(N/A) The manufacture of sulphuric acid by the Contact process involves three main steps:
$1.$ Burning of sulphur or sulphide ores to form $SO_2$.
$2.$ Catalytic oxidation of $SO_2$ to $SO_3$ using $O_2$ in the presence of $V_2O_5$ catalyst: $2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g)$,$\Delta H = -196.6 \ kJ/mol$.
$3.$ Absorption of $SO_3$ in $H_2SO_4$ to produce oleum $(H_2S_2O_7)$,which is then diluted with water.
The key step for maximizing yield is the second step. According to Le Chatelier's principle:
- Since the reaction is exothermic,a low temperature (approximately $720 \ K$) favors the forward reaction.
- Since there is a decrease in the number of moles of gas ($3$ moles of reactants to $2$ moles of product),a high pressure (approximately $2 \ bar$) favors the forward reaction.