🔥 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.