Frequently Asked Questions
Everything you need to know about the Enthalpy Calculator.
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.