🔥 Hess's Law Enthalpy Calculator
Enter each species' coefficient and standard enthalpy of formation to get the reaction enthalpy — and whether it's exothermic or endothermic.
Enter each species' coefficient and standard enthalpy of formation ΔHf° (kJ/mol). Elements in their standard state are 0.
Reactants
Products
Enthalpy of reaction ΔH°rxn
-890.3 kJ/mol
Exothermic — releases heat.
Hess's law: ΔH°rxn = Σ(coeff · ΔHf° products) − Σ(coeff · ΔHf° reactants). Because enthalpy is a state function, the reaction enthalpy depends only on the difference between products and reactants, not the path. Use consistent ΔHf° values (kJ/mol) from one data table, and mind the physical state — H₂O(l) and H₂O(g) differ. 🔒 In your browser.
How the hess's law enthalpy calculator works
Because enthalpy is a state function, the heat of a reaction depends only on the reactants and products, not the route between them (Hess's law). The tool applies ΔH°rxn = Σ(coefficient · ΔHf° of products) − Σ(coefficient · ΔHf° of reactants), summing the formation enthalpies you enter and reporting the reaction enthalpy in kJ/mol with an exothermic/endothermic verdict.
Enter ΔHf° values (kJ/mol) from a single consistent data table, and remember an element in its standard state has ΔHf° = 0. Physical state matters — H₂O(l) and H₂O(g) have different formation enthalpies, so use the state that appears in your equation. The result is the standard enthalpy at 25 °C, per mole of reaction as written.
Frequently asked questions
How do I calculate enthalpy of reaction using Hess's law?
ΔH°rxn = Σ(ΔHf° products × coefficient) − Σ(ΔHf° reactants × coefficient). For methane combustion (CH₄ + 2O₂ → CO₂ + 2H₂O(l)) that's [−393.5 + 2(−285.8)] − [−74.8 + 0] ≈ −890 kJ/mol.
What is Hess's law?
The principle that the total enthalpy change of a reaction is the same whatever path it takes, because enthalpy is a state function. It lets you find a reaction's ΔH from formation enthalpies or by adding known steps.
What is the standard enthalpy of formation?
ΔHf° is the enthalpy change when one mole of a compound forms from its elements in their standard states, at 25 °C and 1 bar. Elements in their standard state (like O₂ or graphite) have ΔHf° = 0 by definition.
How do I know if a reaction is exothermic or endothermic?
By the sign of ΔH°rxn: negative means exothermic (releases heat), positive means endothermic (absorbs heat). The tool shows the value and the verdict.
Does the physical state of water matter?
Yes — H₂O(l) has ΔHf° ≈ −285.8 kJ/mol and H₂O(g) ≈ −241.8 kJ/mol, a difference of the heat of vaporisation. Always use the state written in your equation, or the answer will be off.