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MCAT Content / Energy Changes In Chemical Reactions / Bond Dissociation Energy As Related To Heats Of Formation

Bond dissociation energy as related to heats of formation

Topic: Energy Changes In Chemical Reactions

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.

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