Gibbs Free Energy (G) is a thermodynamic quantity that tells us whether a process or chemical reaction can occur spontaneously at constant temperature and pressure.
Definition:
It combines enthalpy (H), entropy (S), and temperature (T) into a single value: G=H−TSG = H – T S
- H (Enthalpy): Heat content of the system.
- T (Temperature in Kelvin): Absolute temperature.
- S (Entropy): Measure of disorder.
Key Idea:
The change in Gibbs free energy (ΔG) decides spontaneity:
- ΔG < 0 → Negative → Reaction is spontaneous (happens on its own).
- ΔG > 0 → Positive → Reaction is non-spontaneous (needs energy input).
- ΔG = 0 → Reaction is at equilibrium.
Examples:
- Combustion of fuel → ΔG is negative → happens spontaneously, releasing energy.
- Photosynthesis → ΔG is positive → requires sunlight (external energy).
- Melting of ice at 25 °C → ΔG is negative → spontaneous at room temperature.
In short:
Gibbs Free Energy is like the “useful energy” available to do work. It tells us if a reaction will occur naturally or if we need to supply energy.