Hess Law Calculator

Work out hess law instantly with clear inputs, formula shown and shareable results.

About Hess Law Calculator

Enthalpy is a state function, so a reaction's enthalpy depends only on the start and end points and not the route taken. Any set of steps that adds up to the target equation has enthalpies that add up to the target enthalpy. Reversing a step simply flips its sign, which is why the reverse route is the negative of the forward total.

Formulas

Hess's law

dH(overall) = dH1 + dH2 + dH3

Reversed route

dH(reverse) = -dH(overall)

Worked example

With the default values below, Hess Law Calculator returns Overall Reaction Enthalpy of 211.00. Change any input above and every figure here is recalculated in your browser.

Inputs used

Enthalpy of step 1: -393.5 kJ/mol
Enthalpy of step 2: -285.8 kJ/mol
Enthalpy of step 3: 890.3 kJ/mol

Results

Overall Reaction Enthalpy: 211.00 kJ/mol
Enthalpy Of The Reverse Route: -211.00 kJ/mol
Overall Thermal Character: Overall endothermic
Largest Single Step: 890.30 kJ/mol

Step by step

1Values used
2Hess's law
3Reversed route
4Overall Reaction Enthalpy= 211.00 kJ/mol
5Enthalpy Of The Reverse Route= -211.00 kJ/mol
6Overall Thermal Character= Overall endothermic
7Largest Single Step= 890.30 kJ/mol

Assumptions and limits

The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

What if a step must be reversed or doubled?

Enter it with its sign flipped, or with its enthalpy multiplied by the factor you scaled the equation by, before summing.

Why does Hess's law work at all?

Because enthalpy is a state function. The energy difference between two states cannot depend on the path, or you could build a perpetual energy loop.