GATE 2024 PH – Question 14
If a thermodynamical system is adiabatically isolated and experiences a change in volume under an externally applied constant pressure, then the thermodynamical potential minimized at equilibrium is the
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Correct answer: (A) enthalpy
Explanation
The thermodynamic potential that is minimised at equilibrium depends on what is held fixed.
| Potential | Held fixed |
|---|---|
| Internal energy $U$ | $S$, $V$ |
| **Enthalpy $H=U+PV$** | **$S$, $P$** |
| Helmholtz energy $F=U-TS$ | $T$, $V$ |
| Gibbs energy $G=H-TS$ | $T$, $P$ |
The system is **adiabatically isolated** (no heat exchange, so for a reversible process the entropy is fixed) and its volume changes under a **constant external pressure**. So the fixed variables are $S$ and $P$, and the potential that is minimised is the **enthalpy** (option A).