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Hess’s Law

If a specified set of reactants is transformed to a specified set of products by more than one sequence of reactions, the total enthalpy change must be the same for every sequence. The enthalpy change in a chemical or physical process is the same whether the process is carried out in one step or in several steps.

Bond Energies

The amount of energy required to break one mole of a particular type of bond between the atoms in the gaseous state. Energy required to separate the atoms in the gaseous state under 1 atm pressure and the specified temperature is called bond dissociation energy.

H–H(g)  2H(g); ΔH = 433 kJ/mol
H–l(g)  H(g) + I(g); ΔH = 299 kJ/mol


The bond dissociation energy of a diatomic molecule is also called bond energy. However, the bond dissociation energy depends on the nature of bonds and also on the molecule in which the bond is present. When a molecule of a compound contains more than one bond of the same kind, the average value of the dissociation energies of a given bond is taken. This average bond dissociation energy required to break each bond in a compound is called the bond energy which is also the heat of formation of the bond from gaseous atoms constituting the bond with reverse sign.
Consider the dissociation of water molecule,
H2O(g) H(g) + O–H(g); ΔH = 497.8 kJ/mol
O–H(g) H(g) + O(g); ΔH = 428.5 kJ/mol
The average of these two bond dissociation energies gives the value of bond energy of O–H bond. Therefore,
Bond energy of O–H bond Description: 45434.png = 463.15 kJ/mol
Bond energies can be obtained from the data of heats of combustion and heats of dissociation.

Application of bond energies

  • Determination of heat of reactions
    ΔHreaction = Sum of bond energies of reactants
    – Sum of bond energies of products
  • Determination of resonance energy
    Resonance energy = Experimental or actual heat of formation
    – Calculated heat of formation

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