Some reactions are reversible meaning both the forward and reverse reaction can take place.
An example of this is anmonium chloride decomposing to ammonia and hydrogen chloride.
When ammonium chloride is broken down, some of the ammonia and hydrogen chloride formed re-forms to remake ammonium chloride. This is known as an open reaction as the other ammonia and hydrogen chloride can escape.
If the experiment is sealed it is a closed reaction as substances cannot enter or leave, therefore all the product will re-form ammonium chloride.
After some time, the rate at which ammonium chloride breaks down will be equal to the rate it reforms, this is the state of equilibrium. Meaning the amount of each substance remains.
However, this does not mean that the substances on each side of the reaction are the same, sometimes the amount of the substance on one side will be more than the other.
The Effect of Conditions on Equilibria
The position of the equilibrium is changed if the conditions (temperature, pressure) change.
This is useful as we can control the amount of each substance created.
To uphold to law of conservation, a reaction is exothermic one side and endothermic the other so if the temperature rises, more energy would be taken in an endothermic reaction. Therefore there is more yeild for the endothermic reaction. This works the other way as well.
If the pressure was increased then the reaction with more molecules as reactants will produce more product.
The Haber Process - The Reaction
Ammonia is used as it fertilises crops, meaning a bigger yield therefore more can be fed.
Ammonia is made by the reaction with nitrogen and hydrogen. Nitrogen is obtaind by fractional distillation of liquified air as it is 78% nitrogen and hydrogen is obtained by natural gas, methane or oil.
The reaction between the two is reversible and slow meaning a low yeild of ammonia. Fritz Haber was able to produce more ammonium by moving the equilibrium position.
The Haber process passed hydrogen and nitrogen over an iron catylist at 200 atmospheres and 450 degress celcius which are the optimum conditions for making ammonia.
The reaction from hydrogen and nitrogen to ammonia is exothermic so the lower the temperatures the better and side there are more molecules on the nitrogen and hyrdogen side, the higher the pressure the better.
Because the rate of reaction needs to be fast the temperature needs to be increased so that is why 450 degrees is a compromise. The pressure is also a compromise as the higher the pressure the more expensive it is.
The Haber Process - Recycling
Ammonia has a higher boiling point than hydrogen and nitrogen so will liquify first and then be carried off and used.
The rest of the hydrogen and nitrogen is then recycled to make more ammonia.