Le Chatelier's Principle

Le Chatelier's Principle

It is of great practical importance to be able to predict, and to control, the equilibrium composition of a chemical reaction. In an industrial process it may be desirable to convert as much as possible of reactants to prodcuts. At equilibrium the product yield may not be satisfactory. But, the composition of the equilibrium mixture (equilibrium position) is dependent on certain conditions, such as temperature and pressure, and it might be possible to change these to achieve a better yield. Le Chatelier's Principle helps us deal with this.

Now consider another chemical reaction in a state of equilibrium at a constant temperature.

N₂(g)   +   3H₂(g)        2NH₃(g)    DH°_{f, 298} = - 46.0 kJ mol^-1

In a closed system and at a pressure of 250 atmospheres in the presence of a finely divided iron catalyst at 500 °C, about 15 percent of the gases are converted into ammonia at equilibrium.
We can impose the following changes [both an increase and decrease] upon this chemical system at equilibrium:

1. Amount of a substance / concentration of a reactant or product.
2. Pressure / Volume (pressure and volume are inversely related).
3. Temperature.

In general terms, this is what Le Chatelier's Principle says...

If a change is imposed upon a chemical system at equilibrium then it will respond in such a way as to undo, in part, the effect of the change imposed upon it.