What is the position of chemical equilibrium and how can it change? (A-Level/high GCSE)

A dynamic chemical equilibrium refers to a reversible chemical reaction, where the forward and reverse reactions are happening at the same rate. This results in the concentrations of reactant and product being constant. Even though the concentrations are constant at equilibrium, they are normally not the same. We use the term ‘position of equilibrium’ to describe the relative concentrations of reactant and product. When a reaction starts out it might be that the reactants are converted very quickly to products, and the forward reaction is almost finished before the reverse reaction starts to happen. Once the rate of the reverse reaction has increased to match the rate of the forward reaction, equilibrium is established. In this case we say the position of equilibrium lies to the products/to the right-hand side of the reaction. If it happens the other way around, and not very much reactant is converted to product by the time equilibrium is established, we say the position of equilibrium lies to the reactants/to the left. The position of equilibrium can be changed by altering certain reaction conditions: the concentrations of products and reactants, the pressure of reacting gasses, and temperature. Le Chartelier’s principle tells us that if you impose a change, the dynamic equilibrium will act against it to keep everything constant. If you increase the concentration of reactants or decrease the concentration of products, the position of equilibrium will shift to the product side. Reactants are then consumed and the concentration of product increases, so the imposed change is countered. The reverse happens if you increase the concentration of products or reduce the concentration of reactants. Increasing the pressure of the system shifts the position of equilibrium to the side of the reaction with the fewest moles of gas, to lower the pressure. The reverse happens if the pressure of the system is reduced. If a reversible reaction is exothermic (energy-releasing) in one direction, it will be endothermic (energy-absorbing) in the other. Increasing the temperature (which can be thought of as making energy available to be absorbed) will shift the position of equilibrium to favour the endothermic reaction. Lowering the temperature (which can loosely be thought of as creating an absence of energy in the external environment that needs to be filled) favours the exothermic reaction.