A classic example of a reaction at equilibrium is the Haber process for ammonia production:
Nitrogen gas reacts with hydrogen gas to form ammonia gas:
- The reaction is reversible.
- At equilibrium, the rate of formation of ammonia equals the rate of decomposition of ammonia back into nitrogen and hydrogen.
- Concentrations of nitrogen, hydrogen, and ammonia remain constant over time, even though the reaction continues in both directions.
Other examples:
- Dissolution of salts: e.g., sodium chloride in water – the rate of dissolving equals the rate of crystallization at equilibrium.
- Esterification: Acid + alcohol ⇌ ester + water – the forward and backward reactions occur at the same rate when equilibrium is reached.
In short, an equilibrium reaction is any reversible reaction where the forward and backward rates are equal and concentrations stay constant.