Bond Energy & Reaction Enthalpy Calculator
Work out the reaction enthalpy ΔH from average bond energies. Pick the bonds broken and formed from a table, enter the counts, and see the totals, the subtraction and whether the reaction is exothermic or endothermic. A reverse mode finds an unknown bond energy from ΔH.
Bonds
Try an example
Choose bonds and counts. Leave unused rows as "(not used)".
How to use
- 1
Pick a pattern
Choose whether to find ΔH or an unknown bond energy.
- 2
Enter bonds and counts
Pick bonds from the table and enter the total counts (coefficient × bonds per molecule). For a bond not in the table, enter its energy directly.
- 3
Check the working
See the totals for bonds broken and formed, the subtraction, and the exothermic/endothermic verdict.
Features
- One mode for ΔH and a reverse mode for an unknown bond energy
- A table of average bond energies (H—H, C—H, O=O, N≡N and more)
- Enter the energy directly for bonds not in the table
- Shows the totals, the subtraction and the exothermic/endothermic verdict
- Runs entirely in your browser; no sign-up and nothing uploaded
Use cases
Check homework
Verify bond-energy problems with the totals and subtraction shown.
Avoid miscounting bonds
Enter coefficient × bonds per molecule and check the tally.
Find an unknown bond energy
Back-calculate a bond energy such as H—Cl from ΔH and the known bonds.
Details
In a reaction the bonds in the reactants break and new bonds form in the products. Breaking bonds absorbs energy and forming bonds releases it. The net heat change ΔH is estimated as "total energy of bonds broken − total energy of bonds formed".
The values in bond-energy tables are averages over many molecules (average bond energies). Even the same C—H bond differs slightly between methane and ethane, so the result is an estimate and may differ from enthalpies calculated from standard enthalpies of formation (methane combustion: about -890 kJ/mol measured vs about -824 kJ/mol from average bond energies).
Counts are the total number of bonds, including the coefficients: one methane molecule has four C—H bonds, and two oxygen molecules have two O=O bonds. Bond energies refer to gas-phase molecules, so reactions involving liquids or solids need phase-change energy separately. The reverse mode finds one unknown bond energy from a given ΔH and the known bonds.
FAQ
How are bond energy and reaction enthalpy related?
ΔH = (total bonds broken) − (total bonds formed). Breaking bonds absorbs energy and forming bonds releases it, so if more energy is released than absorbed, ΔH is negative (exothermic).
How do I count bonds?
Count the total, including coefficients. For CH₄ + 2O₂ → CO₂ + 2H₂O, four C—H and two O=O bonds break, while two C=O and four O—H bonds form.
Why does the result differ slightly from experimental values?
Table values are average bond energies over many molecules. The same bond type differs slightly between molecules, so the estimate can differ from enthalpies based on standard enthalpies of formation.
Does it work for reactions with liquids or solids?
Bond energies refer to gas-phase molecules. If liquids or solids are involved, add or subtract the phase-change energy separately.
What can I find with the reverse mode?
Given ΔH and the known bond energies, it finds one unknown bond energy. For H₂ + Cl₂ → 2HCl, for example, you can find the H—Cl energy from ΔH and the H—H and Cl—Cl energies.
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Verified: Checked with the table values (H₂+Cl₂→2HCl giving -183 kJ/mol, N₂+3H₂→2NH₃ giving -109 kJ/mol, N₂+O₂→2NO giving +222 kJ/mol, and back-calculating H—Cl as 427 kJ/mol) plus error handling are covered by browser tests
Did you know?
Bond-energy table values are averages over many molecules. Even the same C—H bond differs slightly between methane and ethane, so reaction enthalpies from bond energies are estimates: methane combustion is about -890 kJ/mol measured versus about -824 kJ/mol from average bond energies.