What is bond dissociation energy?

Bond dissociation energy (BDE, D) is the enthalpy change for breaking one mole of a specific bond homolytically in the gas phase, giving two radicals, for example CH₃–H → CH₃• + H• (D = 439 kJ/mol). It is always positive: breaking a bond absorbs energy.

What is the difference between bond dissociation energy and average bond enthalpy?

A BDE refers to one bond in one molecule; it differs from molecule to molecule (C–H is 439 kJ/mol in methane but 400 kJ/mol at a tertiary carbon). Average bond enthalpy is the mean over many compounds (C–H ≈ 415 kJ/mol) and is what general-chemistry ΔH estimates use.

How do you calculate enthalpy of reaction from bond energies?

ΔH ≈ Σ (energies of bonds broken) − Σ (energies of bonds formed). For H₂ + Cl₂ → 2HCl: broken 436 + 243 = 679 kJ, formed 2 × 432 = 864 kJ, so ΔH ≈ −185 kJ/mol; the experimental value is −184.6 kJ/mol.

Why does the bond-energy estimate differ from the real ΔH?

Average bond energies ignore the molecular environment, and the method assumes every species is a gas. Estimates are typically within about 10–50 kJ/mol. For accurate values use standard enthalpies of formation: ΔH° = Σ ΔHf°(products) − Σ ΔHf°(reactants).

What does a negative ΔH mean?

More energy is released forming the product bonds than is absorbed breaking the reactant bonds, so the reaction is exothermic. A positive ΔH means endothermic.

How do I convert bond energies between kJ/mol, kcal/mol and eV?

1 kcal/mol = 4.184 kJ/mol, and 1 eV per bond = 96.485 kJ/mol. The C–H bond in methane is 439 kJ/mol = 104.9 kcal/mol = 4.55 eV.

Which bond is the strongest?

Among common bonds, the triple bonds are strongest: C≡O in carbon monoxide (1076 kJ/mol specific, about 1080 average) and N≡N (945–946 kJ/mol). Single bonds between small electronegative atoms, such as F–F (about 159 kJ/mol) and O–O (about 140 kJ/mol), are among the weakest.

Bond Dissociation Energy

Pick bonds from the average bond-energy table or the specific BDE table, add them as broken or formed, and get ΔH ≈ Σ D(broken) − Σ D(formed) in kJ/mol, kcal/mol or eV. Presets show classic reactions next to the experimental ΔH.

Bond	kJ/mol	kcal/mol
H–H	436	104.2
H–C	415	99.2
H–N	390	93.2
H–O	464	110.9
H–F	569	136.0
H–Si	395	94.4
H–P	320	76.5
H–S	340	81.3
H–Cl	432	103.3
H–Br	370	88.4
H–I	295	70.5
C–C	345	82.5
C=C	611	146.0
C≡C	837	200.0
C–N	290	69.3
C=N	615	147.0
C≡N	891	213.0
C–O	350	83.7
C=O	741	177.1
C=O (in CO₂)	799	191.0
C≡O	1080	258.1
C–F	439	104.9
C–Cl	330	78.9
C–Br	275	65.7
C–I	240	57.4
C–S	260	62.1
C–Si	360	86.0
N–N	160	38.2
N=N	418	99.9
N≡N	946	226.1
N–O	200	47.8
N–F	270	64.5
N–Cl	200	47.8
O–O	140	33.5
O=O	498	119.0
O–F	160	38.2
O–Cl	205	49.0
O–Si	370	88.4
F–F	160	38.2
Cl–Cl	243	58.1
Br–Br	190	45.4
I–I	150	35.9
S–S	215	51.4
Si–Si	230	55.0
P–P	215	51.4
P–Cl	330	78.9

Bond	kJ/mol	kcal/mol
H–Hhydrogen	436	104.2
CH₃–Hmethane	439	104.9
CH₃CH₂–Hprimary C–H (ethane)	421	100.6
(CH₃)₂CH–Hsecondary C–H (propane)	410	98.0
(CH₃)₃C–Htertiary C–H (isobutane)	400	95.6
H₂C=CH–Hvinylic C–H	464	110.9
C₆H₅–Haryl C–H (benzene)	472	112.8
HC≡C–Halkynyl C–H	558	133.4
H₂C=CHCH₂–Hallylic C–H	369	88.2
C₆H₅CH₂–Hbenzylic C–H	375	89.6
HO–Hwater	497	118.8
CH₃O–Hmethanol O–H	440	105.2
CH₃–CH₃ethane C–C	377	90.1
H₂C=CH₂ethene C=C	728	174.0
CH₃–OHmethanol C–O	385	92.0
CH₃–NH₂methylamine C–N	386	92.3
CH₃–Clchloromethane	350	83.7
CH₃CH₂–Clchloroethane	352	84.1
CH₃–Brbromomethane	294	70.3
CH₃–Iiodomethane	239	57.1
HO–OHhydrogen peroxide	211	50.4
H–Fhydrogen fluoride	570	136.2
H–Clhydrogen chloride	431	103.0
H–Brhydrogen bromide	366	87.5
H–Ihydrogen iodide	298	71.2
F–Ffluorine	159	38.0
Cl–Clchlorine	242	57.8
Br–Brbromine	194	46.4
I–Iiodine	152	36.3
O=Ooxygen	498	119.0
N≡Nnitrogen	945	225.9
C≡Ocarbon monoxide	1076	257.2
OC=Ocarbon dioxide (first C=O)	532	127.2

Average bond energies (OpenStax Chemistry 2e, Table 7.2; C=O in CO₂ from Brown et al.). Use these for general-chemistry ΔH estimates.

