# Chemical energy **Chemical energy** is the energy held in the arrangement of [[Electron|electrons]] among nuclei — in [[Chemical_bond|chemical bonds]] and in the relative positions of [[Atom|atoms]] within and between [[Molecule|molecules]] — and released or absorbed when a [[Chemical_reaction|reaction]] rearranges them. It is a form of [[Potential_energy|potential energy]]: nothing moves in bulk while it is being stored, and the ledger is kept in the electronic structure. When the bonds formed are stronger than the bonds broken, the difference leaves as [[Heat|heat]], light or electrical work; when they are weaker, the reaction must be driven. In the microsim below the reader picks a fuel and a mass, and a single table drives every readout: energy per kilogram, energy per litre, energy per atom or per bond, and the mass of carbon dioxide that comes with each megajoule. The governing arithmetic is one division, E_atom = ΔH_c/N_A. Burning carbon releases 394 kJ for every 12 g — one [[Mole_(unit)|mole]] — and dividing by the [[Avogadro_constant|Avogadro constant]] turns that into 6.5×10⁻¹⁹ J, about 4 [[Electronvolt|electronvolts]] per atom, or "about 1 eV per C–O bond."[^murphy-carbon] Every fuel on the ladder lands within a factor of a few of that figure, which is the single most useful fact about chemical energy. On the [[Energy]] flagship this article is the child of Part I — Forms, section *Chemical energy* (row E4), where it carries the fuels half of the shared fuel-and-cell dial; the electrochemical half belongs to [[Electric_battery|batteries]] at row E19. ### The scale of the store The natural unit of chemical energy is the electronvolt, because the store lives in valence electrons and an electronvolt is the energy an electron gains falling through one volt. The whole of practical chemistry therefore occupies a narrow band: bond energies run from roughly one to about six electronvolts, and the energy released per atom of fuel oxidised is of the same order. Expressed in the units an engineer uses, chemical fuels deliver about 10 kcal per gram, which is 41.8 MJ/kg (derived).[^murphy-fission-vs-chem] That band is narrow because the electrostatic forces holding valence electrons are all of similar strength. It is also low, which is the second useful fact. The fission of uranium-235 delivers 16.8 million kcal per gram, about 1.7 million times as much (derived), and deuterium–deuterium fusion 137 million kcal per gram, because those processes tap the [[Nuclear_force|strong force]] rather than the outermost electrons.[^murphy-fission-vs-chem][^murphy-fusion] Chemical energy is the store that is cheap to release, easy to store at ambient conditions and safe to carry in a tank, and that is what it is for; it is not the store with the most joules in it. ### Bonds, enthalpy and the heat of reaction The accounting is done with [[Enthalpy|enthalpy]] rather than with bonds directly, because bond energies are averages while enthalpies of formation are measured. The [[Standard_enthalpy_of_reaction|standard enthalpy of reaction]] is the difference between the tabulated formation enthalpies of products and reactants, and it is path-independent, so a reaction that cannot be run in one step can be assembled from steps that can — which is [[Hess's_law|Hess's law]]. Bond energies remain the right picture for why the numbers come out as they do. Splitting a hydrogen molecule into two atoms costs 436 kJ per mole, which is 4.52 eV per bond (derived).[^cboc-hh] Burning hydrogen to liquid water releases 286 kJ per mole of H₂, which is 142 MJ/kg and about 3.0 eV per molecule (derived) — the highest gravimetric figure of any chemical fuel, and the reason the [[Hydrogen_economy|hydrogen economy]] keeps being proposed.[^haverkort-thermoneutral] What decides whether the reaction actually runs is not the enthalpy but the [[Gibbs_free_energy|Gibbs energy]], and what decides how fast is the [[Activation_energy|activation energy]] and whatever [[Catalysis|catalyst]] is present; a tank of petrol and air is thermodynamically doomed and kinetically stable, which is exactly what makes it useful. ### The fuel ladder The microsim's table is the article's spine. Selecting a fuel and a mass drives four readouts at once, and the point of putting them side by side is that they rank fuels differently. | Store | Energy released | Per unit mass | Per particle | |---|---|---|---| | Carbon → CO₂ | 394 kJ per 12 g | 32.8 MJ/kg (derived) | ≈4 eV per atom[^murphy-carbon] | | Hydrogen → liquid