Chemical Equilibrium Definition

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Chemical equilibrium is the state of a chemical reaction when the concentrations of the products and reactants are unchanged over time. In other words, the forward rate of reaction equals the backward rate of reaction. Chemical equilibrium is also known as dynamic equilibrium.

Concentration and Reaction Constants

Assume a chemical reaction:

aA + bB ⇄ cC + dD, where k1 is the forward reaction constant and k2 is the reverse reaction constant

The rate of the forward reaction may be calculated by:

rate = -k1[A]a[B]b = k-1[C]c[D]d

When the net concentrations of A, B, C, and D are at equilibrium, then the rate is 0. According to Le Chatelier's principle, any change in temperature, pressure, or concentration will then shift the equilibrium to make more reactants or products. If a catalyst is present, it lowers the activation energy, which causes a system to reach equilibrium more quickly. A catalyst does not shift equilibrium.

  • If the volume of an equilibrium mixture of gases is reduced, the reaction will proceed in the direction that forms fewer moles of gas.
  • If the volume of an equilibrium mixture of gases is increase, the reaction proceeds in the direction that yields more moles of gas.
  • If an inert gas is added to a constant volume gas mixture, total pressure increases, the the partial pressures of the components remain the same and equilibrium remains unchanged.
  • Increasing the temperature of an equilibrium mixture shifts equilibrium in the direction of the endothermic reaction.
  • Decreasing the temperature of an equilibrium mixture shifts equilibrium to favor the exothermic reaction.


  • Atkins, Peter; De Paula, Julio (2006). Atkins' Physical Chemistry (8th ed.). W. H. Freeman. ISBN 0-7167-8759-8.
  • Atkins, Peter W.; Jones, Loretta. Chemical Principles: The Quest for Insight (2nd ed.). ISBN 0-7167-9903-0.
  • Van Zeggeren, F.; Storey, S. H. (1970). The Computation of Chemical Equilibria. Cambridge University Press.