114
Sizes and Shapes of Molecules
ment, by dissociation equilibrium measurements, by absorption spectrum measurements, or by mass spectrometry.
We define bond energy as the energy change (A//) for the chemical process in
which one mole of a given bond is broken, when both the reactants and the
products are in the hypothetical ideal-gas state of I atm and 25°C. For a
diatomic molecule, the bond energy is identical to the energy required to dissociate the gaseous molecule into its respective gaseous atoms. For the dissociation of C1 2 gas this corresponds to the reaction
Cl2<9) *^ 2 Cl (9)
for which the Cl-Cl bond energy is 58.0 kcal. We say that A# C i-ci = 58.0 kcal.
For a polyatomic molecule of the type AB n , which possesses n A-B bonds,
our definition of bond energy implies that each bond is the same, and that it
corresponds to \ln of the total energy required to dissociate the gaseous AB n
molecule into A + «B gaseous atoms. This is a useful definition except when
studying the detailed steps of a chemical reaction. For example, the total binding energy in a CH 4 molecule is 397 kcal/mole and, by our definition of bond
energy, the C-H bond energy = 397/4 = 99.3 kcal. Extensive, complicated, and
detailed studies have shown, however, that each H atom is not equally easily
removed from carbon in this molecule; it is estimated that the individual bond
energies are 104 kcal for CH 3 -H, 106 kcal for CHa-H, 106 kcal for CH-H, and
81 kcal for C-H, with a total of 397 kcal. In most cases, such detailed information is not available; neither is it normally needed except in discussion of the
individual steps involved in chemical reactions.
The atoms of some elements (such as C, N, and O) are able to share more
than one pair of electrons between them, to form single, double, or triple
bonds—depending on whether one, two, or three pairs of electrons are shared.
In general, the bonding energy increases and the internuclear distance decreases as the number of bonds between a pair of atoms increases.
By studying the experimentally determined bond energies of hundreds of
compounds, researchers have uncovered some useful generalizations, such as
the following.
1. A single bond between two identical atoms has about the same strength
(A// A _A) in any molecule in which it occurs. Because the atoms are
identical, the bond is purely covalent.
2. A strictly covalent bond between two different atoms (A and B) is about
the same strength as the average of the bond strengths that would be
observed if each atom were bonded to another like itself. That is,
A//A-B = MA//A-A + A//B-B]
(9-1)
Sizes and Shapes of Molecules
ment, by dissociation equilibrium measurements, by absorption spectrum measurements, or by mass spectrometry.
We define bond energy as the energy change (A//) for the chemical process in
which one mole of a given bond is broken, when both the reactants and the
products are in the hypothetical ideal-gas state of I atm and 25°C. For a
diatomic molecule, the bond energy is identical to the energy required to dissociate the gaseous molecule into its respective gaseous atoms. For the dissociation of C1 2 gas this corresponds to the reaction
Cl2<9) *^ 2 Cl (9)
for which the Cl-Cl bond energy is 58.0 kcal. We say that A# C i-ci = 58.0 kcal.
For a polyatomic molecule of the type AB n , which possesses n A-B bonds,
our definition of bond energy implies that each bond is the same, and that it
corresponds to \ln of the total energy required to dissociate the gaseous AB n
molecule into A + «B gaseous atoms. This is a useful definition except when
studying the detailed steps of a chemical reaction. For example, the total binding energy in a CH 4 molecule is 397 kcal/mole and, by our definition of bond
energy, the C-H bond energy = 397/4 = 99.3 kcal. Extensive, complicated, and
detailed studies have shown, however, that each H atom is not equally easily
removed from carbon in this molecule; it is estimated that the individual bond
energies are 104 kcal for CH 3 -H, 106 kcal for CHa-H, 106 kcal for CH-H, and
81 kcal for C-H, with a total of 397 kcal. In most cases, such detailed information is not available; neither is it normally needed except in discussion of the
individual steps involved in chemical reactions.
The atoms of some elements (such as C, N, and O) are able to share more
than one pair of electrons between them, to form single, double, or triple
bonds—depending on whether one, two, or three pairs of electrons are shared.
In general, the bonding energy increases and the internuclear distance decreases as the number of bonds between a pair of atoms increases.
By studying the experimentally determined bond energies of hundreds of
compounds, researchers have uncovered some useful generalizations, such as
the following.
1. A single bond between two identical atoms has about the same strength
(A// A _A) in any molecule in which it occurs. Because the atoms are
identical, the bond is purely covalent.
2. A strictly covalent bond between two different atoms (A and B) is about
the same strength as the average of the bond strengths that would be
observed if each atom were bonded to another like itself. That is,
A//A-B = MA//A-A + A//B-B]
(9-1)