Chemical equilibria are dynamic and therefore respond to changes in the conditions.  The French chemist Henri Le Chatelier found a general principle that lets us predict how the equilibrium composition of a reaction mixture tends to change when the conditions are changed.  A modern statement of Le Chatelier's principle is that when a system in chemical equilibrium is disturbed by a change of temperature, pressure, or a concentration, the system shifts in equilibrium composition in a way that tends to counteract this change of variable.

Adding or Removing Reagents 

Suppose the following reaction has reached equilibrium:

N₂(g)  +  3 H₂(g)  <==>  2 NH₃(g)   ΔH = -91.8 kJ

Now suppose we pump in more hydrogen gas, (we have increased the concentration of H₂) at this point the system does not have an equilibrium composition and is not at equilibrium.  According to Le Chatelier's principle, the reaction will tend to counteract this change.  It does this by consuming hydrogen and thus the equilibrium shifts to the right. In general then we can say that the reaction will move toward the side of the equation opposite the side to which a substance is added or it will move away from what we add.  Conversely if we remove a substance,  Le Chatelier's principle predicts that the reaction will move toward the side of the equation from which a substance is removed.

Compressing the Reaction Mixture 

What happens to the reaction above if we compress the mixture by reducing the volume? In order to counteract this change,  Le Chatelier's principle predicts that the reaction will move toward the side of the equation having the smallest number of moles of gas.  Since the left side has 4 moles of gas and the right side has 2 moles, the reaction will move toward the right, that is more product will form.  If there are the same number of moles of gas on the reactant side and the product side of the reaction, then compressing the mixture will have no effect on the equilibrium.  Also note that increasing the total pressure by adding some inert gas to the mixture, instead of reducing the volume, will have no effect on the equilibrium. The reason being that this does not change the partial pressures and hence concentrations of the reactants and products.

The Effect of a Temperature Change

Finally what would happen to our ammonia equilibria if we raised the temperature of the reaction mixture?  We see that the reaction is exothermic, so lets rewrite the equation so that it shows the heat as part of the reaction. Since it is exothermic we will show heat as a product.

N₂(g)  +  3 H₂(g)  <==>  2 NH₃(g)  + 91.8 kJ

Now if we raise the temperature this must mean that we are adding heat to the reaction mixture.  We can apply Le Chatelier's principle here just like we did for adding a reagent, just consider heat a reagent, therefore the reaction is going to move away from what we add, so it will move to the left.  So in general if the temperature of an exothermic reaction is raised the equilibrium will shift to the left, whereas it the temperature of an endothermic reaction is raised the equilibrium will shift toward the right.  Of course the effect of lowering the temperature will be the opposite of this.