Reversible reactions, dynamic equilibrium in a closed system, the energy changes in the two directions, and Le Chatelier's principle applied to changes in concentration, temperature and pressure.

A C5.2 structured question. Reward: a reversible reaction is one that can go in both directions: the products can react to re-form the reactants. It is shown with a special double arrow. In the example, nitrogen and hydrogen react to form ammonia, and ammonia can break down to re-form nitrogen and hydrogen ($\text{N}_2 + 3\text{H}_2 \rightleftharpoons 2\text{NH}_3$). Dynamic equilibrium is reached in a closed system when the forward and backward reactions are happening at the same rate, so the concentrations of reactants and products stay constant (they do not change), even though both reactions are still occurring. Markers credit the idea that a reversible reaction goes both ways, and that at dynamic equilibrium the forward and backward rates are equal so the concentrations stay constant in a closed system. A common error is to say the reactions have stopped.

A Higher tier Le Chatelier question. Reward: Le Chatelier's principle says that if a change is made to a system at equilibrium, the equilibrium shifts to oppose the change. Increasing the temperature: the equilibrium shifts in the endothermic (backward) direction to take in the extra heat, so less ammonia is made (the yield of ammonia decreases). Increasing the pressure: there are 4 moles of gas on the left (1 nitrogen plus 3 hydrogen) and 2 moles on the right, so the equilibrium shifts towards the side with fewer moles of gas (the right) to reduce the pressure, so more ammonia is made (the yield increases). Markers credit, for temperature, the shift in the endothermic direction giving less ammonia, and for pressure, the shift to fewer moles of gas (the right) giving more ammonia. A common error is to reverse the temperature effect.