Hess's Law Calculator: Sum Steps or Formation Enthalpies

Two routes to reaction enthalpy: sum reaction steps with reverse and multiply, or subtract standard formation enthalpies you can override. Signed kJ, method and sources shown.

At a glance

Computes: Reaction enthalpy by summing reversed and multiplied steps, or from formation enthalpies.
You supply: Step enthalpies with multipliers, or reactants and products with formation values.
Use when: You need a reaction enthalpy that cannot be measured directly in one step.
Assumes: Standard-reference-form elements have zero formation enthalpy at 298.15 K and 1 bar.

Combine the reaction steps you already have, or read a reaction enthalpy straight off a formation table. Both routes give the same ΔH°rxn, because enthalpy is a state function.

Method

Sum reaction steps From formation enthalpies

Reaction steps

One row per step. A negative multiplier reverses that step and its magnitude scales it, so 0.5 is allowed. ΔH is the step enthalpy as written, in kJ.

Step	Multiplier	ΔH (kJ)	Actions

Formation enthalpies

Enter the balanced reaction as reactant and product rows. ΔHf° is kJ/mol at 298.15 K and 1 bar; an element in its standard state is 0. Every inserted value stays editable.

Insert a reactant from the ΔHf° library

OpenStax Appendix G, 298.15 K. Fills a row you can still edit.

Reactant	n	ΔHf° (kJ/mol)	Actions

Insert a product from the ΔHf° library

Choose a species to fill a product row, then edit as needed.

Product	n	ΔHf° (kJ/mol)	Actions

Cumulative enthalpy (kJ) against Reaction step; reference at Starting enthalpy

Reaction enthalpy ΔH°rxn

Enter a reaction

Choose a method, fill the table, and calculate.

Export

ΔH°rxn = Σ n*ΔHf°(products) - Σ n*ΔHf°(reactants) How?

How this is calculated

Method. Enthalpy is a state function, so the enthalpy change of a reaction is independent of the path taken. The summation route adds the enthalpies of any set of steps that combine to the target reaction, reversing a step (which flips the sign of its ΔH) and scaling it (which scales the magnitude) as needed. The formation route reads the same ΔH°rxn from a table of standard formation enthalpies as Σ n*ΔHf°(products) minus Σ n*ΔHf°(reactants).

Conventions. Formation values are tabulated at 298.15 K and 1 bar. 1 bar is the IUPAC standard pressure adopted in 1982; older tables use 1 atm, a difference of about 1.3 percent that is negligible here. ΔHf° for an element in its most stable reference form is 0 by definition: carbon as graphite (diamond is +1.9 kJ/mol), oxygen as O2 (ozone is nonzero), and white phosphorus as the reference for P. A negative ΔH°rxn releases heat (exothermic); a positive value absorbs it (endothermic). The result is per mole of reaction exactly as written, so scaling the equation scales ΔH.

Values shown. The bundled ΔHf° table is OpenStax Chemistry 2e, Appendix G, at 298.15 K. It only pre-fills a field you can overwrite, because published tables disagree at the second decimal and you are graded against your own textbook: OpenStax lists CH4 as -74.6 kJ/mol where other tables use -74.8. The calculation reads only the numbers in the table on this page, never the bundled library.

Formula: ΔH°rxn = Σ n*ΔHf°(products) - Σ n*ΔHf°(reactants)

See Sources below

Sources

OpenStax Chemistry 2e, 5.3 Enthalpy. OpenStax. Retrieved Jul 10, 2026.
OpenStax Chemistry 2e, Appendix G: Standard Thermodynamic Properties. OpenStax. Retrieved Jul 10, 2026.
LibreTexts General Chemistry, 5.6 Hess's Law. LibreTexts. Retrieved Jul 10, 2026.

How we choose and check our formulas