According to the law of mass action, the rate of a chemical reaction is directly proportional to the product of the active masses of the reactants each term raised to its stoichiometric coefficients.

𝑎𝐴 + 𝑏𝐵 ⇌ 𝑐𝐶 + 𝑑𝐷

Now, from the law of mass action, we know that the rate of forward reaction (R_f) and rate of backward reaction (R_b) will be:

𝑅_𝑓 = 𝑘_𝑓 [𝐴]^𝑎[𝐵]^𝑏

𝑅_𝑏 = 𝑘_𝑏 [𝐶]^𝑐[𝐷]^𝑑

Where 𝑘𝑓 and 𝑘𝑏 are the rate constants for the forward and backward reactions, respectively. After equilibrium is reached, we have

𝑅_𝑓 = 𝑅_𝑏

𝑘_𝑓 [𝐴]^𝑎[𝐵]^𝑏 = 𝑘_𝑏 [𝐶]^𝑐[𝐷]^𝑑

\begin{equation} \frac{{(Kf)}}{{Kb}} = \frac{{[C]^c [D]^d}}{{[A]^a [B]^b}} \tag{i} \end{equation}

Since the 𝑘𝑓 and 𝑘𝑏 are also constant at equilibrium, the ratio of the two is also a constant and is typically labelled as K or the equilibrium constant. Therefore, equation (i) is modified as:

\begin{equation} \frac{{K_f}}{{K_b}} = K = \frac{{[C]^c [D]^d}}{{[A]^a [B]^b}} = \frac{{[Product]}}{{[Reactant]}} \end{equation}