A chemical reaction is in equilibrium when the composition of the reactants and products remains constant indefinitely.

The equilibrium constant, $K$, characterises the equilibrium composition of the reaction mixture.

For the general reaction $aA \space + \space bB \rightleftharpoons cC \space + \space dD$, the following is the equilibrium is expression.

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

$[A]$, $[B]$, $[C]$, and $[D]$ are the equilibrium concentrations of $A$, $B$, $C$, and $D$ and $a$, $b$, $c$, and $d$ are the stochiometric coefficients in the balanced reaction equation.

In heterogenous equilibria (reactions where chemicals are in different phases), liquids are considered as solvents and as such their concentration does not alter so they are not included in the equilibrium expression; likewise, any solids are not included — concentrations of pure solids and pure liquids at equilibrium are taken as constant and given a value of $1$ in the equilibrium expression.

The equilibrium constant can be used in calculations.

Equilibrium constants have no units.

The value of an equilibrium constant indicates the position of equilibrium.

→ The equilibrium constant is greater than or equal to $1$ for feasible reactions; for values of $K$ well below $1$ the equilibrium will lie well to the left.

The numerical value of the equilibrium constant depends on the reaction temperature and is independent of concentration and/or pressure.

→ For endothermic reactions, a rise in temperature causes an increase in $K$ and the yield of the product is increased.

→ For exothermic reactions, a rise in temperature causes a decrease in $K$ and the yield of the product is decreased.

The presence of a catalyst does not affect the value of the equilibrium constant.

In water and aqueous solutions there is an equilibrium between the water molecules and hydronium (hydrogen) and hydroxide ions.