Question

Which of the following is produced in the third step in contact process?

• A. Sulphur dioxide
• B. Sulphur trioxide
• C. Pyrosulphuric acid
• D. Sulphurous acid

Answer 1

The correct option is C Pyrosulphuric acid

In the first step, sulphur is converted to sulphur dioxide.
$S+O_2\to S{O}_2$
Second step involves reacting it with more oxygen to form sulphur trioxide in the presence of $V_2{O}_5$ catalyst.
$2S{O}_2+O_2\stackrel{V_2{O}_5}{\rightleftharpoons }2S{O}_3{\Delta }_{r}H=-196.6kJ/mol$
In the third step, instead of reacting sulphur trioxide with water to form sulphuric acid directly, an alternative, safer method is utilized to dissolve sulphur trioxide gas in sulphuric acid to form oleum, also known as pyrosulphuric acid. Oleum then react with water to form sulphuric acid.
$S{O}_3+H_{2}S{O}_4\to \underset{\text{(Oleum)}}{H_2{S}_2{O}_7}{H}_2{S}_2{O}_7+H_{2}O\to 2H_{2}S{O}_4$