Bond Enthalpy
Bond Enthalpy
It is
defined as the amount of energy required to break one mole of bonds of a
particular type between two atoms in a gaseous state. The unit of bond enthalpy
is kJ mol-1. For example, the H-H bond enthalpy in hydrogen molecule
is 435.8kJ mol-1.
H2(g) → H(g)
+ H(g); ∆aHv =
435.kJ mol-1
Similarly the
bond enthalpy for molecules containing multiple bonds, for example O2
and N2 will be as under :
O2
(O=O) (g) → O(g) + O(g); ∆aHv = 498kJ mol-1
N2
(N=N) (g) → N(g) + N(g); ∆aHv = 946.0
kJ mol-1
It is
important that larger the bond dissociation enthalpy, stronger will be the bond
in the molecule. For a heteronuclear diatomic molecules like HCL, we have
HCL (g) → H(g) + Cl(g); ∆aHv = 431.0 kJ mol-1
In case of
polyatomic molecules, the measurement of bond strength is more complicated. For
example in case of H2O molecule, the enthalpy needed to break the two O-H bonds
is not the same.
H2O(g) → H(g)
+ OH(g); ∆aHv1 =
502kJ mol-1
OH(g) → H(g)
+ O(g); ∆aHv2
= 427 kJ mol-1
The difference
in the ∆aHv value shows that the second O-H bond undergoes some change because
of changed chemical environment. This is the reason for some difference in
energy of the same O-H bond in different molecules like C2H5OH (ethanol) and
water. Therefore in polyatomic molecules the term mean or average bond enthalpy is used. It is obtained by dividing
total bond dissociation enthalpy by the number of bonds broken as explained below
in case of water molecule,
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