The reaction between hydrogen and nitrogen
Many of the reactions you observe only go one way - this means that reactants react to form products and eventually the reaction comes to a stop.
However, some reactions are never completed because there are two competing reactions occurring, a forward reaction and a backward reaction.
Take the reaction between nitrogen gas and hydrogen gas, the two competing reactions that occur are:
The forward reaction:
The backward reaction:
This is an example of a reversible reaction, and is often written as:
indicates a reversible reaction.
If the back and forward reaction rates become the same, we say that the reaction is in equilibrium.
If you want to increase the amount of ammonia you make, you have a problem when your reaction mixture reaches equilibrium as the amount of ammonia been produced remains constant.
An important fact about reversible reactions: it is in equilibrium, if you cause a change then it will oppose the change.
You cannot make a reversible reaction go to completion but you can change the conditions so that the equilibrium shifts to the right, producing more product, in this case ammonia.
Since the forward reaction in the production of ammonia is exothermic - gives out heat - by increasing the temperature you are worse off.
This is because the back reaction being endothermic takes the extra energy i.e. it opposes the change. Hence, increasing temperature produces less ammonia!
Pressure is caused by the collision of gas molecules and the container wall.
The fewer molecules you have the lower the pressure. By increasing pressure, the equilibrium mixture will oppose the change - more ammonia will form in order to reduce the pressure.
Adding a catalyst:
Iron acts as a catalyst for this reaction. The catalyst does not affect the equilibrium position since it increases the rate of the forward and back reactions equally, but it does speed up the speed at which equilibrium is reached.