le Chatelier's Principle

When a system is in dynamic equilibrium, the concentration of reactants and products stays the same because the rates of reaction of the forward and reverse reactions are the same.

What if we add an external influence to the system, such as heat, pressure, or more reactants...?

This is where le Chatelier's Principle comes in. It states that the equilibrium system will "shift" to minimise the impact of any external factors (such as those mentioned). What this means is that the rate of one of the reactions will increase (but not the other one!), re-establishing the equilibrium "position" - the concentration of the reactants or products will change until the K_C is correct (for that specific temperature - remember that the K_C for an equilibrium is different at different temperatures).

This is very well explained here (as well as being an overview of everything to do with Equilibrium so far):

We need to be able to explain the effect on an equilibrium of:

1. Increasing the amount (therefore concentration) of a reactant or product
2. Removing some/all (therefore reducing the concentration) of a reactant or product
3. Increasing/decreasing the temperature
4. Increasing/decreasing the pressure
5. Introducing a catalyst

We then need to link these to what we would observe (temperature changes, colour changes etc.)

Changing Concentration of Reactants or Products

When we increase the concentration of a reactant (by adding more of it into the system), the forward reaction is favoured. This means the forward reaction's rate or reaction is increased. This is to "use up" some of the reactants, so re-establishing a dynamic equilibrium system. If the forward reaction is exothermic, the system will also feel warmer. If the forward reaction is endothermic, the system will absorb heat energy from the surroundings, so feel colder.

When we reduced the concentration of a reactant (maybe by reacting some of it with another chemical that we introduce), the reverse reaction is favoured. The rate of reaction of the reverse reaction increases in order to make more reactants, so re-establishing a dynamic equilibrium system. If the forward reaction is endothermic, the system will also feel warmer as the reverse reaction is exothermic. If the forward reaction is exothermic, the system will absorb heat energy from the surroundings, so feel colder, as the reverse reaction must be endothermic.

You can apply both of these to a change in concentration of the products.

Changing the Temperature

If you add heat energy, the system will try to absorb this heat energy, so favour the endothermic reaction until a new dynamic equilibrium position is established.

If you cool the system, the exothermic reaction will be favoured.

Changing Pressure

This only affects gaseous particles. If you increase the pressure, you limit the space for gaseous particles. Therefore, an equilibrium system will favour the reaction which produces fewer gaseous particles until a new dynamic equilibrium is established.

Introducing a Catalyst

This has no effect upon the position of the equilibrium. However, it does mean that dynamic equilibrium is established quicker, as the catalyst increases the rate of both the forward and the reverse reactions.