Enthalpy of reaction calculator

How Hess's law turns formation enthalpies into ΔH_rxn

Each species object must include enthalpy_formation as a finite number; optional coefficient defaults to one when omitted and should match the balanced stoichiometric coefficient (including fractions such as one half for oxygen). Optional name labels such as CH4 or H2O make the JSON readable in reports but do not change the math. Missing formation enthalpy on any species, empty arrays, non-positive coefficients, and non-finite numbers raise invalid_input and are not charged. 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: sums, contributions, and classification

Validation messages that mention missing formation enthalpy name the species index so you can fix a blank cell without reading a stack trace. On success the response includes delta_h, the alias delta_H, and enthalpy_reaction—all equal to the computed reaction enthalpy—plus unit, defaulting to kJ/mol unless you send a display unit string. sum_products and sum_reactants expose the two intermediate Hess sums so students can check their scratch work term by term. The reactants and products arrays echo each species with name when provided, coefficient, enthalpy_formation, and contribution equal to coefficient times formation enthalpy. reactant_count and product_count give array lengths for UI tables. classification is exothermic when ΔH is negative, endothermic when positive, and thermoneutral when zero, which is enough for quick labels without re-deriving the sign in the client. formula restates the Hess identity in a short string, and method is always hess_formation_enthalpies so downstream code can branch without sniffing shapes. Use delta_h alone when a spreadsheet or gradebook needs a single number; use the contribution columns when a lab report must show how carbon dioxide and water each pull the methane combustion enthalpy downward. The engine never converts units: if you mix kJ and kcal you will get a wrong number, so keep one energy unit for the whole request.

API usage, pricing, classroom labs, and practical limits

Call the free in-browser enthalpy of reaction calculator for homework checks, lecture demos, and quick lab prep without installing a thermochemistry package. Authors shipping SDKs can map the contribution arrays into tables of name, coefficient, ΔH_f, and contribution—the same columns students fill by hand when they apply Hess's law from formation data. Send a JSON body with required arrays reactants and products; aliases reactant, left, product, and right are accepted when documented by the SDK. Per-species formation aliases include delta_hf, dhf, hf, formation_enthalpy, and Hf; coefficient aliases include coeff, nu, stoich, and n. Use the prepaid API when a tutoring platform, LIMS integration, process-safety worksheet, 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. Species lists are capped for Worker isolate safety while still covering ordinary multi-reactant classroom and industrial teaching examples. Pair this endpoint with your own formation-table lookup when the next step is filling blanks from a periodic-style ΔH_f chart, or with stoichiometry helpers when you first balance the equation.