Pressure can significantly affect thermodynamic functions like enthalpy, Gibbs free energy, and others, especially for gases. Here’s a breakdown:
1. Internal Energy (U)
- For ideal gases, U depends only on temperature, not pressure.
- For real gases, high pressure increases molecular interactions, slightly affecting U.
2. Enthalpy (H)
- Defined as H = U + P × V.
- Increasing pressure increases the P × V term, so enthalpy can change.
- Important in high-pressure reactions and compressors.
3. Gibbs Free Energy (G)
- G = H – T × S
- Increasing pressure generally increases G for gases, because it reduces entropy (molecules are more “crowded”).
- This affects spontaneity of reactions involving gases.
- Example: Gas-phase reactions may become less favorable at high pressure if the number of gas molecules increases.
4. Helmholtz Free Energy (A)
- A = U – T × S
- At constant volume, pressure changes indirectly affect A through changes in internal energy and entropy.
Simple idea:
- High pressure compresses gases → molecules are closer → entropy decreases → Gibbs and Helmholtz energies usually increase.
- Low pressure → more freedom → energies decrease → reactions may become more spontaneous.