# Gibbs free energy Gibbs free energy combines the two things that decide whether a process happens: G = H − TS. Enthalpy wants systems to shed [[Energy]]; [[Entropy]] wants them to spread out; temperature sets the exchange rate between the two. At constant temperature and pressure a process runs spontaneously exactly when ΔG is negative, and stops when it reaches zero. ## Microsim — p5.js Sidecar: `Gibbs_free_energy.p5.js` · route leaf: `microsim/p5js/Gibbs_free_energy__20260910T0400Z`. Built to the Betterfire Standard v0 (single `ARTICLE` constant, HUD title and Wikitube URL, control hints, parameter readout and equation). | Control | Does | |---------|------| | `ΔH slider` | enthalpy change of the process | | `ΔS slider` | entropy change | | `T slider` | temperature, which weights the entropy term | *What to watch:* Drag temperature across the crossover and watch ΔG change sign — the point where the enthalpy and entropy bars cancel is the transition temperature. ## Temperature picks the winner Because the entropy term carries a factor of T, the same reaction can be spontaneous at one temperature and impossible at another. Ice melts above 273 K and freezes below it — same ΔH, same ΔS, and the sign of ΔG flips at the temperature where they balance, which is the melting point. That crossover is why a [[Phase_diagram]] has the shape it does, and why [[Third_law_of_thermodynamics|low-temperature]] chemistry behaves so differently from furnace chemistry. ## The working number ΔG is what an engineer actually uses. It sets the minimum [[Voltage]] an [[Electrolysis]] cell needs and the maximum a [[Galvanic_cell]] or [[Fuel_cell]] can deliver, through ΔG = −nFE. It fixes the equilibrium constant of a [[Chemical_equilibrium]], and its partial form is the [[Chemical_potential]] that governs where matter moves. Spontaneous does not mean fast — [[Hydrogen]] and [[Oxygen]] sit together indefinitely with a hugely negative ΔG until something supplies the activation energy. **Reads with:** *Thermodynamics and Chemistry, 2nd ed. (Howard DeVoe, 2020)* — [OTL record](https://open.umn.edu/opentextbooks/textbooks/thermodynamics-and-chemistry) · [download](https://www2.chem.umd.edu/thermobook/downloads.htm) · CC BY 4.0. Section 6 of the [[PORTAL_Thury_Hydrodynamics_Apex_Spine|Apex Spine]] book shelf. **On the spine:** [[Enthalpy]] · [[Chemical_potential]] · [[Chemical_equilibrium]] · [[Galvanic_cell]] · [[WT!Thury_Hydrodynamics_Compendium]]. ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Gibbs_free_energy) : [Wikitube](https://en.wikitube.io/wiki/Gibbs_free_energy) ## Previous hub tags Hubs: `Life_Physics`, `Systems`. Portals: [[PORTAL_Thury_Hydrodynamics_Apex_Spine]], [[PORTAL_Physics]], [[PORTAL_Energy]]. --- *Book-section wave · 2026-09-10 · article + p5 microsim shipped together.*