}\right)$
• Hence, the name of the compound produced is oleum or pyrosulphuric acid.