Written by the Fiveable Content Team • Last updated August 2025
Written by the Fiveable Content Team • Last updated August 2025
Definition
Change in entropy is the measure of the disorder or randomness in a system as it undergoes a process. It quantifies the energy dispersal and unavailability for doing work.
Entropy change ($\Delta S$) can be calculated using the formula $\Delta S = \frac{Q_{rev}}{T}$, where $Q_{rev}$ is the reversible heat exchange and $T$ is the absolute temperature.
The Second Law of Thermodynamics states that the total entropy of an isolated system always increases over time.
For irreversible processes, the change in entropy is greater than zero ($\Delta S > 0$).
In a reversible process, the change in entropy of a system and its surroundings sums to zero ($\Delta S_{system} + \Delta S_{surroundings} = 0$).
Entropy can be understood as a measure of molecular disorder or unpredictability within a thermodynamic system.
States that the total entropy of an isolated system can never decrease over time.
$Q_{rev}$ (Reversible Heat): The amount of heat exchanged during a process that occurs infinitely slowly, keeping the system and surroundings in equilibrium.
$T$ (Absolute Temperature): The temperature measured on an absolute scale such as Kelvin, which starts from absolute zero.