ENERGETICS 73
The heats of formation of the gaseous atoms, 4, are not very
different; clearly, it is the change in the bond dissociation energy
of HX, which falls steadily from HF to HI, which is mainly res
ponsible for the changes in the heats of formation, 6. We shall see
later that it is the very high H—F bond energy and thus the less
easy dissociation of H—F into ions in water which makes HF in
water a weak acid in comparison to other hydrogen halides.COVALENT COMPOUNDS. OTHER SYSTEMS
We have just seen that a knowledge of bond energies enables
enthalpies of reaction to be calculated. This is certainly true for
simple diatomic systems. When polyatomic molecules are con-
sidered, however, the position can be more complicated and there
are a number of different dissociation energies for even a two-
element polyatomic molecule. Consider, for example, ammonia.
There are three N—H bond dissociation energies (p. 47) and the
bond dissociation energy is different for each N—H bond and
depends on the environment of the atoms concerned. The same
conditions apply to any polyatomic molecule. However, average
values, called average thermochemical bond energies (or average
standard bond enthalpies) have been determined from a wide
variety of compounds, and tables can be found in most data books.
In spite of the known limitations of these bond energies, they are
useful in estimating enthalpies of reactions, as indicated on p. 63,
and the likely stabilities of covalent compounds. However, special
care is needed when small positive or negative values for enthalpies
are obtained (often as the difference between two larger values),
since the predictions may then be unreliable because of the lack of
precision in the original data.IONIC COMPOUNDS: LATTICE ENERGY
Let us consider the formation of sodium chloride from its elements.
An energy (enthalpy) diagram (called a Born-Haber cycle) for the
reaction of sodium and chlorine is given in Figure 3.7. (As in the
energy diagram for the formation of hydrogen chloride, an upward
arrow represents an endothermic process and a downward arrow
an exothermic process.)
The enthalpy changes evolved are:A/I! the enthalpy of atomisation (or sublimation) of sodium.