Chemical Equilibrium and the Law of Mass Action

Chemical equilibrium is the state in which both the reactants and products are present in concentrations which have no further tendency to change with time. Consider the general reversible chemical reaction:

The Law of Mass Action

The Law of Mass Action states that the rate of a chemical reaction is directly proportional to the product of the active masses (molar concentrations) of the reacting substances.

At equilibrium, the rate of forward reaction equals the rate of backward reaction. The equilibrium constant $K_c$ is expressed as:

Where $[A]$, $[B]$, $[C]$, and $[D]$ are the molar concentrations of reactants and products at equilibrium, and $a$, $b$, $c$, and $d$ are their stoichiometric coefficients.

Equilibrium in Gas Phase ($K_p$)

For reactions involving gases, we can write the equilibrium constant in terms of partial pressures ($K_p$):

The relation between $K_p$ and $K_c$ is defined by:

Where:

• $R$ is the universal gas constant
• $T$ is the absolute temperature in Kelvin
• $\Delta n_g$ is the difference in number of moles of gaseous products and gaseous reactants: $\Delta n_g = (c+d) - (a+b)$