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.
The Haber process 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.