Chemistry Reference
In-Depth Information
2.2.
THERMODYNAMIC STABILITY IN AQUEOUS SOLUTIONS
Silicon, with a standard potential of is a rather active element and
is readily oxidized in water. The stability of silicon in water and aqueous solutions in
the absence of complex formation at 25 °C is determined by the equilibrium conditions
listed in Table 2.5.
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Figure 2.2 shows the pH-potential diagram (Pourbaix diagram) which is a graphic
presentation of the equilibrium conditions listed in Table 2.5. The lines labeled
a
and
b
represent, respectively, the equilibrium conditions of the reduction of water to gaseous
hydrogen and of the oxidation of water to oxygen, when the partial pressure of hydro-
gen or oxygen is 1 atm. According to Fig. 2.2, the stable region of silicon is far below
line
a
(the stability line of water) and thus silicon is thermodynamically not stable in
water and aqueous solutions. It tends to be oxidized with the evolution of hydrogen
and gaseous silicon hydride and the formation of silica and silicates. Silicon, as
a solid substance, is, however, generally stable in most aqueous solutions because of
the formation of a passive oxide film on the surface. Figure 2.2 also shows that hydride
is unstable in the presence of water and tends to decompose into hydrogen and
silica or silicates. The diagram is approximate since it does not take into account the
complex nature of the solutions of silicic acid and silicates. Also, it is only valid in the
absence of substances with which silicon can form compounds of insoluble salts or
soluble complexes.
In the presence of fluoride ions, we find formation of soluble silicon hexafluo-
which is dominant in the acidic region. Figure 2.3 illustrates the stability
ride,
