Bond Dissociation Energy
What if you do not have ΔHf° values for the compounds in your reaction? There is another way to estimate ΔH: use the energies of the individual bonds being broken and formed. This approach is less precise than using ΔHf° (because bond energies are averages), but it gives you a fast estimate - and the MCAT tests it regularly.
The Core Principle
Every chemical bond is a store of energy. Breaking a bond requires energy input (endothermic). Forming a bond releases energy (exothermic). The overall enthalpy change of a reaction depends on the balance between these two processes.
The Bond Energy Formula
Important: This formula uses the convention that bond dissociation energies (BDEs) are always positive numbers (the energy required to break). The subtraction handles the sign:
- If you form stronger bonds than you break → ΔH < 0 (exothermic)
- If you break stronger bonds than you form → ΔH > 0 (endothermic)
Common Bond Energies
You do not need to memorize these - the MCAT will provide them in a table. But knowing the trends helps:
| Bond | Energy (kJ/mol) | Trend |
|---|---|---|
| C-H | 413 | Single bonds |
| C-C | 348 | Weaker than double/triple |
| C=C | 614 | Stronger than single |
| C≡C | 839 | Strongest carbon-carbon |
| O-H | 463 | Strong - explains water’s stability |
| O=O | 498 | Must break this in combustion |
| N≡N | 941 | Very strong - N₂ is hard to break |
| H-H | 436 | |
| C=O | 799 | Strong bond in CO₂ |
| N-H | 391 |
Key trend: Triple bonds > double bonds > single bonds in energy. Stronger bonds = more stable molecules = more energy released when formed.
Worked Example
Estimate ΔH for the combustion of methane: CH₄ + 2 O₂ → CO₂ + 2 H₂O
Bonds broken (reactants):
- 4 × C-H = 4 × 413 = 1,652 kJ
- 2 × O=O = 2 × 498 = 996 kJ
- Total broken = 2,648 kJ
Bonds formed (products):
- 2 × C=O (in CO₂) = 2 × 799 = 1,598 kJ
- 4 × O-H (in 2 H₂O) = 4 × 463 = 1,852 kJ
- Total formed = 3,450 kJ
ΔH ≈ 2,648 - 3,450 = -802 kJ
The actual value is -890 kJ. The estimate is off by about 10% because bond energies are averages across many molecules. For the MCAT, this level of accuracy is expected and acceptable.
Why Combustion Is Always Exothermic
In combustion, you break relatively weak C-H, C-C, and O=O bonds and form very strong C=O and O-H bonds. The bonds formed are stronger than the bonds broken, so energy is released. This is why all combustion reactions are exothermic - the products (CO₂ and H₂O) contain some of the strongest bonds in chemistry.