Equilibrium is an important topic in the A Level Chemistry course. Many A Level Chemistry exam
questions will ask about equilibrium so you need to be very familiar with the topic, especially the law of
equilibrium which tends to be asked again and again.
Chemical Equilibrium Defined
In a chemical reaction, chemical equilibrium is the state in which the concentrations of the
reactants and products have not yet changed with time. It occurs only in reversible reactions , and not in
irreversible reactions Usually, this state results when the forward reaction proceeds at the same rate as the
reverse reaction. The reaction rates of the forward and reverse reactions are generally not zero but, being equal,
there are no net changes in the concentrations of the reactant and product. This process is called dynamic
The Law of Equilibrium
The law of equilibrium states that:
At any constant temperature, the rate of a reaction is proportional to the active mass
of the reacting substances.
You can assume that active mass is the same as molar concentration in mol dm-3
or mol/m3 which is the SI unit but the unit mol/L is sometimes used instead. A solution of concentration
1 mol/L is also denoted as "1 molar" (1 M). In another word:
1 mol/L = 1 mol/dm3 = 1 mol dm−3 = 1 M = 1000 mol/m3
The Law of Mass Action
In A Level Chemistry, the law of mass action is a mathematical model that explains and predicts
behaviors of solutions in dynamic equilibrium. It can be described with two aspects:
1) the equilibrium aspect, concerning the composition of a reaction mixture at equilibrium
2) the kinetic aspect concerning the rate equations for elementary reactions.
Both aspects stem from the research by Guldberg and Waage (1864–1879) in which equilibrium
constants were derived by using kinetic data and the rate equation which they had proposed. Guldberg and Waage also
recognized that chemical equilibrium is a dynamic process in which rates of reaction for the forward and backward
reactions must be equal.
Taken as a statement about kinetics, the law states that the rate of an elementary reaction (a
reaction that proceeds through only one transition state, that is one mechanistic step) is proportional to the
product of the concentrations of the participating molecules. In modern chemistry this is derived using statistical
Taken as a statement about equilibrium, that law gives an expression for the equilibrium
constant, a quantity characterizing chemical equilibrium. In modern chemistry this is derived using equilibrium
Le Chatelier's Principle
In A Level chemistry, Le Chatelier's principle, also called the Chatelier's principle, can be
used to predict the effect of a change in conditions on a chemical equilibrium. The principle is named after Henry
Louis Le Chatelier and sometimes Karl Ferdinand Braun who discovered it independently. It can be summarized as:
If a chemical system at equilibrium experiences a change in concentration, temperature,
volume, or partial pressure, then the equilibrium shifts to counteract the imposed change and a new equilibrium
This principle has a variety of names, depending upon the discipline using it.
You will learn about all of these principles and laws throughout your A Level Chemistry