What is the heat of reaction?

The heat of reaction (ΔH°rxn) is the enthalpy change at constant pressure. A negative ΔH means the reaction is exothermic (releases heat); a positive ΔH means it is endothermic (absorbs heat). Units are typically kJ/mol.

What is the Standard Enthalpies of Formation method?

This method uses tabulated ΔH°f values — the heat to form one mole of a compound from its elements in standard states. The formula is ΔH°rxn = Σ n·ΔH°f(products) − Σ n·ΔH°f(reactants). Elements in their standard state (O₂, H₂, C(graphite)) have ΔH°f = 0.

What is the Bond Energy method?

The bond energy method estimates ΔH by summing energy needed to break all bonds in the reactants, then subtracting energy released when bonds form in the products: ΔH ≈ Σ BE(broken) − Σ BE(formed). It gives an approximation because average bond energies are used.

Why does the Bond Energy method give an approximate result?

Bond dissociation energies are averages across many molecules. The actual energy for a specific bond can differ from the average. The formation method using tabulated ΔH°f values is more precise because it relies on direct calorimetric measurements.

How do I interpret the sign of ΔH?

A negative ΔH means the reaction releases energy to the surroundings (exothermic). A positive ΔH means it absorbs energy (endothermic). A ΔH near zero indicates a thermoneutral reaction.

What units should I use?

Enter all ΔH°f values and bond energies in kJ/mol. The output uses the same unit. If your values are in kcal/mol, multiply by 4.184 to convert.

Heat Of Reaction Calculator

Calculate the heat of reaction (ΔH) using standard enthalpies of formation or bond energies — with a full step-by-step breakdown.

Heat of Reaction

ΔH°rxn = Σ n·ΔH°f(products) − Σ n·ΔH°f(reactants)

Products

nΔH°f (kJ/mol)

Reactants

nΔH°f (kJ/mol)

Load example

Enter values and click Calculate to see ΔH

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Summary

The Heat of Reaction Calculator computes the enthalpy change (ΔH) of a chemical reaction using two methods: the Standard Enthalpies of Formation method applies ΔH = Σ n·ΔH°f(products) − Σ n·ΔH°f(reactants), while the Bond Energy method uses ΔH ≈ Σ BE(bonds broken) − Σ BE(bonds formed). A signed result instantly identifies the reaction as exothermic or endothermic, with a full algebraic breakdown shown for every calculation.

How it works

Select a method: Standard Enthalpies of Formation or Bond Energies.
Formation method: enter each product and reactant with its stoichiometric coefficient (n) and standard enthalpy of formation (ΔH°f) in kJ/mol.
The calculator applies ΔH°rxn = Σ n·ΔH°f(products) − Σ n·ΔH°f(reactants) and displays the result.
Bond Energy method: enter bonds broken (reactants) and bonds formed (products) with their bond dissociation energies.
The calculator applies ΔH ≈ Σ BE(bonds broken) − Σ BE(bonds formed) and shows a full breakdown.
Use preset examples to load a real reaction instantly, then adjust values as needed.

Use cases

Determine whether a combustion reaction releases or absorbs heat.
Verify thermochemistry homework for AP Chemistry or university courses.
Estimate ΔH from bond dissociation energies when formation data is unavailable.
Solve formation-enthalpy problems with a step-by-step equation.
Cross-check textbook ΔH values for common reactions.
Learn the difference between exothermic and endothermic reactions.
Calculate heat released during industrial synthesis reactions.
Compare two calculation methods to understand their accuracy difference.

Frequently Asked Questions

Last updated: 2026-09-30 · Reviewed by Nham Vu