CHAP. 1: BASIC TERMS [CONTENTS] 30
Note:If a system in the state of thermodynamic equilibrium occurs in an external force
field, e.g. the gravitational field, the pressure is not the same in all parts of the system
but it changes continuously. The concentration of the system components also changes
continuously in each phase, with a discontinual change occurring at the phase boundary.1.4.2 System’s transition to the state of equilibrium
If a system is not in the state of equilibrium, its properties change in time in such a way that
it tends toward equilibrium. Thermodynamics postulates that every system under invariable
external conditions is bound to attain the state of thermodynamic equilibrium. The time needed
for a system to attain equilibrium varies considerably, ranging from fractions of a second needed
for pressure equalization up to hundreds of years needed for glass transition to the crystalline
state. A measure of the velocity of a system’s transition to equilibrium is called therelaxation
time.
Example
If we immerse several crystals of copper(II) sulphate pentahydrate (CuSO 4 · 5 H 2 O) into a closed
vessel containing water, the system thus created will be in a non-equilibrium state at the be-
ginning. There will be neither a phase equilibrium between the crystals and the liquid phase
nor a concentration equilibrium. After some time the crystals will dissolve (phase equilibrium).
If we do not mix the system, the dissolved copper(II) sulphate pentahydrate will slowly diffuse
through the solution from the bottom up to the surface, and after many weeks (relaxation time),
concentration in all parts of the system will become equal as well (thermodynamic equilibrium).1.4.3 Thermodynamic process
If the properties of a system change in time, i.e. if at least one thermodynamic quantity changes,
we say that a certainthermodynamic processtakes place in the system. The term process
relates to a very broad range of most varied processes, from simple physical changes such as,
e.g., heating, various chemical reactions, up to complex multistage processes. Individual kinds
of processes may be classified according to several criteria.