Bond Energy Formula
Bond energy measures the strength of a chemical bond.
Learn how to calculate enthalpy changes using bond energies with worked examples.
The Formula
Bond energy (also called bond dissociation energy) is the amount of energy required to break one mole of a particular type of bond in the gas phase. The enthalpy change of a reaction can be estimated by comparing the total energy needed to break all bonds in the reactants with the total energy released when new bonds form in the products.
If the result is positive, the reaction is endothermic (absorbs energy). If the result is negative, the reaction is exothermic (releases energy). Bond energy values are averages because the exact energy depends on the molecular environment.
Common bond energies include: C-H = 413 kJ/mol, O-H = 463 kJ/mol, C=O = 799 kJ/mol, O=O = 498 kJ/mol, and H-H = 436 kJ/mol. These values are tabulated in chemistry reference tables and textbooks.
Variables
| Symbol | Meaning |
|---|---|
| ΔH | Enthalpy change of the reaction (in kJ/mol) |
| Σ(bonds broken) | Sum of bond energies of all bonds broken in reactants |
| Σ(bonds formed) | Sum of bond energies of all bonds formed in products |
Example 1
Estimate ΔH for the reaction H₂ + Cl₂ → 2HCl, given: H-H = 436 kJ/mol, Cl-Cl = 242 kJ/mol, H-Cl = 431 kJ/mol.
Bonds broken: 1 × H-H + 1 × Cl-Cl = 436 + 242 = 678 kJ
Bonds formed: 2 × H-Cl = 2 × 431 = 862 kJ
ΔH = 678 - 862
ΔH = -184 kJ/mol (exothermic)
Example 2
Estimate ΔH for combustion of methane: CH₄ + 2O₂ → CO₂ + 2H₂O. Bond energies: C-H = 413, O=O = 498, C=O = 799, O-H = 463 kJ/mol.
Bonds broken: 4 × C-H + 2 × O=O = 4(413) + 2(498) = 1,652 + 996 = 2,648 kJ
Bonds formed: 2 × C=O + 4 × O-H = 2(799) + 4(463) = 1,598 + 1,852 = 3,450 kJ
ΔH = 2,648 - 3,450
ΔH = -802 kJ/mol (exothermic)
When to Use It
Use bond energy calculations to estimate reaction enthalpies when calorimetric data is not available.
- Predicting whether a reaction is exothermic or endothermic
- Estimating energy released in combustion reactions
- Comparing the stability of different molecules
- Quick thermochemical calculations in organic chemistry