H₂O | 286 kJ per mol H₂ | 142 MJ/kg (derived) | ≈3.0 eV per molecule (derived)[^haverkort-thermoneutral] | | H–H bond (breaking) | 436 kJ/mol | — | 4.52 eV per bond (derived)[^cboc-hh] | | Natural gas | ≈1.036 therm per 100 ft³ | ≈1.09 MJ/ft³ (derived) | —[^murphy-therm] | | Propane | ≈91,500 Btu per gallon | ≈96.5 MJ/gal (derived) | —[^murphy-therm] | | Chemical fuels, generic | ≈10 kcal/g | ≈41.8 MJ/kg (derived) | ≈1–6 eV per bond[^murphy-fission-vs-chem] | | U-235 fission | 16.8 million kcal/g | 7.0×10⁷ MJ/kg (derived) | ≈200 MeV per nucleus[^murphy-fission-vs-chem] | *The sim's preset table. Values in the first column are read from the cited Portal Books; the per-mass and per-particle columns are computed from them using 1 kcal = 4,184 J, 1 Btu = 1,055 J, 1 therm = 1.055×10⁸ J and 1 eV = 1.6×10⁻¹⁹ J.*[^murphy-units] The table is ILLUSTRATIVE in one respect that matters commercially. A [[Heat_of_combustion|heat of combustion]] can be quoted as a higher heating value, with the product water condensed, or as a lower heating value, with it left as vapour, and the two differ by the enthalpy of vaporisation — for hydrogen by about 16 %. The hydrogen row above is a higher heating value, because it is derived from the 286 kJ/mol that corresponds to the thermoneutral voltage of a water electrolyser.[^haverkort-thermoneutral] Comparisons that mix the conventions are the commonest error in fuel tables. ### Energy per kilogram against energy per litre The second readout exists because vehicles are limited by volume at least as often as by mass. [[Energy_density|Energy density]] measured per kilogram and per litre rank fuels in different orders, and the gap is widest exactly where it is most inconvenient. Hydrogen leads every chemical fuel per kilogram at 142 MJ/kg, and as a gas at ambient pressure it is close to worthless per litre; compressing or liquefying it recovers volume at a cost in energy and in tankage.[^kerlin-fuels] Liquid hydrocarbons sit in the comfortable middle of both scales, which is why the transport fleet still runs on them, and the practical constraints — [[Fuel|fuel]] handling, cold-start behaviour, the acid content of the exhaust — are as decisive as the energy content.[^kerlin-fuels] The comparison that puts the ladder in perspective is with electrochemical storage. A 9 V battery rated at 0.5 A·h holds 4.5 W·h, which is 16.2 kJ.[^murphy-battery] At the generic chemical figure of 10 kcal/g, that is the energy in about 0.4 g of fuel (derived). The store in a cell is chemical energy like any other; what a cell buys is not density but the ability to deliver the energy as electrical work directly, without a [[Heat_engine|heat engine]] in the path. ### Where the fuels come from Almost all of the chemical energy burned industrially was put there by [[Photosynthesis|photosynthesis]], either last season in the case of [[Biomass|biomass]] or over geological time in the case of [[Fossil_fuel|fossil fuels]], where buried organic matter was cooked under pressure into coal, oil and [[Natural_gas|natural gas]].[^kerlin-fossil] The store is therefore solar energy that has already been concentrated, which is why its energy density is so much higher than that of any flow humans can intercept directly. Getting it back out of the ground is a flow problem rather than a chemical one. Petroleum sits in the pore space of a rock, and production is governed by pressure-driven flow through a porous medium: porosity fixes how much is there, permeability fixes how fast it can move, and [[Darcy's_law|Darcy's law]] relates the two to the pressure gradient that drives the flow.[^zeidouni] A reservoir that holds a great deal of oil and will not give it up quickly is a common and expensive outcome, and enhanced recovery is the business of changing the mobility term rather than the amount in place. The same accounting, run forward over decades, is what puts a ceiling on fossil supply and makes the substitution question a matter of rates rather than of totals.[^theis-tomkin] ### Carbon dioxide per megajoule The fourth readout follows from stoichiometry alone and needs no extra data. Oxidising carbon produces 44 g of carbon dioxide for every 12 g of carbon, and releases 394 kJ; that is 112 g of CO₂ per megajoule (derived).