# Difference Between Reversible and Irreversible Process

## Main Difference – Reversible vs Irreversible Process

The terms reversible and irreversible are used in thermodynamics to describe the behavior of a system. Here, the process that occurs in that system can be named either as a reversible process or an irreversible process. The main difference between reversible and irreversible process is that a reversible process is a thermodynamic process that can be reversed to get the initial state whereas an irreversible process is a thermodynamic process that cannot be reversed to obtain the initial state of the system.

### Key Areas Covered

1. What is a Reversible Process
– Definition, Explanation of Features with Examples
2. What is an Irreversible Process
– Definition, Explanation of Features with Examples
3. What is the Difference Between Reversible and Irreversible Process
– Comparison of Key Differences

Key Term: Finite Changes, Infinitesimal, Irreversible, Reversible, Thermodynamics

## What is a Reversible Process

A reversible process is a process that can be reversed in order to obtain the initial state of a system. After the completion of a certain chemical reaction (a process), the initial state of a system is changed. In other words, when the system undergoes a certain process, the system is changed from its initial state to the final state. If we can reverse the process to obtain the initial state back, we call that process a reversible process.

Let us consider a system that has been changed from state A to state B. The initial state of the system is A and the final state is B. If we can reverse the process that caused this change, this process is a reversible process. However, this reverse of the process should give the original state A and the properties of the surrounding should be the same (no change in thermodynamic properties).

Figure 01: Reversible Process

A reversible process can be completely reversed to restore the initial state which does not show any trace of evidence that the system was subjected to that process. There are two important things for a reversible process to occur. The process should occur in an infinitesimally small time. This means the process should take a time that cannot even be measured.  The initial and final state of the system should be in equilibrium with each other. Otherwise, the process is not reversible.

## What is an Irreversible Process

An irreversible process is a thermodynamic process that cannot be reversed in order to obtain the initial state of a system. That means, if a system undergoes an irreversible process, the system is changed to the final state and we cannot obtain the initial state back by reversing the process.

These types of processes are also called natural processes because almost all processes that occur in nature are irreversible. Irreversible processes take a measurable time to occur and there are no equilibriums in that system. The entropy of the system is changed due to the irreversible process and this change cannot be reversed. An irreversible process can be described as a thermodynamic process that departs from equilibrium. Here, finite changes are made.

Figure 2: Irreversible Process

Some examples of irreversible processes include water flow from a high level to a lower level, the heat flow from a warmer place to a cooler place, releasing a gas into a vacuum system, etc.

## Difference Between Reversible and Irreversible Process

### Definition

Reversible Process: A reversible process is a process that can be reversed in order to obtain the initial state of a system.

Irreversible Process: An irreversible process is a thermodynamic process that cannot be reversed in order to obtain the initial state of a system.

### Reversibility

Reversible Process: Reversible processes can be reversed.

Irreversible Process: Irreversible processes cannot be reversed.

### Changes in the System

Reversible Process: Infinite changes occur in the system in reversible processes.

Irreversible Process: Finite changes occur in the system in irreversible processes.

### Equilibrium

Reversible Process: There is an equilibrium between the initial state and the final state of a system where a reversible process has taken place.

Irreversible Process: There are no equilibriums in systems where an irreversible process has occurred.

### Conclusion

Reversible and irreversible processes are thermodynamic processes that can happen in a thermodynamic system. The main difference between reversible and irreversible process is that a reversible process is a thermodynamic process that can be reversed to get the initial state whereas an irreversible process is a thermodynamic process that cannot be reversed to obtain the initial state of the system.

##### References:

1. “Reversible process (Thermodynamics).” Wikipedia, Wikimedia Foundation, 30 Sept. 2017, Available here.
2. “What are Reversible and Irreversible Processes in Thermodynamics?” Brighthub Engineering, 22 Apr. 2011, Available here.

##### Image Courtesy:

1. “Adjabatic-revisible-state-change” By Andlaus – Own work (CC0) via Commons Wikimedia
2. “Adiabatic-irrevisible-state-change” By Andlaus – Own work (CC0) via Commons Wikimedia