PHYSICAL CHEMISTRY IN BRIEF

CHAP. 1: BASIC TERMS [CONTENTS] 29

1.4 The state of a system and its changes

Any system may be in any moment characterized using a certain number of quantities. These
quantities define thestateof a given system. The degree of generality at which we observe
a given system has to be taken into account at the same time. In terms of a microscopic
scale, the state of a system is defined by the position and velocity of all its particles. In terms
of thermodynamics, however, it is enough to know only a few quantities, e.g. temperature,
pressure and composition.

1.4.1 The state of thermodynamic equilibrium

The state of thermodynamic equilibrium (equilibrium state, equilibrium) is a state in which no
macroscopic changes occur in the system and all quantities have constant values in time.

Note:In the state of thermodynamic equilibrium, changes take place at the microscopic

level. For instance, when the liquid and vapour phases are in equilibrium, some molecules

continuously move from the liquid to the vapour phase and others from the vapour to the

liquid phase. However, the temperature and pressure of the system do not change.

The state of thermodynamic equilibrium embraces the following partial equilibria:

• mechanical (pressure) equilibrium—the pressure in all parts of the system is the same^1 ,


• thermal (temperature) equilibrium—the temperature in all parts of the system is equal-
  ized,


• concentration equilibrium—the concentration of the system components is the same in
  all parts of eachphaseof the system, but the composition of individual phases is usually
  different,


• chemical equilibrium—no changes in composition occur as a result of chemical reactions,


• phase equilibrium—if a system is heterogeneous (see1.1.4), the components of its phases
  are in equilibrium.

(^1) The osmotic equilibrium is an exception.