Calculating the Energy Released by a Reaction
Calculating a Reaction Enthalpy from Bond Energies
Breaking Bonds Costs Energy, Forming Them Releases Energy
A chemical reaction transforms reactants into products by breaking certain bonds and forming others. Breaking a covalent bond always requires energy (the attraction between atoms must be overcome); forming a bond always releases energy (the atoms become more stable). It is the difference between these two quantities that gives the sign of ΔrH.
The Formula
ΔrH = Σ E(bonds broken) - Σ E(bonds formed)
where each E is a bond energy (also called dissociation energy), always positive, expressed in kJ/mol, and tabulated for common bonds.
| Bond | Energy (kJ/mol) |
|---|---|
| C-H | 413 |
| O=O | 498 |
| C=O (in CO2) | 799 |
| O-H | 463 |
| H-H | 436 |
Interpreting the Result
- If the total energy released in forming the new bonds is greater than the energy needed to break the old ones, then ΔrH < 0: the reaction is exothermic.
- If it is the other way around, ΔrH > 0: the reaction is endothermic.
Practical Method
- Identify all the bonds present in the reactants, and count them.
- Identify all the bonds present in the products, and count them.
- Sum the energies of the bonds broken, then those of the bonds formed.
- Subtract: ΔrH = Σ(broken) - Σ(formed).
Common Mistake
A frequent error is reversing the subtraction, writing ΔrH = Σ(formed) - Σ(broken). This flips the sign of the result and makes an exothermic reaction look endothermic. Remember the logical order: bonds are broken first (energy is spent), then formed (energy is recovered), hence "broken minus formed".