When it comes to equilibrium in AQA A level chemistry, it all starts with Le Chatelier's principle. This principle helps us predict what will happen to a dynamic equilibrium when we change the reaction conditions.
"When an equilibrium undergoes a change to its conditions, The equilibrium will shift to oppose the change."
So when we lower the concentration of a reactant the equilibrium will move to the left to oppose the loss of the reactant.
If we were to increase the pressure of a system, the equilibrium would move to the side with the least number of gaseous moles. In this case of this equilibrium it would shift to the right as there are only 2 moles of gas on the right compared with 3 moles on the left.
2SO2(g) + O2(g) ⇌ 2SO3(g)
Temperature is a bit more complicated as we have to look at the enthalpy change to determine if the forward reaction is exothermic (Negative ΔH) or endothermic (Positive ΔH). If the temperature is decreased the reaction will shift in the exothermic direction to increase the temperature. So in the case below the equilibrium will shift to the right to oppose the change as the forward reaction is exothermic.
CO(g) + 3H2(g) ⇌ CH4(g) + H2O(g)
Understanding how Le Chatelier's principle needs applying to different exam questions can be tricky. Use the module on Bale's Chemistry to help you practice!
