Reversible Reactions and Equilibrium
Reversible Reactions
Some chemical reactions are reversible — the products can react to re-form the original reactants. A reversible reaction is shown with a double arrow (\(\rightleftharpoons\)):
$$A + B \rightleftharpoons C + D$$
A simple example is the thermal decomposition of ammonium chloride:
$$NH_4Cl \rightleftharpoons NH_3 + HCl$$
When heated, ammonium chloride decomposes into ammonia and hydrogen chloride. When cooled, these gases recombine to form ammonium chloride again.
In a reversible reaction, if the forward reaction is exothermic, the reverse reaction is endothermic by the same amount of energy, and vice versa.
Dynamic Equilibrium
When a reversible reaction occurs in a closed system (where nothing can enter or leave), it eventually reaches dynamic equilibrium. At equilibrium:
- The rate of the forward reaction equals the rate of the reverse reaction.
- The concentrations of reactants and products remain constant (but are not necessarily equal).
- The reaction has not stopped — both forward and reverse reactions are still occurring.
If conditions are changed, the equilibrium shifts to oppose the change (Le Chatelier’s principle):
- Temperature: Increasing temperature shifts equilibrium in the endothermic direction. Decreasing temperature shifts it in the exothermic direction.
- Concentration: Increasing the concentration of a reactant shifts equilibrium towards the products. Increasing the concentration of a product shifts it towards the reactants.
- Pressure (gases only): Increasing pressure shifts equilibrium towards the side with fewer moles of gas.
A catalyst does not change the position of equilibrium — it speeds up both the forward and reverse reactions equally, helping equilibrium to be reached faster.
Le Chatelier’s principle: if conditions change, equilibrium shifts to oppose the change. Catalysts speed up both reactions equally — they do not change the equilibrium position.
Separate Chemistry only. The Haber process is not examined on Combined Science, so treat this box as background reading. It manufactures ammonia from nitrogen and hydrogen:
$$N_2 + 3H_2 \rightleftharpoons 2NH_3$$
The forward reaction is exothermic. Conditions used: temperature of about 450°C, pressure of about 200 atmospheres, and an iron catalyst. The temperature is a compromise — lower temperature would give a higher yield but the rate would be too slow. The iron catalyst speeds up the reaction without changing the yield.