Law of Mass Action

Law of Mass Action

The rate at which a substance reacts is proportional to its active mass and hence the rate of a chemical reaction is proportional to the product of the active masses of the reactants.

The active mass means molar concentration i.e. number of moles dissolved per litre of the solution.

Suppose x g of NaOH are dissolved in V litres of solution. Then we can say:

Concentration of NaOH solution = x g in V litre = x/40 moles in V litres = x / 40 × V  moles/Litre

Consider the reaction

A + B → products

According to Law of mass action,

Rate at which A reacts ∝ [A]

Rate at which B reacts ∝ [B]

Rate at which A and B react together ∝ [A] [B] = K [A] [B]

where K is a constant of proportionality and is called velocity constant.

aA + bB → products
rate ∝ [A]^m [B]ⁿ
rate = k [A]^m [B]ⁿ

The rate of a reaction is proportional to the product of the active masses of the reactants, each raised to the power equal to its stoichiometric coefficient as represented by the balanced chemical equation.