Short Answer
Gibbs free energy G is the best answer.
Thermodynamics questions usually test sign conventions, state variables, or what is being held constant. Before calculating, decide whether the system is exchanging heat, doing work, or both.
G = U + PV − TS is minimised for spontaneous processes at constant T and P; ΔG < 0 for spontaneous reactions.
Why This Answer Is Correct
This is a Medium-level question in Thermodynamics. The prompt is really testing whether you can connect the concept to its defining physical relationship instead of picking a nearby-but-wrong term.
Write the system boundary first. Many thermodynamics mistakes disappear once you know what counts as heat, work, and internal-energy change.
Choices At A Glance
- A. Internal energy U
- B. Helmholtz free energy F
- C. Gibbs free energy G
- D. Enthalpy H
When similar options appear on an exam, eliminate the ones that break the core law, use the wrong units, or confuse a definition with a consequence.
Topic Snapshot
Topic: Thermodynamics
Difficulty: Medium
Best next move: Re-state the governing law in your own words, then solve one more example from the same topic before moving on.