Hess Law Reaction Enthalpy Calculator
Because enthalpy is a state function, the heat change of a reaction depends only on the initial and final states, not the path. Hess law lets you build a target reaction from a series of known steps and add their enthalpy changes to get the overall value. This calculator takes up to three step enthalpies, each with its own multiplier (use negative to reverse a step, larger values to scale it), and returns the weighted sum. Enter your own thermochemical data because those values are empirical and source dependent.
Hess law formula
delta H(reaction) = sum( multiplier_i * delta H_i )
= c1*dH1 + c2*dH2 + c3*dH3
Reverse a step with a negative multiplier
Scale a step with a multiplier greater than 1
The example combines the formation of carbon dioxide, twice the formation of water, and the reversed combustion of methane to recover the methane formation enthalpy: -393.5 + 2*(-285.8) - (-890.3) = -74.8 kJ.
Thermochemistry facts
- Enthalpy is a state function, so reaction enthalpy is path independent.
- Reversing a reaction reverses the sign of its enthalpy change.
- Scaling a reaction by a factor scales its enthalpy by the same factor.
- Standard enthalpies of formation are tabulated at 298.15 K and 1 bar.
- Keep every step in the same energy unit before summing.
Hess law: frequently asked questions
What is Hess law?
Hess law states that the total enthalpy change for a reaction is the same regardless of the route taken, because enthalpy is a state function. This means you can add the enthalpy changes of individual steps, each multiplied by how many times it is used, to obtain the overall reaction enthalpy.
How do I use multipliers and reversed steps?
Multiply a step enthalpy by the factor needed to match the target equation. If a step must be reversed, enter a negative multiplier (for example -1), which flips the sign of its enthalpy. If a step is doubled, use a multiplier of 2. The calculator sums multiplier times step enthalpy across all steps.
What sign convention does enthalpy use?
A negative enthalpy change means the reaction releases heat (exothermic) and a positive value means it absorbs heat (endothermic). Reversing a reaction flips the sign, and scaling a reaction scales the enthalpy by the same factor. Both behaviours fall out of the multiplier you enter.
What units should the enthalpies be in?
Use consistent energy units, most commonly kilojoules (kJ) or kilojoules per mole (kJ/mol). The output carries the same units you enter. Because the calculation is a weighted sum, mixing units would give a meaningless result, so keep every step in the same unit.
Where do the step enthalpy values come from?
Step enthalpies are measured or tabulated thermochemical data, such as standard enthalpies of formation or combustion. Because these are empirical and source dependent, you enter them yourself; the calculator only performs the deterministic Hess law summation.
Official sources
- NIST: NIST Chemistry WebBook thermochemistry data.
- IUPAC: IUPAC Gold Book: enthalpy and state functions.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.