[^murphy-carbon] Hydrogen produces none. Every real fuel lies between those two limits in proportion to its hydrogen-to-carbon ratio, so natural gas, which is mostly CH₄, does better per unit energy than liquid hydrocarbons, which in turn do better than coal — a ranking fixed by chemistry before any question of efficiency arises. This is the readout that turns the fuel ladder from a physics exercise into an energy-policy one. A fuel's usefulness is judged on four axes at once: joules per kilogram, joules per litre, carbon dioxide per joule and the cost and difficulty of handling it, and no chemical fuel is best on all four.[^theis-tomkin] The figure is also a floor rather than a forecast, because it counts only the [[Combustion|combustion]] step; the carbon spent extracting, refining and moving the fuel is additional, and for the marginal barrel it is not small.[^theis-tomkin] ### Chemical energy as electrical work When a reaction's electrons can be made to travel through an external circuit rather than passing directly between reactants, the chemical energy appears as electrical work instead of heat, and the ceiling is the Gibbs energy rather than the enthalpy. This is the operating principle of [[Electric_battery|batteries]], of [[Fuel_cell|fuel cells]] and, run backwards, of [[Electrolysis_of_water|water electrolysis]].[^haverkort-batteries] For the water reaction the two numbers differ by a useful amount: ΔG° = 237 kJ/mol gives an equilibrium cell voltage of 1.23 V, while the enthalpy of 286 kJ/mol corresponds to a thermoneutral voltage of 1.48 V, and a cell run between the two draws heat from its surroundings.[^haverkort-thermoneutral] The practical consequence is that electrochemical conversion is not bound by the Carnot limit, because no heat reservoir is involved. A [[Lithium-ion_battery|lithium-ion cell]] stores chemical energy in the electrode materials and returns it at high round-trip efficiency but at an energy density one to two orders of magnitude below liquid fuel; a fuel cell keeps the fuel's density and converts it directly, at the cost of a catalyst and a gas-handling system.[^haverkort-batteries] Direct conversion of chemical energy to electricity, without an intermediate mechanical stage, is the whole subject of the direct-energy literature.[^mitofsky] ### Chemical energy in living systems Biology runs on the same ledger at smaller denominations. Photosynthesis drives the endergonic assembly of carbohydrate from carbon dioxide and water using light, and [[Cellular_respiration|respiration]] runs the reaction back to release the store, with [[Adenosine_triphosphate|ATP]] serving as the intermediate currency in which small amounts of chemical energy are moved between reactions. A human adult on 2,000 kcal a day is consuming 8.368 MJ, which averaged over 86,400 seconds is 96.85 W — "very close to 100 W," and a useful anchor for how small a biological power demand is against an industrial one.[^murphy-diet] Against that, the average resident of the United States commands about 10,000 W, roughly a hundred people's worth of [[Food_energy|food energy]].[^murphy-diet] The chemistry of food and the chemistry of fuel are the same chemistry, which is why dietary energy is quoted in kilocalories and fuel energy in megajoules without any change of physics; only the units and the [[Basal_metabolic_rate|metabolic]] pathway differ. What biology adds is control — enzymes lowering activation barriers so that reactions which would otherwise need a flame proceed at 37 °C. *See also:* [[Fuel]] · [[Fossil_fuel]] · [[Energy_density]] · [[Heat_of_combustion]] · [[Combustion]] · [[Enthalpy]] · [[Fuel_cell]] ## References [^murphy-carbon]: Murphy, Thomas (2021). *Energy and Human Ambitions on a Finite Planet*. Chapter 5 "Energy and Fossil Fuels", p. 98 (394 kJ per 12 g of carbon, divided by 6×10²³, is 6.5×10⁻¹⁹ J, "about 4 eV per atom" and "about 1 eV per C–O bond"; 1 eV = 1.6×10⁻¹⁹ J). The 32.8 MJ/kg and 112 g CO₂/MJ figures are derived from it. Portal Book 097, https://open.umn.edu/opentextbooks/textbooks/energy-and-human-ambitions-on-a-finite-planet [^murphy-units]: Murphy (2021), *Energy and Human Ambitions on a Finite Planet*, Chapter 5, pp. 92–98 (1 kWh = 3.6 MJ; 1 cal = 4.184 J and 1 kcal = 4,184 J; 1 Btu ≈ 1,055 J; 1 therm = 10⁵ Btu = 1.055×10⁸ J; 1 eV = 1.6×10⁻¹⁹ J). Portal Book 097. [^murphy-therm]: Murphy (2021), *Energy and Human Ambitions on a Finite Planet*, Chapter 5, p. 96 (100 ft³ of natural gas ≈ 1.036 therm; 1 gal of propane ≈ 91,500 Btu; 10,000 kcal = 41.84 MJ ≈ 40,000 Btu = 0.4 therm ≈ 40 ft³ of gas, or a little under half a gallon of propane). The MJ/ft³ and MJ/gal conversions are derived. Portal Book 097. [^murphy-fission-vs-chem]: Murphy (2021), *Energy and Human