🔥Enthalpy Calculator
Enter each species' coefficient and standard formation enthalpy to find ΔH of reaction via Hess's law.
Default: CH₄ + 2O₂ → CO₂ + 2H₂O. One species per line: "coefficient, ΔHf° (kJ/mol)". Elements in their standard state have ΔHf° = 0.
Free Enthalpy Calculator
This enthalpy calculator applies Hess's law to find the overall enthalpy change of a reaction: ΔH°rxn = Σ(n × ΔHf° products) − Σ(n × ΔHf° reactants). Enter each species' stoichiometric coefficient and standard enthalpy of formation, and the calculator sums both sides and subtracts.
Hess's law states that the total enthalpy change for a reaction is the same regardless of the path taken — it only depends on the initial and final states. This makes it possible to calculate ΔH for reactions that are difficult to measure directly, using tabulated formation enthalpies.
A negative ΔH means the reaction is exothermic (releases heat); a positive ΔH means it's endothermic (absorbs heat). Free, runs entirely in your browser.
About the Enthalpy Calculator
Features
Any number of species
Enter as many reactants and products as your equation needs.
Exo/endothermic label
Instantly see which way heat flows.
Step-by-step
See the Hess's law sum spelled out.
Instant, private
Runs entirely in your browser. No data is sent anywhere.
Frequently Asked Questions
What is Hess's law?
Hess's law states that the total enthalpy change for a chemical reaction is the same regardless of the pathway taken, since enthalpy is a state function depending only on initial and final states.
What is the formula for ΔH of reaction?
ΔH°rxn = Σ(n × ΔHf° products) − Σ(n × ΔHf° reactants), where n is each species' stoichiometric coefficient and ΔHf° is its standard enthalpy of formation.
What is standard enthalpy of formation?
ΔHf° is the enthalpy change when one mole of a compound forms from its elements in their standard states. Elements in their standard state (like O₂ gas or solid graphite) have ΔHf° = 0 by definition.
What does a negative ΔH mean?
A negative ΔH means the reaction is exothermic — it releases heat to the surroundings, and the products have lower total energy than the reactants.
What does a positive ΔH mean?
A positive ΔH means the reaction is endothermic — it absorbs heat from the surroundings, and the products have higher total energy than the reactants.
Where do I find ΔHf° values?
Standard formation enthalpies are tabulated in chemistry textbooks and reference tables (like the NIST WebBook), usually in kJ/mol at 25°C and 1 atm.
Why does the coefficient matter?
Formation enthalpy is per mole, so if a balanced equation has 2 moles of a species, its contribution is multiplied by 2 — the coefficient must match your balanced chemical equation.
Can I have multiple reactants or products?
Yes — enter as many lines as needed for reactants and products, one species per line.
Does this account for reaction conditions like temperature?
No — this calculator uses standard formation enthalpies at standard conditions (25°C, 1 atm), so results represent ΔH under those conditions only.
Is this calculator free to use?
Yes, completely free with no sign-up, and it runs entirely in your browser.