What is Le Chatelier's Principle?

By defintion, Le chatelier's principle states that if a reaction at equilibrium (i.e a reversible reaction) is subject to a change in concentration, pressure or temperature the system will act in a way to counteract the change. How each condition affects the reaction depends upon the conditions of the reaction. For example, if the temperature is changed the counteraction depends on whether or not the reaction is endothermic (absorbs heat) or exothermic (gives out heat). If the forward reaction is exothermic and the temperature is increased the reverse reaction will be favoured in order to absorb the extra heat. Similarly, if the temperature is decreased and the forward reaction is exothermic, the forward reaction will be favoured in order to produce more heat to bring the reaction back to equilibria. If you increase the concentration of reactants in a reaction, the forward reaction will be favoured in order to produce more products. If you decrease the concentration of reactants in a reaction, the reverse reaction will be favoured in order to produce more reactants. Finally, a change in pressure depends upon the amount of moles in the reaction. If you increase the pressure, and there are more moles of reactants than products, the forward reaction will be favoured in order to 'decrease' the pressure. If you decrease the pressure, and there are more moles of reactants, than products then the reverse reaction will be favoured to 'increase' the pressure again.