Chemistry Reference
In-Depth Information
but also look better to the chemical structures. If one wants to emphasize that
chemical symbols always symbolize smallest structural units of substances
(see Fig.
10.21
), unit cells and their symbols like Na
4
Cl
4
,Li
8
O
4
,orZn
4
S
4
should
be used and studied [
14
].
In other compounds the molecules are the smallest structural units, for example,
the molecular symbol C
6
H
6
is chosen for the benzene molecule. This unit is obvious
for the smallest particle of benzene and nobody would recommend shortening this
symbol to the stoichiometric symbol C
1
H
1
or CH. It would be consequent to discuss
according to salts those formulae of the unit cells.
10.5.4 Reactions of Salts: Regrouping Ions
The dissolution of salts in water is usually accompanied by energy transfer -
therefore, it is a chemical reaction. It can be interpreted as a separation of ions
from the salt's ionic lattice and simultaneous hydration of the ions by water
molecules:
ammoniumnitrate (s)
!
aq
!
ammoniumnitrate (aq)
;
endothermic
!
NH
4
þ
Þ
1
ð
NO
3
Þ
1
NH
4
þ
(aq)
NO
3
ð
ð
aq
!
þ
aq
Þ; D
H
>
0
sodium hydroxide (s)
!
aq
!
sodiumhydroxide (aq)
;
exothermic
!
Na
þ
Þ
1
ð
OH
Þ
1
Na
þ
(aq)
OH
ð
ð
aq
!
þ
aq
Þ; D
H
<
0
For the dissolution of salts in water, lattice energy is used for separating the ions
of an ionic structure. Parallel to the separation of ions, a shell of H
2
O molecules
forms around the ions: this process releases the hydration energy. The first shown
reaction above is endothermic - therefore, the lattice energy has to be higher than
the released hydration energy. For exothermic dissolution processes, this is the
other way around. One example shows the dissolution of sodium hydroxide in
water: the released hydration energy is higher than the lattice energy required.
Precipitation reactions
: If solutions that react to insoluble salts are mixed, the
corresponding solids precipitate, while the ions of easily soluble substances remain
dissolved. Such a precipitation process is a regrouping of ions (see Fig.
10.22
). The
formation of characteristically looking precipitates confirms the existence of certain
kinds of ions: these reactions can therefore be employed as analytical tests for ions
(qualitative analysis). Drying and weighing the obtained precipitates allows calcu-
lating the amounts of precipitates (quantitative analysis, gravimetry). Some
examples for precipitation reactions are listed below:
