IB Chemistry home > Syllabus 2016 > Equilibrium > Physical equilibria

Syllabus ref: 7.1

The term "equilibrium" refers to a situation where there is a balance between the forces acting upon a system, resulting in no overall change to the system. There are three different types of equilibrium:

  • Stable equilibrium
  • Unstable equilibrium
  • Dynamic equilibrium

The first two apply to physical situations where forces acting on a body are balanced in some way and the body does not move. The third case, dynamic equilibrium, refers to a situation of continual change, where the overall concentrations of the components remain unchanged.

Nature of science:

Obtaining evidence for scientific theories. Isotopic labelling and its use in defining equilibrium.

Common language across different disciplines-the term dynamic equilibrium is used in other contexts, but not necessarily with the chemistry definition in mind.


Essential idea: Many reactions are reversible. These reactions will reach a state of equilibrium when the rates of the forward and reverse reaction are equal. The position of equilibrium can be controlled by changing the conditions.

A state of equilibrium is reached in a closed system when the rates of the forward and reverse reactions are equal.

The equilibrium law describes how the equilibrium constant (Kc) can be determined for a particular chemical reaction.

The magnitude of the equilibrium constant indicates the extent of a reaction at equilibrium and is temperature dependent.

The reaction quotient (Q) measures the relative amount of products and reactants present during a reaction at a particular point in time. Q is the equilibrium expression with non-equilibrium concentrations. The position of the equilibrium changes with changes in concentration, pressure, and temperature.

A catalyst has no effect on the position of equilibrium or the equilibrium constant.

Applications and skills

The characteristics of chemical and physical systems in a state of equilibrium.

Deduction of the equilibrium constant expression (Kc) from an equation for a homogeneous reaction.

Determination of the relationship between different equilibrium constants (Kc) for the same reaction at the same temperature.

Application of Le Châtelier's principle to predict the qualitative effects of changes of temperature, pressure and concentration on the position of equilibrium and on the value of the equilibrium constant.

In Chapter 7.1