Category : NEET
Bond Energy or Bond Enthalpies
When a bond is formed between atoms, energy is released. Obviously same amount of energy will be required to break the bond. The energy required to break the bond is termed bond dissociation energy. The more precise definition is,
?The amount of energy required to break one mole of bond of a particular type between the atoms in the gaseous state, i.e., to separate the atoms in the gaseous state under 1 atmospheric pressure and the specified temperature is called bond dissociation energy.?
For example, \[H-H(g)\to 2H(g);\] \[\Delta H=+\,433\,kJ\,mo{{l}^{-1}}\]
\[Cl-Cl(g)\to 2Cl\,(g);\] \[\Delta H=+\,242.5\,kJ\,mo{{l}^{-1}}\]
\[H-Cl(g)\,\to H(g)+Cl(g);\]\[\Delta H=+\,431\,kJ\,mo{{l}^{-1}}\]
\[I-I(g)\to 2I(g);\] \[\Delta H=+\,15.1\,kJ\,mo{{l}^{-1}}\]
\[H-I(g)\to H(g)+I(g);\] \[\Delta H=+\,299\,kJ\,mo{{l}^{-1}}\]
The bond dissociation energy of a diatomic molecule is also called bond energy. However, the bond dissociation energy depends upon the nature of bond and also 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 bond energy.
Bond energy is also called, the heat of formation of the bond from gaseous atoms constituting the bond with reverse sign.
\[H(g)+Cl(g)\to H-Cl\,(g);\]\[\Delta H=-\,431\,kJ\,mo{{l}^{-1}}\]
Bond energy of \[H-Cl=-\] (enthalpy of formation) \[=-(-431)=+\,431\,kJ\,mo{{l}^{-1}}\]
Consider the dissociation of water molecule which consists of two \[O-H\] bonds. The dissociation occurs in two stages.
\[{{H}_{2}}O(g)\to H(g)+OH(g);\,\]\[\Delta H=497.89\,kJ\,mo{{l}^{-1}}\]
\[OH(g)\to H(g)+O(g);\] \[\Delta H=428.5\,kJ\,mo{{l}^{-1}}\]
The average of these two bond dissociation energies gives the value of bond energy of \[O-H.\]
Bond energy of \[O-H\] bond \[=\frac{497.8+428.5}{2}=463.15\,kJ\,mo{{l}^{-1}}\]
Similarly, the bond energy of \[N-H\] bond in \[N{{H}_{3}}\] is equal to one ? third of the energy of dissociation of \[N{{H}_{3}}\] and those of \[C-H\] bond in \[C{{H}_{4}}\] is equal to one ? fourth of the energy of dissociation of \[C{{H}_{4}}.\]
Bond energy of \[C-H=\frac{1664}{4}=416\,kJ\,mo{{l}^{-1}}\]
\[[C{{H}_{4}}(g)\to C(g)\,+4H(g);\]\[\Delta H=1664\,kJ\,mo{{l}^{-1}}]\]
Applications of bond energy
(1) Heat of a reaction \[=\Sigma \]Bond energy of reactants ? \[\Sigma \] Bond energy of products.
Note : In case of atomic species, bond energy is replaced by heat of atomization.
Order of bond energy in halogen \[Cl>Br>{{F}_{2}}>{{I}_{2}}\]
(2) Determination of resonance energy : When a compound shows resonance, there is considerable difference between the heat of formation as calculated from bond energies and that determined experimentally.
Resonance energy = Experimental or actual heat of formation ~ Calculated heat of formation.
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