A lot of chemistry that is studied at GCSE considers complete reactions, reactions during which the reactant produce a product.
It must be noted that the reality is that not all reactions are complete reaction, and a number of reversible reactions do exist, with equilibrium being created between reactants and products.
In an equilibrium reaction, the rate at which a reactant reacts is the same as the rate that it is being produced. This would leave the concentration of such a reactant as constant.
An example of such a reaction is the production of ammonia:
Le Chatelier’s Principle
Equilibria are governed by one simple principle, also known as Le Chatelier’s Principle. This states that:
If a chemical system at equilibrium experiences a change in concentration, temperature, volume, or partial pressure, then the equilibrium shifts to counteract the imposed change and a new equilibrium is established.
which means that if some interference is performed on the reaction, the reaction would try to oppose the interference. This can be seen in:
- If the temperature is changed the reaction would change as to oppose the reaction, according to how exothermic or endothermic the reaction is. It must be noted that if the forward reaction is exothermic, the backward reaction would be endothermic.
- If the concentration is changed the equilibrium would change as to produce a new ratio between the concentrations, removing the forced change.
- Pressure change only affects gases, and this would affect the equilibrium only if the number of moles of gases in the reactants is different then that of products. The pressure change would have to be calculated, if more moles are found in the reactants then the forward reaction would reduce the pressure.
NB: Catalysts do not affect the equilibrium, they will only affect the rate at which the equilibrium is reached.