{ "node": { "id": "term:gibbs-free-energy", "type": "Term", "label": "Gibbs free energy", "properties": { "id": "gibbs-free-energy", "name": "Gibbs free energy", "abbr": "ΔG", "section": "topic-electrochemistry", "definition": "The maximum useful work a reaction can deliver at constant temperature and pressure. For a cell it equals the charge transferred times the cell voltage, with a negative sign, which ties the thermodynamics of the materials to the voltage on the terminals.", "formula": "ΔG = −nFE", "unit": "kJ/mol", "related": [ "cell-voltage", "nernst-equation", "faradays-constant", "entropy-coefficient" ], "example": { "title": "Convert voltage to molar free energy", "text": "For a one-electron reaction with equilibrium voltage 3.0 V, ΔG = −nFE is approximately −289 kJ/mol. The sign refers to the reaction direction and the equilibrium electrical work." }, "url": "https://lemonergy.com/learn/glossary/gibbs-free-energy/", "mentions": [] } }, "edges": [ { "id": "edge:1862", "source": "term:gibbs-free-energy", "target": "topic:topic-electrochemistry", "type": "belongsTo", "properties": { "sourceFile": "terms.json" } }, { "id": "edge:1863", "source": "term:gibbs-free-energy", "target": "term:cell-voltage", "type": "relatedTo", "properties": { "sourceFile": "terms.json", "meaning": "Related context; not a causal claim" } }, { "id": "edge:1864", "source": "term:gibbs-free-energy", "target": "term:nernst-equation", "type": "relatedTo", "properties": { "sourceFile": "terms.json", "meaning": "Related context; not a causal claim" } }, { "id": "edge:1865", "source": "term:gibbs-free-energy", "target": "term:faradays-constant", "type": "relatedTo", "properties": { "sourceFile": "terms.json", "meaning": "Related context; not a causal claim" } }, { "id": "edge:1866", "source": "term:gibbs-free-energy", "target": "term:entropy-coefficient", "type": "relatedTo", "properties": { "sourceFile": "terms.json", "meaning": "Related context; not a causal claim" } }, { "id": "edge:1867", "source": "term:gibbs-free-energy", "target": "visual:reaction-energy", "type": "illustratedBy", "properties": { "sourceFile": "visual-pages.json" } }, { "id": "edge:1868", "source": "term:gibbs-free-energy", "target": "reference:iupac-green", "type": "cites", "properties": { "sourceFile": "wiki-references.json", "scope": "direct technical reference" } } ], "neighbors": [ { "id": "topic:topic-electrochemistry", "type": "Topic", "label": "Electrochemistry fundamentals", "properties": { "id": "topic-electrochemistry", "name": "Electrochemistry fundamentals", "lede": "The reactions and potentials that make a cell a battery.", "context": "Connect reaction sites and transport paths.", "example": "A circuit can predict terminal voltage while leaving electrode potentials and reaction kinetics unresolved.", "lesson_url": "https://lemonergy.com/learn/fundamentals/cell-anatomy/", "lesson_label": "Open cell anatomy" } }, { "id": "term:cell-voltage", "type": "Term", "label": "Cell voltage", "properties": { "id": "cell-voltage", "name": "Cell voltage", "aka": [ "electromotive force", "EMF" ], "section": "topic-electrochemistry", "definition": "The difference between the positive and negative electrode potentials at equilibrium: for NMC against graphite about 3.7 V. It is set by the chemistry, and the difference in Gibbs free energy of the cell reaction per unit charge.", "formula": "E = E₊ − E₋ = −ΔG / nF", "unit": "V", "related": [ "electrode-potential", "gibbs-free-energy", "open-circuit-voltage", "nominal-voltage" ], "example": { "title": "Subtract equilibrium electrode potentials", "text": "If two illustrative electrode potentials are 4.0 V and 0.2 V versus the same reference, their equilibrium cell voltage is 3.8 V. Under load, resistance and polarization change the measured terminal voltage." }, "url": "https://lemonergy.com/learn/glossary/cell-voltage/", "mentions": [ "electrode", "negative-electrode", "graphite" ] } }, { "id": "term:nernst-equation", "type": "Term", "label": "Nernst equation", "properties": { "id": "nernst-equation", "name": "Nernst