Equilibrium

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Alexander Hood
Equilibrium
LE CHATELIER'S PRINCIPLE
This page looks at Le Chatelier's Principle and explains how to apply it to reactions in a state of
dynamic equilibrium. It covers changes to the position of equilibrium if you change concentration,
pressure or temperature. It also explains very briefly why catalysts have no effect on the position of
equilibrium.
It is important in understanding everything on this page to realise that Le Chatelier's Principle is no
more than a useful guide to help you work out what happens when you change the conditions in a
reaction in dynamic equilibrium. It doesn't explain anything. I'll keep coming back to that point!
Equilibrium considerations
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 ammonia in the equilibrium mixture.
The forward reaction (the production of ammonia) is exothermic.
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.
In order to get as much ammonia as possible in the equilibrium mixture, you need as low a
temperature as possible. However, 400 - 450°C isn't a low temperature
Equilibrium considerations
Notice that there are 4 molecules on the left-hand side of the equation, but only 2 on the right.
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 ammonia as possible in the equilibrium mixture, you need as high a pressure
as possible. 200 atmospheres is a high pressure, but not amazingly high
Equilibrium considerations
The catalyst has no effect whatsoever on the position of the equilibrium. Adding a catalyst doesn't
produce any greater percentage of ammonia in the equilibrium mixture. Its only function is to speed
up the reaction.

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