The Equilibrium Constant Kc

Le Chatelier's principle gives qualitative predictions, but chemists need a quantitative measure of how far a reaction proceeds. This is provided by the equilibrium constant, $K_c$. For a general reversible reaction $aA + bB \rightleftharpoons cC + dD$, the equilibrium constant expression is

$$K_c = \frac{[C]^c[D]^d}{[A]^a[B]^b}$$

where each square bracket denotes the equilibrium concentration in mol dm$^{-3}$, and each concentration is raised to the power of its stoichiometric coefficient. Products appear in the numerator and reactants in the denominator. This relationship, known as the law of mass action, holds regardless of the path or starting amounts, and at a fixed temperature $K_c$ is a constant.

Writing the expression correctly requires attention to a few rules. Only species whose