Reaction Enthalpy (Hess's Law) Calculator
Hess's law lets you find the enthalpy change of any reaction from tabulated standard enthalpies of formation, without measuring it directly. The reaction enthalpy is the sum of the products' formation enthalpies minus the sum of the reactants' formation enthalpies, each weighted by its stoichiometric coefficient. This calculator takes the two summed totals and returns the standard reaction enthalpy along with whether the reaction is exothermic or endothermic. Look up formation enthalpies in a reference table, multiply by coefficients, and enter the two sums.
Hess's law formula
delta H_rxn = sum(delta H_f products) - sum(delta H_f reactants)
each term = coefficient * formation enthalpy
negative result = exothermic
positive result = endothermic
Enter the already-summed totals. Each species term is its stoichiometric coefficient times its standard enthalpy of formation. Elements in their standard state contribute zero.
Worked example: burning carbon
- C(s) + O2(g) gives CO2(g).
- Products: CO2 formation enthalpy is -393.5 kJ/mol.
- Reactants: C and O2 are elements in standard state, sum is 0.
- delta H = -393.5 minus 0 = -393.5 kJ/mol.
- The negative sign confirms combustion is exothermic.
Reaction enthalpy: frequently asked questions
What is Hess's law?
Hess's law states that the total enthalpy change of a reaction is the same regardless of the path taken, because enthalpy is a state function. In practice this means the standard reaction enthalpy equals the sum of the standard enthalpies of formation of the products minus the sum for the reactants.
What is a standard enthalpy of formation?
The standard enthalpy of formation (delta H f) is the enthalpy change when one mole of a compound forms from its elements in their standard states at 298.15 K and 1 bar. By definition, elements in their standard states have a formation enthalpy of zero.
How do I include stoichiometric coefficients?
Multiply each species formation enthalpy by its coefficient in the balanced equation before summing. This calculator takes the already-summed product total and reactant total, so multiply and add the individual values first, then enter the two sums.
What does the sign of the reaction enthalpy mean?
A negative reaction enthalpy means the reaction releases heat (exothermic); a positive value means it absorbs heat (endothermic). For example, combustion reactions have large negative enthalpies because they release energy.
Official sources
- NIST Chemistry WebBook: standard enthalpy of formation data.
- IUPAC Gold Book: Hess's law and enthalpy of formation.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.