1
The Strengths and Lengths of Bonds
–bond strengths and lengths depend largely on the bond
order (single, double or triple) and have similar values
in different molecules
9.6 Bond Strengths
•Bond enthalpy (∆∆HB) –the enthalpy change for
the dissociation of one mole bonds from
molecules in the gas phase
A-B(g)→→ A(g) + B(g) ∆∆Ho= ∆∆HB> 0
•∆∆HBis a measure of the strength and stability of
chemical bonds
Large ∆∆HB⇔stronger bonds
•Bond strength (∆∆HB)
increases with increasing
the bond order
:N≡≡N: :O=O:
:F–F:
Fig. 9.17
•Bond strength (∆∆HB)
increases as the atomic
radius of the bonded atoms
decreases
•Lone pairs on neighboring
atoms decrease bond strength
(∆∆HB)
H–H436 kJ/mol
:F–F: 158 kJ/mol
Fig. 9.18
9.7 Bond Strengths in Polyatomic
Molecules
•The strength of the bond between a given pair of
atoms varies slightly in different molecules
•Average bond enthalpies (∆∆HB) –averaged
over many compounds
•Average bond enthalpies can be used to estimate
the enthalpy changes of reactions in the gas
phase (only approximate values)
∆∆Ho(reaction) = ∆∆HB(broken) -∆∆HB(formed)
–energy is absorbed (+) to break the bonds of the reactants
and emitted (-) during forming the bonds of the products
Fig. 9.22
