Handbook of Solvents - George Wypych - ChemTech - Ventech!

1462 D.W. Rooney, K.R. Seddon
Figure 21.2.2. Showing the relationship between the
size of similar anions containing halides and melting
point of various sodium salts.
increasing the ionic charge will tend to increase
the lattice energy of the crystal. However
the effect on melting point is
complicated by the fact that according to
Fajans’ rules an increasing charge also results
in increasing covalency particularly for
small cations and large anions. The effect of
size can even be observed in the melting
points of the lithium halide series, where the
small size of the lithium ion leads to a greater
covalent character in the lithium halide
bond, reducing the melting point when compared
to the other Group 1 salts.
For a given cation, and singly charged
cations and ions, equation [21.2.1] demon-
strates that the lattice energy will become only be a function of r 0, and since r 0 is the sum of
r + and r - , then the lattice energy is only dependant on anion size. The radius for simple anions
can be found in a number of different sources. 27,29 However for more complex anions it
becomes more difficult to assign unique values to the radii. Yatsimirskii 30 has shown that it
is possible to determine the apparent values of the ionic radii of these ions indirectly from
the lattice energy of the compounds containing them. These values are generally referred to
as the thermochemical radii and a number are shown in Table 21.2.2. In most cases the fact
that ions such as [CNS] - and [CH 3COO] - , are markedly non-spherical makes these radii of
limited use. However for the case of tetrahedral ions, the symmetry is sufficiently high
enough for comparison purposes. If one were to plot the melting points a number of sodium
salts against the opposing anions’ thermochemical radius a clear relationship is observed
(see Figure 21.2.2)
Table 21.2.2. Anionic and thermochemical radii
Name Anion r, pm Name Anion r, pm
Tetrachloroferrate 31
[FeCl4] -
358 Chlorate 33
[ClO3] -
201
Tetrachloroborate 31
[BCl4] -
310 Thiocyanate 33
[CNS] -
195
Tetrachloroaluminate 31
[AlCl4] -
295 Borate 33
[BiO4] -
191
Tetrachlorogallate 31
[GaCl4] -
289 Bromate 33
[BrO3] -
191
Trifluoromethylsulfonate 32
[CF3SO3] -
267 Nitrate 33
[NO3] -
188
Hexafluoroarsenate 32
[AsF6] -
259 Cyanide 33
[CN] -
181
Hexafluorophosphate 32
[PF6] -
253 Iodate 33
[IO3] -
181
Periodate 33
[IO4] -
249 Cyanate 33
[CNO] -
160
Perchlorate 33
[ClO4] -
236 Formate 33
[HCOO] -
160
Permanganate 33
[MnO4] -
240 Acetate 33
[CH3CO2] -
156
Tetrafluoroborate 33
[BF4] -
229 Nitrite 33
[NO2] -
156
[C6H2(NO2) 3O] -
222 Amide 33
[NH2] -
129
Trinitrophenoxide 33