Chemical Equilibrium and Le Chatelier's Principle

Many chemical reactions do not go to completion. Instead, they reach a state of dynamic equilibrium where the forward and reverse reactions occur at equal rates, and the concentrations of reactants and products remain constant over time. Understanding equilibrium — and how to manipulate it — is central to both A-level chemistry and industrial chemistry.

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Dynamic Equilibrium

A system is in dynamic equilibrium when:

1. The rate of the forward reaction equals the rate of the reverse reaction.
2. The concentrations of reactants and products remain constant (but are not necessarily equal).
3. The reaction has not stopped — both forward and reverse reactions continue to occur at the molecular level. It is "dynamic," not static.

For equilibrium to be established, two conditions must be met:

• The system must be closed (no substances can enter or leave).
• The temperature must be constant.

At the start of a reaction, only reactants are present and the forward rate is high. As products form, the reverse rate increases. Eventually, the two rates become equal and the system reaches equilibrium.

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Le Chatelier's Principle