What is a chemical equilibrium?

If a chemical reaction happens in a container where one or more of the reactants [reactants: substances present at the start of a chemical reaction ] or products [product: A product is a substance formed in a chemical reaction. ] can escape, you have an open system. If a chemical reaction happens in a container where none of the reactants or products can escape, you have a closed system. Reversible reactions [Reversible reactions: Reversible reactions are chemical reactions which can go both ways. The direction of the reaction depends on the condition of the reactants. ] that happen in a closed system eventually reach an equilibrium.

Chemical equilibrium

In a chemical equilibrium, the concentrations of reactants and products do not change. But the forward and reverse reactions have not stopped - they are still going on at the same rate as each other.

Imagine walking the wrong way on an escalator - at the same speed as the escalator, but in the opposite direction. Your legs would still be walking forwards, and the escalator would continue to move backwards. However, the net result would be that you stay in exactly the same place. This is what happens in an equilibrium.

Other factors

If we remove the products from an equilibrium mixture, more reactants are converted into products. If a catalyst [catalyst: A catalyst changes the rate of a chemical reaction without being changed by the reaction itself. ] is used, the reaction reaches equilibrium much sooner, because the catalyst speeds up the forward and reverse reactions by the same amount. The concentration of reactants and products is nevertheless the same at equilibrium as it would be without the catalyst.