Ambitions on a Finite Planet*, Chapter 15 "Nuclear Energy", pp. 273–275 (16.8 million kcal/g for fission against ≈10 kcal/g for chemistry; the graphical fission estimate of ≈210 MeV against an exact 172 MeV per ²³⁵U nucleus). The MJ/kg conversions and the 1.7-million-fold ratio are derived. Portal Book 097. [^murphy-fusion]: Murphy (2021), *Energy and Human Ambitions on a Finite Planet*, Chapter 15, p. 285 (energy densities of 153, 137 and 81 million kcal/g for proton–proton, D–D and D–T fusion). Portal Book 097. [^murphy-battery]: Murphy (2021), *Energy and Human Ambitions on a Finite Planet*, Chapter 5, pp. 97–98 (a 9 V, 0.5 A·h battery holds 4.5 W·h = 16.2 kJ, or 4.5 h at 1 W; A·h × V = W·h). The 0.4 g fuel equivalent is derived. Portal Book 097. [^murphy-diet]: Murphy (2021), *Energy and Human Ambitions on a Finite Planet*, Chapter 5, pp. 94–95 (2,000 kcal/day = 8.368 MJ over 86,400 s = 96.85 W, "very close to 100 W", and the book's warning not to quote it to four figures; the US at ≈100 quads/yr ≈ 3 TW ≈ 10,000 W per person). Portal Book 097. [^cboc-hh]: Blackstock, Lindsay; Brewer, Sharon; Cinel, Bruno (2022). *Chemical Bonding and Organic Chemistry*. Chapter 4 "Chemical Bonding", p. 231 (H₂ → 2H, +436 kJ/mol). The 4.52 eV per bond is derived. Portal Book 054, https://open.umn.edu/opentextbooks/textbooks/chemical-bonding-and-organic-chemistry [^haverkort-thermoneutral]: Haverkort, Willem (2024). *Electrolysers, Fuel Cells and Batteries: Analytical Modelling*. Chapter 1 "Electrochemistry", pp. 25–27 (ΔG = −n·F·V_eq; 237 kJ/mol gives 1.23 V and 286 kJ/mol gives the thermoneutral 1.48 V, with the cell drawing heat from its surroundings between them). The 142 MJ/kg and 3.0 eV per molecule figures are derived from the 286 kJ/mol. Portal Book 053, https://open.umn.edu/opentextbooks/textbooks/electrolysers-fuel-cells-and-batteries-analytical-modelling [^haverkort-batteries]: Haverkort (2024), *Electrolysers, Fuel Cells and Batteries*, Chapter 7 "Batteries", pp. 110–129, and Chapter 9 "Fuel cells", pp. 132–153 (electrode processes, transport limitations and the energy-density comparison; page to pin). Portal Book 053. [^kerlin-fuels]: Kerlin, Thomas (2013). *Future Energy: Opportunities & Challenges*. Chapter 4 "Fuels for transportation", pp. 192–216 (liquid and gaseous transport fuels, diesel, range, and the handling constraints that accompany each; page to pin). Portal Book 048, https://open.umn.edu/opentextbooks/textbooks/future-energy-opportunities-challenges [^kerlin-fossil]: Kerlin (2013), *Future Energy*, Chapter 1 "Fossil fuels", pp. 30–155 (the origin, extraction and combustion of coal, oil and natural gas; page to pin). Portal Book 048. [^zeidouni]: Zeidouni, Mehdi (2025). *Petroleum Reservoir Dynamics*. Chapter 3, pp. 2–167 (single- and multi-phase flow in porous media: porosity, permeability, compressibility, capillarity and Darcy flow as the governing description of reservoir production; page to pin). Portal Book 084, https://open.umn.edu/opentextbooks/textbooks/petroleum-reservoir-dynamics [^theis-tomkin]: Theis, Tom; Tomkin, Jonathan, eds. (2015). *Sustainability: A Comprehensive Foundation*. Chapter 10 "Sustainable Energy Systems" (the comparison of fuels on supply rate, emissions and substitutability; page to pin). Portal Book 098, https://open.umn.edu/opentextbooks/textbooks/sustainability-a-comprehensive-foundation [^mitofsky]: Mitofsky, Andrea (2018). *Direct Energy*. Part I "Survey of Energy Conversion Devices", pp. 33–254 (devices that convert chemical, thermal or optical energy to electricity without an intermediate mechanical stage; page to pin). Portal Book 055, https://open.umn.edu/opentextbooks/textbooks/direct-energy <!-- MATTERSIM:BEGIN g33 — Matter & Energy Cluster microsim (framework build, specs/sims/Chemical_energy.json); do not hand-edit inside --> **Microsim — three.js (Wikitube framework):** *Chemical energy* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/matter/Chemical_energy.html" data-title="Chemical energy"></div> *Built from `MICROSIM_GUIDE/specs/sims/Chemical_energy.json`; part of the [[PORTAL_Matter|Matter portal]] spine (section sims and See-also variants).* <!-- MATTERSIM:END --> ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Chemical_energy) : [Wikitube](https://en.wikitube.io/wiki/Chemical_energy) · pinned revision [1359877719](https://en.wikipedia.org/w/index.php?oldid=1359877719) · 2026-09-11 ## Previous hub tags Hubs: `Life_Physics`. 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