(i.e., bond breaking is endothermic). Bond energy, or bond dissociation energy, is an average of the
energy required to break a particular type of bond in one mole of gaseous molecules. It is tabulated
as the magnitude of the energy absorbed as the bonds are broken. For example:
H 2 (g) → 2H (g) ∆H = 436 kJBASIC CONCEPT
Bond breaking is endothermic; bond formation is exothermic.A molecule of H 2 gas is cleaved to produce two gaseous, unassociated hydrogen atoms. For each
mole of H 2 gas cleaved, 436 kJ of energy is absorbed by the system. This is the bond energy of the H
—H bond. For other types of bonds, the energy requirements are averaged by measuring the
enthalpy of cleaving many different compounds with that bond. The averaging is needed because
the energy required to break a bond is not uniquely determined by what atoms are being separated;
it also depends on what else may be bonded to those atoms. For example, the energy needed to
break a C—H bond is different on going from CH 4 to CH 3 Cl to CCl 3 H, et cetera. The C—H bond
dissociation energy one would find in a table (415 kJ/mol) was compiled from measurements on
thousands of different organic compounds.
Bond energies can be used to estimate enthalpies of reactions. The enthalpy change of a reaction is
given by:
Example: Calculate the enthalpy change for the following reaction:
C (s) + 2H 2 (g) → CH 4 (g)Bond dissociation energies of H—H and C—H bonds are 436 kJ/mol and 415 kJ/mol,
respectively.