What is Le Chatelier's Principle?

Le Chatelier's Principle states that for a reaction which is reversible, where there is a change made to the reaction conditions the equilibrium position will shift to oppose the change.

In answering a question on the principle then, it is important to first state what the principle is, as above, and then apply it to the facts given to you. If you approach this in a logical way then there is no reason to get the question wrong.

For example, where the forward part of a reversible reaction is exothermic, what is the effect of increasing the termperature on the yield of the reaction?

Le Chatelier's Principle states that where a change is made to the conditions of a reversible reaction the equilibrium position will shift to oppose the change. In this case the temperature has been increased therefore the equilibrium position will shift to decrease the temperature. As the forward reaction is exothermic the reaction will shift left, in the direction of the backwards reaction (and the reactants) thus decreasing the yield.