What Bond Energy Means
Bond energy is the approximate energy required to break one mole of a particular type of bond in the gas phase. Breaking bonds requires energy, while forming bonds releases energy.
How to Read the Sign of ΔH
If ΔH is negative, the reaction releases more energy through bond formation than it absorbs through bond breaking. That means the reaction is exothermic.
If ΔH is positive, the reaction absorbs more energy to break bonds than it releases when forming product bonds. That means the reaction is endothermic.
| ΔH result | Meaning |
|---|---|
| ΔH < 0 | Exothermic reaction, heat released. |
| ΔH > 0 | Endothermic reaction, heat absorbed. |
| ΔH ≈ 0 | Approximately thermoneutral by this bond-energy estimate. |
Example: Hydrogen Combustion
For the reaction:
2H₂ + O₂ → 2H₂O
Bonds broken:
2 × H–H 1 × O=O
Bonds formed:
4 × O–H
Using average bond energies:
ΔH = [2(436) + 1(498)] − [4(463)] ΔH = 1370 − 1852 ΔH = −482 kJ
Example: Ammonia Formation
For the reaction:
N₂ + 3H₂ → 2NH₃
Bonds broken:
1 × N≡N 3 × H–H
Bonds formed:
6 × N–H
Approximate calculation:
ΔH = [945 + 3(436)] − [6(391)] ΔH = 2253 − 2346 ΔH = −93 kJ
Why Bond Energy Results Are Approximate
Bond energies are usually average values across many molecules. The same bond type can have slightly different energy depending on neighboring atoms, resonance, molecular geometry, and phase.
Common Mistakes
- Counting atoms instead of bonds.
- Forgetting to multiply by coefficients in the balanced equation.
- Using product bonds as “broken” instead of “formed.”
- Using average bond energies for liquids or solids without noting the approximation.
- Mixing bond energy methods with formation enthalpy methods in the same calculation.
Frequently Asked Questions
What is the formula for bond energy reaction enthalpy?
The estimate is ΔH ≈ energy of bonds broken minus energy of bonds formed.
Why is energy required to break bonds?
Bonds stabilize atoms in molecules. Energy must be supplied to separate bonded atoms.
Why does forming bonds release energy?
When a bond forms, atoms move into a lower-energy arrangement, and the energy difference is released.
Are bond energies exact?
No. Most listed bond energies are averages, so calculated ΔH values are estimates.
Can I add my own bond energy?
Yes. Use the custom bond rows to enter a bond name, energy, bonds broken, and bonds formed.