Many equilibria have been studied to find the concentrations of reactants and products at equilibrium. This has led to a general statement called the Equilibrium Law or Law of Chemical Equilibrium, as follows:

If an equilibrium mixture contains substances A, B, C and D related by the equation aA + bB _∏ cC + dD,

it is found by experiment that   is constant. The constant is given the symbol K_c, the equilibrium constant. K_c is constant at a given temperature.

Points to notice:

• Square brackets are used to represent the concentration of a substance in mol dm^–3. So [X] means 'concentration of X in mol dm^–3. 
• Concentrations of substances on the right-hand side of the equation are written in the numerator. This means that they're written as the top of the fraction in the expression for K_c.
• Concentrations of the substances on the left-hand side of the equation are written in the denominator. This means that they're written as the bottom of the fraction in the expression for K_c.
• The concentrations referred to in the expression for K_c are the equilibrium concentrations.
• The subscript 'c' indicates that K_c is expressed in concentrations, not just in amounts such as a number of moles.
• At a different temperature the value of K_c will be different.

It is important that the equation that K_c relates to is given.

What would be the expression for K_c for the equilibrium written as 2HI _∏ H₂ + I₂?

How is this value of Kc related to the value of Kc for the equilibrium written as H₂ + I_2 ∏ 2HI?

The two values for K_c are inversely related.

K _c for the equilibrium 2HI(g) _∏ H₂(g) + I₂(g) at 440 °C = 0.02. What would be the value of K_c at the same temperature for the equilibrium H₂(g) + I₂(g) _∏ 2HI(g)?