Chemistry Reference
In-Depth Information
The energy necessary to remove the ions from the position that they hold in
a crystal to the infinite is equal, but of opposite sign, to the lattice energy (
U
).
During the separation of ions, the energy (-
U
) is high and positive. Nevertheless,
the ions in solution are not separated through an infinite distance as is the case in
a gaseous phase, but they are surrounded by, and strongly associated with, water
dipoles. This association occurs with energy liberation, called the
free enthalpy of
hydration
(Δ
G
h
o
).
The dissolution capacity of salts depends on the energies of opposite signs, -
U
and
G
h
o
, which make the solubility characteristics irregular. The free energy of dis-
solution
Q
o
can be considered as the sum of lattice energy -
U
of the salt and the
free
energy of hydration
Δ
G
h
o
of metal ions (Figure 3.14). The free enthalpy of hydration
(Δ
G
h
o
), is the favoring factor of the dissolution process and is largely determined by
the enthalpy of hydration of ions.
The hydration process is an exothermic one. In the case of cations, the enthalpy of
hydration is dependent on the size and charge of metal ions. The smaller and higher
charged a metal ion is, the more energy is released at its hydration. In Table 3.12,
the enthalpies of hydration of some metal ions are presented. The enthalpy of hydra-
tion
Δ
h
H
was used in QSAR studies for the analysis of metal ion toxicity (Enache
et al. 1999; Enache et al. 2003).
-
U
M
+
X
-
(s)
M
+
(g)
X
-
(g)
+
Q
o
∆
G
h
o
M
+
(aq)
X
-
(aq)
+
FIGURE 3.14
Born-Haber cycle for the dissolution process of a salt.
TABLE 3.12
Enthalpies of Hydration (kJ
mol
−1
) of Selected Metal Ions
H
+
−1091
Cr
2+
−1904
Cd
2+
−1807
Li
+
−519
Be
2+
−2494
Mn
2+
−1841
Hg
2+
−1824
Na
+
−406
Mg
2+
−1921
Fe
2+
−1946
Sn
2+
−1552
K
+
−322
Ca
2+
−1577
Co
2+
−1996
Pb
2+
−1481
Rb
+
−293
Sr
2+
−1443
Ni
2+
−2105
Al
3+
−4665
Cs
+
−264
Ba
2+
−1305
Cu
2+
−2100
Fe
3+
−4430
Ag
+
−473
Zn
2+
−2046
Tl
+
−326
Source:
Data from W.L. Jolly,
Modern Inorganic Chemistry,
2
nd
edition
(New York: McGraw-Hill, 1991).
