Bond Energy Calculator

Last updated: June 2026

Use this bond energy calculator to estimate the enthalpy change of a reaction from the bonds broken in the reactants and the bonds formed in the products. It is designed for A-Level and Singapore H2 Chemistry energetics questions involving average bond enthalpies.

Key takeaway: Breaking bonds absorbs energy, while forming bonds releases energy. Therefore: ΔH = total bond energy of bonds broken minus total bond energy of bonds formed.
How to use this calculator
  1. Write a balanced reaction equation first.
  2. Count all bonds broken in the reactants, including the reaction coefficients.
  3. Count all bonds formed in the products.
  4. Select each bond and edit the displayed energy when your question gives a different value.
  5. Press Calculate ΔH and follow the numbered working.

How bond energy calculations work

A chemical reaction requires existing bonds in the reactants to be broken before new bonds can form. Bond breaking is always endothermic because energy must be supplied to overcome the attraction between the bonded atoms.

Bond formation is always exothermic because energy is released when atoms enter a lower-energy bonded arrangement.

ΔH = ΣE(bonds broken) − ΣE(bonds formed)

If the energy released during bond formation is greater than the energy absorbed during bond breaking, ΔH is negative and the reaction is exothermic.

Why bond enthalpy answers are approximate

Most data booklets list average bond enthalpies. A C–H bond, for example, does not have exactly the same strength in every molecule. The listed value is an average obtained from several gaseous compounds.

Important: Use the values supplied in the examination question or data booklet. The calculator dropdown contains typical values for practice and checking only.

Worked example: combustion of methane

CH4 + 2O2 → CO2 + 2H2O

Four C–H bonds and two O=O bonds are broken. Two C=O bonds in carbon dioxide and four O–H bonds are formed.

ΔH = [4(C–H) + 2(O=O)] − [2(C=O in CO2) + 4(O–H)]

Using the typical values in this calculator gives approximately −818 kJ mol−1.

Counting bonds correctly

Count bonds from displayed or structural formulae rather than from the molecular formula alone. For example, one molecule of water contains two O–H bonds, so two molecules of water contain four O–H bonds.

Bonds that remain unchanged may be included on both sides, but they cancel. It is usually more efficient to count only bonds that are broken and newly formed.

Common examination traps

Trap 1: Do not reverse the formula. It is bonds broken minus bonds formed.
Trap 2: Multiply the number of bonds by the coefficient in the balanced equation.
Trap 3: The C=O bond in CO2 may have a different listed value from a typical carbonyl C=O bond.
Trap 4: Average bond enthalpies normally apply to gaseous molecules, so calculated reaction enthalpies are estimates.

Frequently asked questions

Why is breaking a bond endothermic?

Energy is needed to separate two atoms that attract each other through a chemical bond.

Why is forming a bond exothermic?

Bond formation places atoms in a more stable, lower-energy arrangement, so energy is released.

Can I edit the dropdown bond energy?

Yes. Selecting a bond inserts a typical value, but you should replace it with the value given in your question whenever necessary.

Why is the calculated enthalpy different from an experimental value?

Average bond enthalpies are approximate and may not account for the exact molecular environments and physical states in the real reaction.

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