What is bond energy (bond dissociation energy)?

Bond energy (also called bond dissociation energy, BDE) is the average energy required to break one mole of a specific type of bond in the gas phase. For example, the C–H bond energy is approximately 413 kJ/mol, meaning it takes 413 kJ to break one mole of C–H bonds homolytically.

Why is ΔH calculated as bonds broken minus bonds formed?

Breaking bonds requires energy input (endothermic), while forming bonds releases energy (exothermic). The net ΔH is the difference: energy put in to break reactant bonds minus energy given out when product bonds form. If more energy is released than absorbed, the reaction is exothermic (ΔH < 0).

Why are bond energy results approximate?

Tabulated bond energies are averages across many compounds. The exact energy of a bond depends on the molecular environment, hybridization, and neighboring atoms. For precise ΔH values, use standard enthalpies of formation (Hess's law) instead.

What units should I use?

Enter bond energies in kJ/mol, which is the standard unit in most chemistry textbooks. All results will be in kJ/mol. If your table uses kcal/mol, multiply each value by 4.184 before entering it.

Where do I find bond energy values?

Average bond dissociation energies are listed in the appendices of most general chemistry textbooks (Zumdahl, Silberberg, Chang). Common values: H–H 436, C–H 413, C–C 347, C=C 614, C–O 358, C=O 799, O–H 459, N–N 163, N=N 418, N≡N 941 kJ/mol.

Can I use this for ionic compounds or aqueous reactions?

No. Bond energy calculations apply to gas-phase covalent bond breaking and forming. Ionic interactions, solvation energies, and lattice enthalpies are not captured. Use Hess's law with enthalpies of formation for ionic or aqueous reactions.

Bond Energy Calculator

Enter bonds broken and bonds formed with their average bond energies to calculate the reaction enthalpy change (ΔH).

Bond Energy Calculator

ΔH = Σ BE(bonds broken) − Σ BE(bonds formed)

Bonds Broken (Reactants)

Bond type Count BE (kJ/mol)

Bonds Formed (Products)

Bond type Count BE (kJ/mol)

Load example reaction

Common Bond Energies (kJ/mol)

H–H436

O=O498

C–H413

O–H459

C–C347

C=C614

C≡C839

C–O358

C=O799

C–N305

N≡N941

N–H391

Cl–Cl243

C–Cl339

F–F159

H–Cl432

Add bonds and click Calculate to see ΔH

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Summary

The Bond Energy Calculator estimates the enthalpy change (ΔH) of a chemical reaction from average bond dissociation energies. It applies the formula ΔH = Σ BE(bonds broken) − Σ BE(bonds formed), where energy is absorbed to break bonds and released when bonds form. Because tabulated values are averages, results are approximate estimates rather than exact thermodynamic measurements.

How it works

List each bond type broken in the reactants along with the number of such bonds and the average bond energy in kJ/mol.
List each bond type formed in the products the same way.
The calculator sums the total energy required to break all reactant bonds.
It then sums the total energy released when all product bonds form.
ΔH = Energy(bonds broken) − Energy(bonds formed) is computed and displayed.
A negative ΔH indicates an exothermic reaction; a positive ΔH indicates an endothermic reaction.

Use cases

Estimate reaction enthalpy when standard enthalpies of formation are unavailable.
Verify hand calculations for AP Chemistry or university general chemistry.
Understand why some reactions are exothermic and others endothermic at the bond level.
Practice using bond dissociation energy tables in textbook problems.
Quickly compare the energy balance of different proposed reaction pathways.
Teach or learn the concept that bond breaking absorbs energy and bond forming releases energy.
Estimate ΔH for organic reactions such as combustion, halogenation, and hydrogenation.
Cross-check thermochemical estimates against Hess's law calculations.

Frequently Asked Questions

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

Bond Energy Calculator

Use the Bond Energy Calculator

Bond Energy Calculator

Summary

How it works

Use cases

Frequently Asked Questions