Reaction spontaneity calculator

How Gibbs free energy decides reaction spontaneity

At constant temperature and pressure, the change in Gibbs free energy is the thermodynamic criterion for spontaneity of a process as written. Collect a reaction enthalpy ΔH and a reaction entropy ΔS in consistent bookkeeping, choose the absolute temperature T in kelvin, and evaluate ΔG equals ΔH minus T times ΔS. When ΔG is less than zero the forward process is thermodynamically spontaneous under those conditions; when ΔG is greater than zero the reverse direction is favored as written; when ΔG is exactly zero the system sits at equilibrium for that temperature. The engine does not look up formation tables: you supply the three numbers (or their aliases enthalpy, entropy, temperature and the common delta_h, delta_s, T forms). Optional energy_unit is a display label only, defaulting to kJ/mol for ΔH and ΔG. Optional entropy_unit defaults to J so that tabulated J per mole kelvin entropies are scaled by one thousandth before multiplication by T, matching kilojoule enthalpies used in most classroom problems; set entropy_unit to kJ or same when your ΔS is already in the same energy unit as ΔH. Negative temperature is rejected with invalid_input because absolute temperature cannot be negative on the kelvin scale for this calculator. Identical inputs always produce identical JSON because the path is pure arithmetic with no network call, no model, no random seed, and no wall clock.

Reading the response: ΔG, T·ΔS, and classification

On success the response includes delta_g, the alias delta_G, and gibbs_energy—all equal to the computed free energy—plus enthalpy and delta_h echoes of the input ΔH after stable rounding. delta_s_input preserves the raw entropy you sent; delta_s, delta_S, and entropy report the value after optional joule-to-kilojoule scaling; temperature and T echo kelvin; t_delta_s is the T times scaled ΔS product subtracted from enthalpy. energy_unit and entropy_unit document the labels used, and entropy_scale is one for same-unit entropy or zero point zero zero one when joule scaling applied. classification is spontaneous, non_spontaneous, or equilibrium; spontaneous is a boolean true only for the first case so UI badges need no extra sign logic. formula restates ΔG = ΔH − T·ΔS, and method is always gibbs_free_energy so downstream code can branch without sniffing shapes. Use delta_g alone when a spreadsheet or gradebook needs a single number; use t_delta_s when a lab report must show how the entropy term competes with enthalpy at high temperature.

API usage, pricing, classroom labs, and practical limits

Call the free in-browser reaction spontaneity calculator for homework checks, lecture demos, and quick lab prep without installing a thermochemistry package. The capability never converts Celsius to kelvin for you—add two hundred seventy-three point fifteen yourself—and never invents missing thermochemical table values. Send a JSON body with required numbers enthalpy, entropy, and temperature; aliases delta_h, dH, H, delta_s, dS, S, T, temp, kelvin, and temp_K are accepted when documented by the SDK. Use the prepaid API when a tutoring platform, process-safety worksheet, materials screening helper, or curriculum generator needs a stable contract under KIT balance at $0.002 per successful item; validation failures never consume balance after the engine rejects the request. Pair this endpoint with enthalpy-of-reaction or entropy-of-reaction helpers when you first build ΔH and ΔS from formation or standard molar data, then feed those aggregates here with a process temperature. Authors shipping SDKs can map classification and spontaneous into badges, and expose delta_g with energy_unit as the primary numeric column students compare to hand work.