# Enthalpy Enthalpy is [[Internal_energy]] plus the pressure–volume term, H = U + pV. The definition looks arbitrary until you notice what it buys: at constant pressure — which is how almost every reaction on a bench or in a plant actually runs — the heat exchanged equals the enthalpy change exactly. That single fact is why chemists tabulate H rather than U. ## Microsim — p5.js Sidecar: `Enthalpy.p5.js` · route leaf: `microsim/p5js/Enthalpy__20260910T0400Z`. Built to the Betterfire Standard v0 (single `ARTICLE` constant, HUD title and Wikitube URL, control hints, parameter readout and equation). | Control | Does | |---------|------| | `p slider` | external pressure | | `V slider` | system volume | | `U slider` | internal energy | *What to watch:* Watch the three bars: internal energy, the pV term, and their sum. Expand the gas at fixed pressure and pV grows — H climbs even where U does not. ## The pV term is the price of room A reaction that produces gas must push the atmosphere back to make space, and that work is real [[Energy]] leaving the system. Folding pV into the definition means the bookkeeping happens automatically: measure the heat at constant pressure and you have ΔH without separately accounting for the shoving. [[Combustion]] enthalpies, bond enthalpies, and latent heats of [[Phase_transition|phase change]] are all tabulated this way, and Hess's law lets them be added along any path because H is a state function. ## Sign, and what it does not tell you Negative ΔH means heat comes out — exothermic, like burning [[Hydrogen]]. Positive means heat goes in. But enthalpy alone does not decide whether a reaction proceeds: endothermic processes happen constantly, from ice melting to salts dissolving. Direction needs [[Entropy]] as well, combined in the [[Gibbs_free_energy]]. Enthalpy answers "how much heat", not "will it go". **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:** [[First_law_of_thermodynamics]] · [[Internal_energy]] · [[Gibbs_free_energy]] · [[Combustion]] · [[WT!Thury_Hydrodynamics_Compendium]]. ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Enthalpy) : [Wikitube](https://en.wikitube.io/wiki/Enthalpy) ## 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.*