Library / Foundations: Chemistry / FCH-11
Foundations: Chemistry
Enthalpy change from bond energies
Energy change of a reaction from the bonds broken and formed.
- Ref
- FCH-11
- Rev
- v1
- Field
- Foundations: Chemistry
- Result
- ΔH [kJ/mol]
Add up the bond energies for all bonds broken in the reactants and for all bonds formed in the products using the same balanced equation. A negative ΔH is exothermic.
Breaking bonds takes energy in; making bonds gives energy out. The difference is the heat of the reaction.
Calculator
- Energy released if exothermic822 kJ/mol
- Broken as share of formed76.26 %
What drives the result
Each variable is moved across its typical range while the others stay at the values above. The longest bar is the key driver.
- More energy released than absorbed means exothermic, with a negative ΔH.
- Bond energies are averages, so values from them differ from measured enthalpies by a few per cent.
Chart data as tables
Ranked variables
| Variable | Typical range | Lowest result | Highest result | Swing (kJ/mol) | Local step | Change in result |
|---|---|---|---|---|---|---|
| broken Energy to break bonds | 100 to 20,000 kJ/mol | −3,362 | 16,540 | 19,900 | +10% | −32.1 % |
| formed Energy released forming bonds | 100 to 20,000 kJ/mol | −17,360 | 2,540 | 19,900 | +10% | +42.1 % |
Sweep of Energy released forming bonds
| Energy released forming bonds (kJ/mol) | Enthalpy change (kJ/mol) |
|---|---|
| 100 | 2,540 |
| 193.9 | 2,446 |
| 376.1 | 2,264 |
| 729.3 | 1,911 |
| 1,414 | 1,226 |
| 2,742 | −102.5 |
| 5,318 | −2,678 |
| 10,310 | −7,673 |
| 20,000 | −17,360 |
Sweep one variable
Variables and how each one contributes
Enthalpy change
What the equation gives you. Everything else feeds it.
Energy to break bonds
Total energy absorbed breaking reactant bonds. Raises ΔH one-for-one.
Energy released forming bonds
Total energy released forming product bonds. Lowers ΔH one-for-one.
Worked example with the values above
- Energy in to break bonds: 2,640 kJ/mol.
- Energy out from forming bonds: 3,462 kJ/mol.
- ΔH = 2,640 − 3,462 = −822 kJ/mol (negative, so exothermic; this is CH₄ + 2O₂ → CO₂ + 2H₂O).
Assumptions and limits
- Gaseous reactants and products.
- Average bond energies.
Common mistakes
- Subtracting in the wrong order.
- Forgetting the number of each bond from the coefficients.
Statistics and uncertainty
Average bond energies differ from the real bond energy in a given molecule, so expect an error of around 5–10% in ΔH.
Where it is used
- Estimating the energy of fuels and propellants.
- Judging whether a reaction is exothermic.
Maths you need
- Exothermic and endothermic: energy in and out
- Balanced equations: bond counts