What Is Gibbs Free Energy? Spontaneity Made Simple
Enthalpy votes for releasing heat. Entropy votes for spreading out. When the two disagree — and they constantly do — chemistry needs a referee. That referee has a name, an equation, and one beautifully simple rule. The short answer: Gibbs free energy change (ΔG) combines enthalpy, entropy, and temperature into one number: ΔG = ΔH − TΔS . If ΔG is negative, the process is spontaneous (it can happen on its own); if positive, it isn't; if zero, the system sits at equilibrium. What "free energy" actually means The "free" in free energy doesn't mean it costs nothing — it means available . ΔG measures how much of a reaction's energy change is available to do useful work, after entropy has taken its share. A reaction with ΔG = −100 kJ/mol could, in principle, deliver up to 100 kJ of work per mole — driving a battery, contracting a muscle, powering a cell. And spontaneous is a technical word, not a speed claim. It means "thermodynamically allowed to ...