Thermodynamic equilibrium with illustrations

A system is in thermodynamic equilibrium when it is simultaneously in mechanical, thermal and chemical equilibrium, so that its properties are uniform and do not change with time, and no spontaneous change occurs if it is isolated from the surroundings.

Components

1. Mechanical equilibrium: no unbalanced force; pressure is the same everywhere and equal to external pressure.
2. Thermal equilibrium: no temperature difference; no heat flow.
3. Chemical equilibrium: no net chemical reaction or mass diffusion; composition is constant.

Illustrations

 (a) Not in equilibrium       (b) In equilibrium
 +------------------+       +------------------+
 |  hot gas | cold  |       |   gas, uniform   |
 |  400 K   | 300 K |       |   T = 350 K      |
 |  5 bar   | 1 bar |       |   P = 3 bar      |
 +------------------+       +------------------+
 heat and mass flow          no change with time

• A gas in a cylinder with a weighted piston: if the gas is at uniform T and its pressure balances the piston weight plus atmospheric pressure, it is in equilibrium. If the weight is suddenly removed, the piston moves and the system is out of equilibrium until it settles at a new equilibrium state.
• Hot metal dropped in cold water: the system is not in equilibrium until both reach the same temperature.
• Water and its vapour at the same T and P in a closed vessel (no net evaporation/condensation) is in phase (chemical) equilibrium.

Thermodynamic properties are defined only for equilibrium states.