Hess's Law Calculator
Combine reaction steps, reversing and scaling as needed.
Work out Hess's Law. Combine reaction steps, reversing and scaling as needed. Shows every step's contribution, not just the total.
ΔH for each step in kJ/mol, comma separated.
Scale factors to balance the target equation. Must line up with the enthalpies.
1 to reverse a step and flip its sign, 0 to leave it as written.
Overall ΔH
-74.6 kJ/mol
Exothermic — releases heat
Hess's law works because enthalpy is a state function: the change depends only on where you start and where you finish, never on the route. That is what lets you build a reaction you cannot measure out of several you can. The two operations are reversing, which flips the sign, and scaling, which multiplies it. Both must be applied to the enthalpy exactly as they are applied to the equation. The commonest failure is reversing an equation and forgetting the sign, which gives an answer wrong by twice that step — and often still plausible-looking, which is what makes it dangerous.
How the Hess's Law Calculator works
Enter each step's enthalpy, what to multiply it by, and whether to reverse it. The page shows every step's contribution separately so a sign error is visible rather than buried — reversing a step and forgetting the sign is the classic way to get an answer wrong by exactly double.
Also known as: hess's law formula · enthalpy from reaction steps · thermochemical equations calculator · how to use hess's law
Frequently asked questions
What is Hess's law?
The total enthalpy change of a reaction is the same regardless of the route taken. Enthalpy is a state function, so you can build an unmeasurable reaction from measurable steps and simply add them.
What happens when I reverse a step?
The sign of its ΔH flips. If forming a compound releases 400 kJ, decomposing it absorbs 400 kJ. Forgetting this is the commonest error, and it produces an answer wrong by twice that step — often still plausible-looking.
What if a step needs to be doubled?
Double its ΔH too. Enthalpy is extensive, so scaling the equation scales the enthalpy by exactly the same factor.
Why does Hess's law work?
Because enthalpy depends only on the initial and final states, not on the path. The same reason applies to internal energy, entropy and Gibbs energy, so the same trick works for all of them.
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