What is 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 its reactant concentrations raised to the power as their respective stoichiometric coefficients at constant temperature and constant pressure.

Mathematical Expression of the Law of Mass Action

Let us see that how law of mass action can be expressed mathematically. For instance, a chemical reaction is represented as follows,

aA + bB → cC + dD

Derivation of Law of Mass Action

According to the law of mass action,

Rate of reaction ∝ [A]^a[B]^b

Replacing the proportionality sign with equal sign by introducing the proportionality constant (K_f), we get,

Rate of forward reaction = K_f [A]^a[B]^b

K_f is called the velocity constant for forward reaction. The above equation represents the rate of forward reaction. Similarly, rate for backward reaction can be written as,

Rate of backward reaction = K_b[C]^c[D]^d

K_b is called the velocity constant for backward reaction.

Equilibrium constant

Equilibrium constant (K_c) is the ratio of velocity factors for forward reaction (K_f) and backward reaction (K_b). At equilibrium, the rate of forward reaction equals the rate of backward reaction. Mathematically,

Rate of Forward reaction = Rate of Backward reaction

K_f [A]^a[B]^b = K_b[C]^c[D]^d

Rearranging the terms, we get,

K_c = K_f/K_b = [C]^c[D]^d/[A]^a[B]^b

K_c is also called as reaction constant. Another related term is reaction quotient or concentration quotient denoted as Q_c, which is obtained by substituting concentrations of reactants and products at any instant in the expression of K_c.

Concentration Quotient (Q_c) versus Equilibrium Constant (K_c)

The relationship between K_c and Q_c gives information about the direction in which reaction would proceed.

• If Q_c < K_c, the reaction proceeds in forward direction.
• If Q_c = K_c, the reaction is at equilibrium.
• If Q_c > K_c, the reaction proceeds in backward direction.

Law of Mass Action relates to the rate of a chemical reaction. It states that the rate of a reaction is directly proportional to the concentrations of its reactants. More precisely, the rate of a chemical reaction is directly proportional to the product of its reactant concentrations raised to their respective stoichiometric coefficients at constant temperature and pressure. This implies that an increase in reactant concentration would lead the reaction to move forward at a faster rate. The law of mass action forms the basis for equilibrium constant expression, which helps in quantifying the dynamics of the reaction.

In this article, we will discuss what is meant by the Law of Mass Action, Equilibrium Constant, Chemical Equilibrium, applications of the law and related frequently asked questions.