Modern inorganic chemistry

74 ENERGETICS

A/I 2 the first ionisation energy of sodium.

A/i 3 the enthalpy of atomisation of chlorine, which is also half the

bond dissociation enthalpy.

A/i 4 the electron affinity of chlorine.

A/i 5 the lattice energy of sodium chloride; this is the heat liberated

when one mole of crystalline sodium chloride is formed from

one mole of gaseous sodium ions and one mole of chloride ions.

AHf" the enthalpy of formation of sodium chloride.

Hence

AHf = A/it + A/I 2 + A/i 3 4- A/i 4 4- A/i 5

Of these enthalpies, all can be determined experimentally except

the lattice energy. Ionisation energies, electron affinities, bond

Na+(g)+e-+Cl(g)

Na^4 (g)-r-e-r|Cl 2 (g)

Ah, Ah^4 Na*(q)+cng)

"Ah-

Ah,

A//f

Ah*

Figure 3.7

dissociation energies and heats of atomisation have all received some
discussion previously. The lattice energy can be determined by
using the Born-Baber cycle as shown above, or by calculation,
summing the attractive and repulsive energies between all the ions
in 1 mole of crystal. Details of the calculation are outside the scope
of this book. However, it may be noted that the calculation is based
on the assumption that ionic crystals are made up of discrete
spherical ions which exert non-directional electrostatic attractive
or repulsive forces on their neighbours in the crystal. The calculation
gives a result which is most simply represented as follows:

Lattice energy (A/is) = A --

"-f- r~

• B