Le Chatelier's Principle
Le Chatelier's principle suggests that when an external condition is placed upon a reaction, it will try and oppose any change made by shifting the equilibrium position.
Change in Temperature
An increase in temperature shifts the equilibrium position to the endothermic side of the reaction to oppose the change.
In exam questions, they usually give you the molar enthalpy change so if the forward reaction was endothermic and temperature was increased, equilibrium would shift to the right hand side so that more product is formed to oppose the change.
Change in Pressure
An increase in pressure will cause the equilibrium to shift to the side with the fewer moles to oppose the change.
In the reaction ; CO + 2H2 <---> CH3OH
There are 3 moles on the left side and 1 on the right side so if the pressure was increased equilibrium would shift to the right side and more product (methanol) would be formed.
But if pressure was decreased equilibrium would shift to the side with more moles so in this case to the left side meaning less product will be formed.
Change in Concentration
An increase in concentration of one if the reactants in a reaction will make equilibrium shift to the right hand side.
So for example in this reaction;
A + B <~~~> 2C + D
If we increased the concentration of B equilibrium would shift to the right side to oppose the change
If we decreased the conc of B equilibrium would shift to the left side to oppose the change and will try and increase the conc of B.
But if we increased the conc of D equilibrium would shift to the oppose side and would shift to the left side.