What Is Chemical Equilibrium?
Many chemical reactions are reversible — they can proceed in both the forward and reverse directions. When a reversible reaction takes place in a closed system, the reaction eventually reaches a state called chemical equilibrium.
A dynamic state in which the rate of the forward reaction equals the rate of the reverse reaction, so the concentrations of reactants and products remain constant over time.
It is crucial to understand that equilibrium is not the same as the reaction stopping. Both the forward and reverse reactions are still happening — they are just occurring at the same rate, so there is no overall change in concentrations.
Imagine a busy escalator in a shopping centre. People are walking up at the same rate that others are walking down. The number of people on each floor stays constant, but individuals are constantly moving. This is like a system at equilibrium — constant concentrations, but constant movement at the molecular level.
We represent a reversible reaction with a double arrow (⇌):
The position of equilibrium describes whether the mixture contains more products or more reactants at equilibrium. If it lies to the right, there are more products; if it lies to the left, there are more reactants.
Le Chatelier's Principle — The Key Idea
When conditions in an equilibrium system are changed, the system responds in a predictable way. This behaviour is described by Le Chatelier's Principle, named after the French chemist Henri Le Chatelier.
When a stress (change in conditions) is applied to a system at equilibrium, the system will shift its position of equilibrium to partially counteract or oppose that stress.
Think of Le Chatelier's Principle as the system's way of "pushing back" against any change you make to it. The system always tries to restore balance.
Le Chatelier's Principle does not mean the system returns to exactly the same equilibrium position — it means the system shifts to partially oppose the change. A new equilibrium is established at different concentrations.
The factors that can change the position of equilibrium are:
- Concentration of reactants or products
- Temperature
- Pressure (for reactions involving gases)
Note: Adding a catalyst does not change the position of equilibrium — it only speeds up how quickly equilibrium is reached.
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