Untitled - Kelly Walsh High School

Chemical Bonding 137
9-6 Bond Energy and Bond Length
The reason bonds form is that bonded species are more stable than nonbonded
species. Since bonds are stabilizing factors, it requires energy to break a bond.
The bond energy is the energy required to break a bond. Since bond energy is
the energy required, the values are always positive (endothermic). We also
know that if energy is necessary to break a bond, the reverse process, the formation
of a bond releases an equivalent amount of energy.
Bond energies, found in tables such as your textbook, are average values. In a
particular molecule, the actual bond energies may be larger or smaller than the
average. The values will always be greater than the average if resonance is present.
The strength of a bond depends upon the identity of the atoms involved
and the type of bond, increasing in the order: single, double, or triple bond.
When we use bond energies to estimate the heat of reaction, we need to have the
values for all the bonds broken and for all the bonds formed. The bonds broken
are all the bonds in the reactants, and the bonds formed are all the bonds in the
products. The difference between these energy totals is the heat of reaction:
H = energies of bonds broken energies of bonds formed
The bond length is the distance between the two atomic nuclei of the atoms
involved in the bond. The bond length is an average distance and varies with the
atoms involved and the type of bond, decreasing in the order: single, double, or
triple bond. It is important to realize that a double bond is not twice the strength
or half the length of a single bond. Without looking at tabulated values, we can
simply say that double bonds are stronger and shorter than single bonds.
9-7 Utterly Confused About
Lewis Structures
Let’s work two examples illustrating the steps necessary to produce a good
Lewis structure. We will take HNO 2 and XeF 4 for these examples. The first molecule
is nitrous acid. It is an example of an oxyacid. (Oxyacids are compounds
containing hydrogen, oxygen, and one other element.) The other compound is
xenon tetrafluoride.
There is more than one way to arrange the atoms in nitrous acid. However, only
one will lead to a good Lewis structure. You should avoid any arrangement that
places identical atoms adjacent. Identical atoms, other than carbon, rarely bond