Question

In a thermodynamic system, define -- (a) Mechanical equilibrium (b) Chemical equilibrium and (c) Thermal equilibrium

Hint:

Think of equilibrium as "no change." {Mechanical:} No change in pressure/motion. {Chemical:} No change in composition. {Thermal:} No change in temperature (no heat flow). A system is in full thermodynamic equilibrium only when all three are true.

Answer 1

For a thermodynamic system to be in thermodynamic equilibrium, it must satisfy three types of equilibrium simultaneously:
Mechanical Equilibrium: A system is in mechanical equilibrium when there are no unbalanced forces acting on any part of the system or between the system and its surroundings. This means the pressure within the system is uniform and does not change with time.
Chemical Equilibrium: A system is in chemical equilibrium when its chemical composition is constant and there are no net chemical reactions occurring within it. The rate of forward reactions equals the rate of reverse reactions.
Thermal Equilibrium: A system is in thermal equilibrium when there is no net flow of heat energy between its parts or between the system and its surroundings. This implies that the temperature is uniform throughout the system and is the same as that of its surroundings.