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Hess's Law Enthalpy Calculator

Enter each step's enthalpy change to find the total reaction ΔH.

Result

-110.50 kJ
Exothermic (releases heat)
-393.50 + 283.00 + 0.00 kJ

How it is calculated

The tool adds the enthalpy changes you type: ΔHtotal = ΔH1 + ΔH2 + ΔH3, all in kJ. Steps 1 and 2 are required; step 3 is optional and counts as 0 if left empty. The total is shown to two decimals, and a negative total is labelled exothermic, a positive one endothermic, and exactly zero thermoneutral. The tool does no stoichiometry for you: if you reverse a reaction, enter its ΔH with the opposite sign, and if you multiply a reaction by a coefficient, multiply its ΔH by the same number before typing it in. The values are taken as kJ for the equation as written, not per mole of anything unless you enter them that way.

Worked example

Find ΔH for C(graphite) + ½O₂ → CO. Step 1: C + O₂ → CO₂ has ΔH = −393.5 kJ. Step 2 is the reverse of CO + ½O₂ → CO₂ (−283.0 kJ), so CO₂ → CO + ½O₂ is +283 kJ. Add them: −393.5 + 283 + 0 = -110.50 kJ. The sign is negative, so the tool reports the reaction as exothermic (releases heat).

Try this example →

Common mistakes

  • Forgetting to flip the sign when a step is written in reverse. The tool simply adds what you type, so a reversed reaction must be entered with the opposite sign.
  • Not multiplying ΔH when you scale an equation. If a step needs two moles of a reaction, enter twice its ΔH, otherwise the total is wrong.
  • Mixing units. Entering one step in kJ and another in J or kcal gives a meaningless sum, because the tool assumes everything is kJ.
  • Leaving a needed third step blank. Step 3 defaults to 0, so a missing step silently drops out of the total instead of raising an error.

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.

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Comments (1)

  • Editorial team

    Hess's law only adds what you give it, so the work is in preparing the steps. Reverse a reaction and its enthalpy changes sign, and multiply the equation by 2 and you must double the value. With -285.80 and -393.50 the tool totals -679.30 kJ and labels it exothermic. Which reaction are you trying to build from known equations?

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