Reversible vs Irreversible Processes | chemca

chemca.in

Home Articles Quizzes About

Thermodynamics

Reversible and Irreversible Processes

The theoretical limit vs. the reality of natural processes.

By chemca Team • Updated Jan 2026

In thermodynamics, the path taken to go from state A to state B determines the work done and the heat exchanged. We categorize these paths into two fundamental types: Reversible (ideal) and Irreversible (real/natural).

1. Reversible Process

A reversible process is carried out infinitesimally slowly (quasi-statically) so that the system remains in thermodynamic equilibrium with its surroundings at every instant.

• Driving Force $\approx$ Opposing Force: The driving force differs from the opposing force by an infinitesimal amount ($dP$).
• Reversibility: The process can be reversed completely at any point by a tiny change in conditions, restoring both system and surroundings to their initial states.
• Work Done: Produces Maximum Work ($W_{max}$) during expansion.

$$ P_{ext} = P_{int} \pm dP $$ $$ W_{rev} = -\int_{V_1}^{V_2} P_{int} \, dV $$

2. Irreversible Process

An irreversible process occurs rapidly in a finite time. These are spontaneous processes where the system is not in equilibrium during the transition.

• Driving Force $\gg$ Opposing Force: There is a finite difference between the internal pressure and external pressure.
• Natural Process: All naturally occurring processes (e.g., free expansion, waterfall, heat flow) are irreversible.
• Work Done: The work done is less than the maximum possible work.

$$ P_{ext} = \text{Constant} $$ $$ W_{irrev} = -P_{ext} (V_2 - V_1) $$

3. Detailed Comparison

Reversible Process	Irreversible Process
Occurs in infinite steps (infinitesimally slow).	Occurs in finite steps (rapid process).
System and surroundings are always in equilibrium.	Equilibrium exists only at the initial and final stages.
Driving force is infinitesimally larger than opposing force.	Driving force is significantly larger than opposing force.
Maximum work is obtained (in expansion).	Work done is less than maximum work.
It is an ideal/theoretical concept.	It is a real/practical concept.

4. Work Done Visualization

On a PV Diagram:

• Reversible: The work done is the total area under the PV curve (Integral).
• Irreversible: The work done is the area of the rectangle defined by $P_{ext} \times \Delta V$. Since $P_{ext} < P_{int}$ during expansion, this area is smaller.

Entropy Note: In a reversible process, the entropy change of the universe is zero ($\Delta S_{total} = 0$). In an irreversible process, the entropy of the universe increases ($\Delta S_{total} > 0$).

Knowledge Check

Test your understanding of Reversible vs Irreversible

© 2026 chemca.in. All rights reserved.

Optimized for learning Thermodynamics.

Quiz Revision Notes Downloads Class Notes Video Lectures