The Contact Process is the name given for the manufacture of Sulphuric Acid (H2SO4(l)). There must also be specific conditions of temperature, pressure and catalyst for the reaction to occur effectively. I will also look at the effect of temperature, pressure and catalyst on the composition of the equilibrium mixture, with respect to Le Châtelier’s principle.
Firstly, Sulphur Dioxide (SO2(g)) is manufactured by burning Sulphur (S(s)) in air. The sulphur dioxide is then purified.
S(s) + O2(g) →SO2(g)
Secondly, Sulphur trioxide (SO3(g)) is produced by reacting sulphur dioxide with O2(g) over a vanadium(v) oxide, V2O5 catalyst at 450 oC and 2 atm.
2SO2(g) + O2(g) ⇌2SO3(g)
Thirdly, oleum is formed by dissolving concentrated sulphuric acid in the sulphur trioxide.
SO3(g) + H2SO4(l) → H2S2O7(l)
Lastly, oleum is diluted with water, forming concentrated sulphuric acid.
H2S2O7(l)+ H2O(l) → 2H2SO4(l)
It should be noted that due to the reaction being extremely exothermic, causing the sulphuric acid to vaporise forming a highly corrosive mist, the sulphur trioxide cannot be directly added to the water.
According to Le Chatelier’s Principle, Increasing the concentration of oxygen in the mixture causes the position of equilibrium to shift towards the right. There are 3 molecules in the reactants and 2 in the product. There is more concentration on the left so increasing the concentration will favour the forward reaction. Since the oxygen comes from the air, this is a very cheap way of increasing the conversion of sulphur dioxide into sulphur trioxide.
You need to shift the position of the equilibrium as far as possible to the right in order to produce the maximum possible amount of sulphur trioxide in the equilibrium mixture.
The forward reaction (the production of sulphur trioxide) is exothermic, and according to Le Chatelier’s Principle, this will be favoured if you lower the temperature. The system will respond by moving the position of equilibrium to counteract this – in other words by producing more heat.
According to Le Chatelier’s Principle, if you increase the pressure the system will respond by favouring the reaction which produces fewer molecules. That will cause the pressure to fall again. In order to get as much sulphur trioxide as possible in the equilibrium mixture, you need as high a pressure as possible. High pressures also increase the rate of the reaction.
The catalyst has no effect whatsoever on the position of the equilibrium. Adding a catalyst doesn’t produce any greater percentage of sulphur trioxide in the equilibrium mixture. Its only function is to speed up the reaction.
Courtney from Study Moose