Bond Energy Calculator

Reaction label

Temperature (°C)

Temperature is informational; average bond energies are used.

Energy unit kJ/mol kcal/mol

If you switch units, re-check energy values.

Display precision

Quick bond picker Select a common bond to auto-fill energy H–H — 436 kJ/molC–H — 413 kJ/molC–C — 348 kJ/molC–O — 358 kJ/molC–N — 305 kJ/molC–Cl — 338 kJ/molC–Br — 276 kJ/molC–I — 240 kJ/molO–H — 463 kJ/molN–H — 391 kJ/molF–F — 158 kJ/molCl–Cl — 243 kJ/molBr–Br — 193 kJ/molI–I — 151 kJ/molO–O — 146 kJ/molN–N — 163 kJ/molS–H — 347 kJ/molS–S — 266 kJ/molC=C — 614 kJ/molC=O (carbonyl) — 743 kJ/molC=O (in CO₂) — 799 kJ/molC=N — 615 kJ/molN=O — 607 kJ/molO=O — 498 kJ/molS=O — 523 kJ/molC≡C — 839 kJ/molC≡N — 891 kJ/molN≡N — 945 kJ/molC–C (aromatic) — 518 kJ/mol

Use the picker above, then click “Add to Broken” or “Add to Formed”.

Add to Bonds Broken Add to Bonds Formed

Bonds Broken

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Bonds Formed

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Calculate Formula used How to use

Tip: Count each bond once per molecule. Example: CH₄ has four C–H bonds.

Formula used

This calculator estimates reaction enthalpy from average bond energies:

• n is the number of bonds of a given type.
• E is the average bond energy (per mole of bonds).
• Positive ΔH suggests endothermic; negative suggests exothermic (approx.).

How to use this calculator

1. Write the balanced reaction (for your reference).
2. List bonds broken: all bonds in reactants that must be broken.
3. List bonds formed: all new bonds in products.
4. For each bond type, enter its energy and the bond count.
5. Click Calculate to see ΔH above the form.
6. Use CSV/PDF to save the computed breakdown for reports.

Example data table

Example: methane combustion estimate (average bond energies, kJ/mol).

This example is an estimate; real values depend on phase and conditions.

Bond energy concepts

Bond energy is the average energy required to break one mole of a specific bond in the gas phase. Because molecules have different environments, tabulated values are averages, yet they remain useful for quick thermochemical estimates and classroom calculations.

Reaction enthalpy by bond accounting

This calculator applies a practical approximation: sum the energies of bonds that must be broken in the reactants and subtract the sum of energies released when new bonds form in the products. The result estimates the reaction enthalpy change, ΔH, in energy per mole of reaction.

What “broken” and “formed” mean

“Broken” includes every bond present in reactants that is absent in products. “Formed” includes every bond present in products that is absent in reactants. Use the balanced chemical equation as your checklist, then count each bond type multiplied by stoichiometric coefficients.

Interpreting the sign of ΔH

If ΔH is negative, the estimate suggests an exothermic process, where more energy is released forming bonds than is required to break bonds. If ΔH is positive, the estimate suggests an endothermic process requiring net energy input.

Units and reporting

Results are commonly reported in kJ/mol; some references use kcal/mol. The calculator lets you enter energies in either unit and keeps a consistent report. Use the export buttons to attach a bond-by-bond breakdown to lab reports, problem sets, or internal notes.

Data quality and limitations

Average bond energies do not capture solvent effects, phase changes, ionic character, resonance stabilization, or strain. For strongly polar bonds, metals, or reactions with significant intermolecular interactions, the approximation can deviate from measured enthalpies.

Worked example with methane combustion

For CH₄ + 2 O₂ → CO₂ + 2 H₂O, you break four C–H bonds and two O=O bonds, then form two C=O bonds in CO₂ and four O–H bonds in water. Using typical average values, the estimate yields a negative ΔH, consistent with combustion.

When to use formation enthalpies instead

For publication-grade numbers, prefer standard enthalpies of formation and Hess’s law, especially when reliable thermodynamic tables exist. Bond-energy estimates are best for rapid comparisons, checking plausibility, and understanding which bond changes dominate the heat released or absorbed.

FAQs

1) Why is this result approximate?

Bond energies are averaged over many molecules and environments. Real enthalpy depends on phase, temperature, resonance, and intermolecular interactions, so the estimate may differ from experimental values.

2) Do I enter bonds from the balanced equation?

Yes. Balance the equation first, then count bonds per molecule and multiply by coefficients. This prevents undercounting and keeps the broken and formed totals consistent.

3) Should I include bonds that appear on both sides?

No. If a bond type remains unchanged from reactants to products, it cancels out conceptually. Only list bonds that are effectively broken or newly formed.

4) Which C=O value should I pick?

Choose the entry that best matches your product environment. For CO₂, use the specific “in CO₂” value. For aldehydes and ketones, use the general carbonyl value.

5) What does a negative ΔH mean here?

It indicates the estimate is exothermic: more energy is released in bond formation than is consumed breaking bonds. Magnitude indicates the approximate heat change per mole of reaction.

6) Can I use custom bond energies?

Yes. Add a row and enter your preferred literature value. Ensure the unit matches your selection, and keep counts as integers for chemically meaningful totals.

7) When should I avoid this method?

Avoid it for ionic solids, metal complexes, strongly solvated reactions, or cases needing high accuracy. Use standard formation enthalpies or calorimetry-based data instead.