Chemistry » Chemical Equilibrium » Le Chatelier's Principle

Summary and Main Ideas


  • A reaction is reversible when reactants can react to form products, and products can react to form the reactants again.

  • A reaction is in chemical equilibrium when the rate of the forward reaction equals the rate of the reverse reaction.

  • In an open system energy and matter can enter and leave the system. In a closed system energy can enter and leave the system, but matter cannot.

  • The equilibrium constant (\(\text{K}_{\text{c}}\)) relates to the concentration of the reactants and products at equilibrium, and can be calculated using the following expression:

    \({K}_{c}=\frac{{\left[C\right]}^{c}{\left[D\right]}^{d}}{{\left[A\right]}^{a}{\left[B\right]}^{b}}\) where \(\text{aA} + \text{bB}\) \(\rightleftharpoons\) \(\text{cC} + \text{dD}\)

    A and B are reactants, C and D are products and a, b, c, and d are the coefficients of the respective reactants and products.

  • A high \(\text{K}_{\text{c}}\) value means that the concentration of products at equilibrium is high and the reaction has a high yield of the products. A low \(\text{K}_{\text{c}}\) value means that the concentration of products at equilibrium is low and the reaction has a low yield of the products.

  • Le Chatelier’s Principle states that if an external stress (change in pressure, temperature or concentration) is applied to a system in chemical equilibrium, the equilibrium will change in such a way as to reduce the effect of the stress.

  • Although a change in temperature, concentration or total pressure will affect the equilibrium position, only temperature will affect the equilibrium constant \(\text{K}_{\text{c}}\).

  • The principles of equilibrium are very important in industrial applications such as the Haber process, so that productivity can be maximised.

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