Example reactions

Bonds Broken (reactants)

Bonds Formed (products)

Custom bond

Bond

kJ/mol

Estimated Reaction Enthalpy

Σ D broken (in)

kJ/mol

Σ D formed (out)

kJ/mol

ΔH (approx.)

kJ/mol

Add bonds above to calculate ΔH.

ΔH ≈ Σ D(bonds broken) − Σ D(bonds formed) · gas-phase estimate

How to estimate ΔH from bond energies

Write the balanced equation with every species as a gas and draw the Lewis structures.
List every bond broken in the reactants and every bond formed in the products (bonds that survive unchanged can be left out of both lists).
ΔH ≈ Σ D(broken) − Σ D(formed). Breaking bonds absorbs energy; forming bonds releases it.

Worked example, methane combustion CH₄ + 2O₂ → CO₂ + 2H₂O(g): broken 4 × H–C (415) + 2 × O=O (498) = 2,656 kJ; formed 2 × C=O in CO₂ (799) + 4 × H–O (464) = 3,454 kJ; ΔH ≈ 2,656 − 3,454 = −798 kJ/mol. From enthalpies of formation the experimental value is −802.3 kJ/mol. Using the generic C=O value (741) instead gives −682 kJ/mol, which shows why the CO₂-specific value matters.

Reaction (gas phase)	Bond-energy estimate	Experimental ΔH°
H₂ + Cl₂ → 2HCl	−185 kJ/mol	−184.6 kJ/mol
CH₄ + 2O₂ → CO₂ + 2H₂O	−798 kJ/mol	−802.3 kJ/mol
N₂ + 3H₂ → 2NH₃	−86 kJ/mol	−91.8 kJ/mol
CO + 2H₂ → CH₃OH	−107 kJ/mol	−90.5 kJ/mol
C₂H₄ + H₂ → C₂H₆	−128 kJ/mol	−136.4 kJ/mol
CH₄ + Cl₂ → CH₃Cl + HCl (specific BDEs)	−100 kJ/mol	−99.3 kJ/mol

Experimental values from standard enthalpies of formation at 298 K (NIST / CRC). Sources for bond energies: OpenStax Chemistry 2e Table 7.2 (averages), OpenStax Organic Chemistry Table 6.3 and CRC Handbook (specific D°298).

Summary

Bond dissociation energy (BDE) is the enthalpy needed to break one mole of a specific covalent bond homolytically in the gas phase. This tool lists average bond enthalpies (OpenStax Chemistry 2e) and specific D°298 values (OpenStax Organic Chemistry, CRC Handbook) and estimates reaction enthalpy as the energy of bonds broken minus bonds formed, in kJ/mol, kcal/mol or eV. Preset reactions compare each estimate with the experimental ΔH.

How it works

Choose the data set: average bond enthalpies (general chemistry, averaged over many molecules) or specific bond dissociation energies (D°298 for a named bond in a named molecule).
Click +B next to each bond broken in the reactants and +F next to each bond formed in the products; use + and − to set counts, or add a custom bond energy.
The calculator sums energy absorbed (bonds broken) and released (bonds formed): ΔH ≈ Σ D(broken) − Σ D(formed).
Negative ΔH is exothermic; positive ΔH is endothermic. Switch the unit between kJ/mol, kcal/mol and eV.
Load a preset reaction to see a worked estimate next to the experimental ΔH from enthalpies of formation.

Use cases

Estimate the enthalpy of gas-phase reactions for general and organic chemistry homework.
Look up a bond dissociation energy table in kJ/mol and kcal/mol without opening a textbook.
Compare primary, secondary, tertiary, allylic and benzylic C–H bond strengths for radical reactions.
Check a hand calculation of combustion, hydrogenation or halogenation enthalpy.
Teach the difference between endothermic bond breaking and exothermic bond formation.

Frequently Asked Questions

Last updated: 2026-10-09 · Reviewed by Nham Vu

Bond Dissociation Energy

Use the Bond Dissociation Energy

Example reactions

Bonds Broken (reactants)

Bonds Formed (products)

Custom bond

Estimated Reaction Enthalpy

How to estimate ΔH from bond energies

Summary

How it works

Use cases

Frequently Asked Questions