Spontaneity - Gibbs Energy and Spontaneity

Gibbs free energy (ΔG) is a thermodynamic quantity that determines spontaneity at constant temperature and pressure.

It is defined as: ΔG = ΔH − TΔS, 
where ΔH = enthalpy change, T = temperature, and ΔS = entropy change.

• A process is spontaneous when ΔG < 0 because free energy is released.
• If ΔG > 0, the process is non-spontaneous and requires external energy.
• At equilibrium, ΔG = 0 and no net change occurs in the system.

Temperature plays a key role when ΔH and ΔS have the same sign:

• ΔH < 0 and ΔS > 0 → always spontaneous
• ΔH > 0 and ΔS < 0 → never spontaneous
• ΔH < 0 and ΔS < 0 → spontaneous at low T
• ΔH > 0 and ΔS > 0 → spontaneous at high T

Gibbs free energy combines both enthalpy (heat changes) and entropy (disorder), making it a more practical criterion than entropy alone.