Law of mass action

The law of mass action is a mathematical model that explains and predicts behaviours of solutions in dynamic equilibrium. Simply put, it states that the rate of a chemical reaction is directly proportional to the product of the masses of the reactants. Necessarily, this implies that for a chemical reaction mixture that is in equilibrium, the ratio between the concentration of reactants and products is constant.

The law of mass action is universal, applicable under any circumstance. However, for reactions that are complete, the result may not be very useful. We introduce the mass action law by using a general chemical reaction equation in which reactants A and B react to give product C and D.

a X + b Y --> c P + d Q

where a, b, c, d are the coefficients for a balanced chemical equation.

The mass action law states that if the system is at equilibrium at a given temperature, then the following ratio is a constant.

[P]c[Q]d / [X]a[Y]b = K eq

The square brackets "[ ]" around the chemical species represent their concentrations. This is the ideal law of chemical equilibrium or law of mass action. The units for K depend upon the units used for concentrations. If M is used for all concentrations, K has units

M c+d-(a+b)