Gibbs Free Energy Explained: Predicting Reaction Spontaneity

Gibbs Free Energy quantifies the amount of enthalpy that can be converted into useful work. Josiah Willard Gibbs defined this thermodynamic property to predict whether a chemical or biological reaction will proceed without external input. The governing equation

[ \Delta G = \Delta H - T\Delta S ]

relates the change in free energy (ΔG) to the change in enthalpy (ΔH), temperature (T), and the change in entropy (ΔS). By evaluating ΔG, one can determine the spontaneity of a reaction.

The Spontaneity Criterion

A reaction is spontaneous when ΔG < 0, at equilibrium when ΔG = 0, and non‑spontaneous when ΔG > 0. This simple sign rule provides a direct test for reaction feasibility. As one quote from the lecture states, “If Delta G is less than zero, our reaction is going to be spontaneous.”

Intuitive Analysis of Variables

The interplay of ΔH and ΔS determines how temperature influences spontaneity.

• When ΔH < 0 (energy released) and ΔS > 0 (entropy increases), the reaction is always spontaneous.
• When ΔH > 0 and ΔS < 0, the reaction never becomes spontaneous.
• When both ΔH and ΔS are negative, the reaction is spontaneous only at low temperatures because the enthalpy term dominates.
• When both ΔH and ΔS are positive, the reaction is spontaneous only at high temperatures as the entropy term (TΔS) outweighs the enthalpy cost.

Temperature therefore acts as a “weighing factor” on entropy, a point emphasized by the speaker: “Temperature here is the weighing factor on entropy.” At low T, ΔH controls ΔG; at high T, TΔS controls ΔG.

Practical Applications

The Gibbs free energy equation assumes constant pressure and temperature, conditions that match test‑tube experiments and many biological systems. Interpreting ΔG values under these constraints allows chemists and biochemists to predict reaction direction, design pathways, and assess energy efficiency. Even when a local entropy decrease occurs, the overall entropy of the universe rises because released heat increases the surrounding entropy, satisfying the Second Law of Thermodynamics. As noted, “The entropy of the universe is going to increase, because of this released heat.”