ΔHrxn can represent both the bond dissociation energy and the enthalpy of formation. The
The energy required to completely separate bonded atoms is represented by a value called bond dissociation energy. Dissociation energies are given values in kJ/mol.
Two values are represented by ΔHrxn: Bond dissociation energy and the Enthalpy of formation.
ΔHrxn for bond dissociation energy is represented by:
ΔHrxn = bond dissociation energy of reactant bonds – bond dissociation energy of product bonds
ΔHrxn = enthalpy of formation of product bonds – enthalpy of formation of reactant bonds.
Values for the calculations are given in tabulated values. For instance, the bond enthalpy, or bond-dissociation energy, for one of the C-H bonds in ethane (C2H6) is defined by the process:
Key Points
• Bond dissociation energy will be positive; energy is required to break bonds.
• Enthalpy of formation will be negative; heat is released to break bonds.
Key Terms
enthalpy of formation: change of enthalpy during the formation of one mole of a substance. Given in kJ/mol
bond dissociation energy: is the energy required for a bond between two atoms to break completely; this value can be calculated for a given reaction from tabulated values, given in kJ/mol.
