Le chateliers principle
Le Chatelier's Principle states that an equilibrium's position will shift to counteract any constraints put upon it.
This principle can be used to predict the outcome of applying a change in condition to a reaction for example:
For a reaction [P] ⇌ [R]
If the forward reaction is exothermic it releases energy as products are formed. Increasing temperature will cause the equilibrium position to shift to the left to counteract this change and remove the increase in temperature from the system by taking in energy [P] -> [R].
For a reaction [P](g) ⇌ 2[R](g)
The forward reaction increases pressure as there are a greater number of gaseous moles on the RHS (right-hand side) of the equilibrium. Therefore, if the pressure of the system increases the equilibrium position will shift to the left to counteract the change (reduce system pressure)