Gibbs free energy answers the question every chemist eventually asks about a reaction: will it happen on its own? By combining how much heat a reaction releases or absorbs (ΔH) with how much disorder it creates or removes (ΔS) at a given temperature, ΔG gives a single number whose sign settles the question — at least for whether the reaction is thermodynamically favorable, not necessarily how fast it runs.

How ΔG is calculated

The Gibbs free energy equation, ΔG = ΔH − TΔS, weighs two competing tendencies: reactions tend to release energy (favoring negative ΔH) and tend to increase disorder (favoring positive ΔS), and temperature controls how much the entropy term matters. At low temperature, the ΔH term dominates; at high temperature, the TΔS term dominates. The unit mismatch between ΔH (kJ) and ΔS (J/K) is the calculation's classic trap — ΔS must be converted to kJ/K (divide by 1000) before it can be subtracted from ΔH, and this calculator does that conversion automatically.

Reading the sign of ΔG

A negative ΔG means the reaction is spontaneous (exergonic) — it can proceed forward without outside energy input, though it may still be slow if the activation energy is high. A positive ΔG means the reaction is non-spontaneous (endergonic) as written; it requires continuous energy input to proceed, or it may run spontaneously in the reverse direction instead. A ΔG at or near zero means the system is at or near equilibrium, where forward and reverse rates roughly balance.

Why spontaneity can flip with temperature

Because temperature multiplies the entropy term, a reaction's spontaneity can flip as temperature changes whenever ΔH and ΔS have the same sign. If ΔH > 0 and ΔS > 0 (absorbs heat, increases disorder), the reaction becomes spontaneous only above a crossover temperature. If ΔH < 0 and ΔS < 0 (releases heat, decreases disorder), it's spontaneous only below a crossover temperature. When ΔH and ΔS have opposite signs, the reaction is spontaneous at every temperature (ΔH < 0, ΔS > 0) or never spontaneous at any temperature (ΔH > 0, ΔS < 0) — the Crossover Temp tab flags this by returning a negative or otherwise non-physical temperature.