# Thermodynamic Processes

– Thermodynamic Processes involve the change of conditions (temperature, pressure, and volume).

## Thermodynamic Processes

– When a thermodynamic system changes from one state to another, the operation is called a Process.

– Thermodynamic Processes involve the change of conditions (temperature, pressure, and volume).

– The various types of thermodynamic processes are:

(1) Isothermal Processes

(3) Isobaric Processes

(4) Isochoric Processes

(5) Cyclic Process

## (1) Isothermal Processes

– Those processes in which the temperature remains fixed, are termed isothermal processes.

– This is often achieved by placing the system in a thermostat (a constant temperature bath).

– For an isothermal process dT = 0

– Those processes in which no heat can flow into or out of the system, are called adiabatic processes.

– Adiabatic conditions can be approached by carrying the process in an insulated container such as ‘thermos’ bottle.

– High vacuum and highly polished surfaces help to achieve thermal insulation.

– For an adiabatic process dq = 0

## (3) Isobaric Processes

– Those processes which take place at constant pressure are called isobaric processes.

– For example, the heating of water to its boiling point and its vaporization take place at the same atmospheric pressure.

– These changes are, therefore, designated as isobaric processes and are said to take place isobarically.

– For an isobaric process dp = 0

## (4) Isochoric Processes

– Those processes in which the volume remains constant are known as isochoric processes.

– The heating of a substance in a non-expanding chamber is an example of an isochoric process.

– For isochoric processes dV = 0.

## (5) Cyclic Process

– When a system in a given state goes through a number of different processes and finally returns to its initial state, the overall process is called a cycle or cyclic process.

– For a cyclic process dE = 0, dH = 0.

## Reversible and Irreversible Processes

– A thermodynamic reverse process is one that takes place infinitesimally slowly and its direction at any point can be reversed by an infinitesimal change in the state of the system.

– In fact, a reversible process is considered to proceed from the initial state to the final state through an infinite series of infinitesimally small stages.

– At the initial, final, and all intermediate stages, the system is in an equilibrium state.

– This is so because an infinitesimal change in the state of the system at each intermediate step is negligible.

– When a process goes from the initial to the final state in a single step and cannot be carried in the reverse order, it is said to be an irreversible process.

### Explanation with Example

– Here the system is in an equilibrium state in the beginning and at the end, but not at points in between.

– Consider a certain quantity of gas contained in a cylinder having a weightless and frictionless piston.

– The expansion of the gas can be carried by two methods illustrated in the following figure:

– Let the pressure applied to the piston be P and this is equal to the internal pressure of the gas.

– Since the external and internal pressures are exactly counterbalanced, the piston remains stationary and there is no change in the volume of the gas.

– Now suppose the pressure on the piston is decreased by an infinitesimal amount of dP.

– Thus the external pressure on the piston being P – dP, the piston moves up and the gas will expand by an infinitesimal small amount.

– The gas will, therefore, be expanded infinitely slowly i.e., by a thermodynamically reversible process.

– At all stages in the expansion of the gas, dP being negligibly small the gas is maintained in a state of equilibrium throughout.

– If at any point of the process, the pressure is increased by dP, the gas would contract reversibly.

– On the other hand, the expansion is irreversible in the figure above (b) if the pressure on the piston is decreased suddenly.

– It moves upward rapidly in a single operation.

– The gas is in an equilibrium state in the initial and final stages only.

– The expansion of the gas, in this case, takes place in an irreversible manner.

## Difference between Reversible and Irreversible Processes

Reference: Essentials of Physical Chemistry /Arun Bahl, B.S Bahl and G.D. Tuli / multicolor edition.