Hess's Law Enthalpy Calculator

About this calculator

Hess’s law states that enthalpy is a state function — the total enthalpy change of a reaction depends only on the initial and final states, not on the path taken to get there. That means if you can break a reaction into a series of steps whose enthalpy changes you already know, the sum of those individual ΔH values equals the ΔH of the overall reaction.

This is invaluable in chemistry because many reactions are difficult or dangerous to measure directly (like combustion of graphite into carbon monoxide, which tends to also form CO₂), so chemists instead sum well-established step reactions to find the target value, then read off whether the overall reaction is exothermic (ΔH negative, releases heat) or endothermic (ΔH positive, absorbs heat). This calculator sums the enthalpy changes you enter for each step to give the total reaction ΔH and its exothermic/endothermic sign.