Hess's Law states that the total enthalpy change for a reaction is the same regardless of the pathway taken, as long as initial and final states are the same. This lets you add known ΔH values to find an unknown one.

What does reversing a reaction do to ΔH?

Reversing a reaction flips the sign of its ΔH. For example, if the forward reaction has ΔH = −286 kJ/mol, the reverse has ΔH = +286 kJ/mol.

Why would I use a multiplier?

If a reaction must be used twice (e.g., to cancel an intermediate that appears in two moles), set its multiplier to 2. The ΔH scales proportionally with the multiplier.

How many steps can I combine?

This calculator supports two or three reaction steps. Most standard Hess's Law problems use two or three known reactions to reach the target.

What units does this calculator use?

Enthalpy values are entered and displayed in kJ/mol. Ensure all your source values use the same units before combining them.

Is this the same as using standard enthalpies of formation?

Yes — ΔH°rxn = Σ ΔH°f (products) − Σ ΔH°f (reactants) is a special case of Hess's Law where each formation reaction is a step.

Hess's Law Calculator

Name: Hess's Law Calculator
Availability: InStock
Author: Nham Vu

Enter two or three known reactions with their enthalpy values, set multipliers and reversal flags, and instantly get the net ΔH for the target reaction.

Number of reaction steps

Step 1

Reverse

ΔH (kJ/mol)

Multiplier

Step 2

Reverse

ΔH (kJ/mol)

Multiplier

Enter your reaction steps and click Calculate ΔH to see the result.

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Summary

Enter two or three known reactions with their enthalpy values, set multipliers and reversal flags, and instantly get the net ΔH for the target reaction.

How it works

Enter the chemical equation and ΔH (kJ/mol) for each known reaction step.
Set a multiplier (default 1) to scale a step if it appears more than once.
Toggle 'Reverse' to flip a reaction, which negates its ΔH.
The calculator sums (multiplier × ΔH) across all active steps, applying sign changes for reversed steps.
The result shows the net ΔH for the target reaction plus a step-by-step breakdown.

Use cases

Calculate combustion enthalpy from formation enthalpies of products and reactants.
Verify thermochemical cycles in general chemistry homework.
Find the enthalpy of a reaction that is hard to measure directly.
Check whether a multi-step synthesis pathway is exothermic or endothermic.
Practice Hess's Law problems with instant feedback.
Combine standard enthalpies of formation to get ΔH of reaction.

Frequently Asked Questions

Last updated: 2026-06-18 · Reviewed by Nham Vu