equation", "section": "topic-electrochemistry", "definition": "The relation between an electrode’s potential and the concentrations, or activities, of the species in its half-reaction. It explains why open-circuit voltage changes with state of charge and temperature.", "formula": "E = E° − (RT/nF)·ln Q", "related": [ "electrode-potential", "standard-potential", "open-circuit-voltage", "gibbs-free-energy" ], "example": { "title": "Change a reaction quotient by a decade", "text": "At 298.15 K with n = 1, increasing the dimensionless reaction quotient from 1 to 10 lowers E by about 59.2 mV. Use activities or a justified concentration approximation." }, "url": "https://lemonergy.com/learn/glossary/nernst-equation/", "mentions": [ "electrode", "state-of-charge", "half-reaction" ] } }, { "id": "term:faradays-constant", "type": "Term", "label": "Faraday’s constant", "properties": { "id": "faradays-constant", "name": "Faraday’s constant", "abbr": "F", "section": "topic-electrochemistry", "definition": "The charge carried by one mole of electrons, 96 485 coulombs or 26.8 ampere-hours. It links the moles of lithium that move in a reaction to the ampere-hours a cell delivers.", "unit": "C/mol", "related": [ "faradays-law", "theoretical-capacity", "coulomb", "ampere-hour" ], "url": "https://lemonergy.com/learn/glossary/faradays-constant/", "mentions": [] } }, { "id": "term:entropy-coefficient", "type": "Term", "label": "Entropy coefficient", "properties": { "id": "entropy-coefficient", "name": "Entropy coefficient", "section": "topic-electrochemistry", "definition": "The change of open-circuit voltage with temperature, dE/dT, usually a fraction of a millivolt per kelvin and varying with state of charge. It sets the reversible heat a cell absorbs or releases, which can be a large share of total heat at low current.", "formula": "q_rev = I·T·(dE/dT)", "unit": "mV/K", "related": [ "heat-generation", "gibbs-free-energy", "open-circuit-voltage" ], "example": { "title": "Track reversible heat with the sign convention", "text": "With positive charging current of 5 A, temperature 300 K and dE/dT = 0.1 mV/K, q_rev = IT(dE/dT) gives +0.15 W. Reversing the current reverses this contribution." }, "notes": [ "The glossary plot takes positive heat as heat released. Other sources may use positive-discharge current and a different written sign." ], "url": "https://lemonergy.com/learn/glossary/entropy-coefficient/", "mentions": [ "state-of-charge" ] } }, { "id": "visual:reaction-energy", "type": "Visual", "label": "Voltage is reaction free energy per charge", "properties": { "title": "Voltage is reaction free energy per charge", "summary": "A spontaneous forward cell reaction has negative Gibbs energy and positive equilibrium voltage under this convention.", "readout": "At 3.7 V, ΔG = -357 kJ/mol reaction for n = 1.", "formula": "ΔG = −nFE", "note": "Equilibrium relation at fixed temperature and pressure. No reaction chemistry, activity model or loaded voltage is calculated.", "software": { "status": "reference", "label": "Reference concept", "body": "This thermodynamic identity explains equilibrium cell voltage. Terminal voltage under load additionally depends on losses and dynamics.", "href": "../chapter-1/" }, "html": "
Reaction free energy
Reaction free energyEquilibrium cell voltage (V) versus ΔG (kJ/mol reaction). Free-energy change. Interpretation follows the graphs.-4820-3621.25-2412.5-1213.7505Equilibrium cell voltage (V)ΔG (kJ/mol reaction)
Free-energy change
" } }, { "id": "reference:iupac-green", "type": "Reference", "label": "Quantities, Units and Symbols in Physical Chemistry, 3rd edition, 2nd printing", "properties": { "title": "Quantities, Units and Symbols in Physical Chemistry, 3rd edition, 2nd printing", "publisher": "IUPAC", "url": "https://media.iupac.org/publications/books/gbook/IUPAC-GB3-2ndPrinting-Online-22apr2011.pdf", "type": "Technical reference", "scope": "Physical-chemistry notation, thermodynamics, electrochemistry and transport quantities.", "year": 2011, "accessed": "2026-10-27" } } ] }