Law of Mass Action in a Gaseous System
For chemical reactions involving gaseous components, pressure plays more important role than concentration as gases can be more conveniently expressed in terms of pressure. Thus, for a gaseous system, the expression for law of mass action involves partial pressures of the respective components instead of concentrations. For instance, consider a general reversible gaseous reaction as follows:
aA(g) + bB(g) ⇌ cC(g) + dD(g)
The expression for equilibrium constant for the above reaction would be written as follows:
Keq = (PC)c(PD)d/(PA)a(PB)b
where, PA, PB, PC and PD represent the partial pressures of gases A, B, C, and D respectively at equilibrium.
This formulation in terms of partial pressures is useful for gas phase reactions as pressure is relatively easier to measure for gases and thus shift in equilibrium can be predict based on pressure changes.
Law of Mass Action
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.