# Change in entropy in reversible and irreversible processes

The change in entropy for a reversible process is given as-

$$\Delta$$S = $$\frac{q_{rev}}{T}$$

Where qrev is the heat supplied isothermally and reversibly.

The change in entropy for a irreversible process is given as-

$$\Delta$$S = $$\frac{q_{irrev}}{T}$$

Where qrev is the heat supplied isothermally and irreversibly.

Could someone please explain these equations while giving the meaning of qrev and qirrev? I didn't really get a proper understanding of them.

For a reversible process that is not isothermal, you need to integrate between the initial and final states of the system: $$\Delta S=\int{\frac{dq_{rev}}{T}}$$where $$dq_{rev}$$ is the differential amount of heat transferred across the boundary of the system during the process and T is the temperature.
The change in entropy for a system experiencing an irreversible process is not $$q_{irrev}/T$$ or even $$\int{\frac{dq_{irrev}}{T}}$$. To get the entropy change for a system that experiences an irreversible process, you need to devise (i.e., dream up) an alternate reversible process path between the same initial and final states as the irreversible process, and evaluate $$\int{\frac{dq_{rev}}{T}}$$ for that reversible process path.