Bond Energy
Definition and meaning of Bond Energy in chemistry.
Bond energy is the average energy required to break one mole of a particular covalent bond in gaseous molecules, homolytically separating the bonded atoms into neutral fragments.
In more detail
Because the exact energy of a given bond type (like C–H) varies slightly depending on the rest of the molecule, tabulated bond energies are averages taken over many different compounds containing that bond. Bond energies are always positive, since breaking a bond always requires energy input (endothermic), while forming a bond releases energy (exothermic). Chemists use these values to estimate reaction enthalpies via Hess's law: ΔH(reaction) ≈ Σ(energy of bonds broken) − Σ(energy of bonds formed). Higher bond energy generally corresponds to a stronger, shorter bond.
Key facts
| Field | Physical Chemistry |
|---|---|
| Typical Units | kJ/mol (or kcal/mol) |
| Common Symbol | D or BE |
| Also Called | Bond dissociation energy (for a specific bond) |
The average C–H bond energy is about 413 kJ/mol, meaning roughly 413 kJ must be supplied per mole of C–H bonds broken to separate methane (CH4) into carbon and hydrogen atoms in the gas phase.
Frequently asked questions
Is bond energy the same as bond dissociation energy?
For a diatomic molecule they are identical. For polyatomic molecules, bond dissociation energy refers to breaking one specific bond in one specific compound, while bond energy is typically an average of that bond dissociation energy across several different molecules containing the same bond type.
Why do textbooks list only one C–H bond energy if C–H bonds differ slightly between molecules?
The listed value is an average bond energy, a convenient approximation used for estimating reaction enthalpies rather than an exact value for every single molecule.