A lot of large-scale industrial processes use reactions which are reversible. They are allowed to reach equilibrium as opposed to going to completion. The conditions applied, however, dictate the position of this equilibrium. In order to maximise the amount of product formed Le Chatelier’s principle can be used.

When finding the best conditions the position of equilibrium is not the only factor to be considered. Other important economic factors include:

• the yield of the product
• the rate of reaction
• the maintenance costs involved with reaction condition.

Each of these factors should be considered separately and conditions which maximise the product yield at a reasonable cost used.

Highlighted below are a few of the industrial processes which involve equilibrium reactions.

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The Contact Process

This process is important for manufacturing sulphuric acid. A number of stages are involved but the main step is the conversation of sulphur dioxide into sulphur trioxide:

2SO₂(g) + O₂(g) ? 2SO₃(g)

The reaction is exothermic. This means that an increase in temperature will mean a decrease in the yield of sulphur trioxide. Therefore, the position of equilibrium is favoured by a low temperature.

However, if the temperature is too low then the rate of reaction will be too slow and no longer viable commercially. Therefore a compromise temperature has to be reached: low enough for the yield to be reasonable yet high enough for the speed of the reaction to be reasonable.

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Pressure

Due to the fact that the reaction involves the number of gas moles being reduced, a higher yield of sulphur trioxide will be achieved by increasing the pressure. In other words: high pressure favours the position of equilibrium.

The rate of reaction is also favoured by high pressure. However, the cost of maintaining a high pressure is expensive so there are financial limits as to the maximise pressure possible.

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Catalysts

As a catalyst can increase the rate of reaction without affecting the position of equilibrium, the catalyst V₂O₅ is also used. However, at a low temperature this catalyst is no longer effective so more constraint is placed on the minimum temperature that can be used.

The optimum conditions for this reactions with a yield of 98% are:

• 400