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Hydrolysis of Tetrachlorosilane |
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  1
Materials, Chemicals, Time Needed |
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- 2 test tubes
- Test-tube rack
- Glass rod
The experiment takes about 3 minutes.
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| 2
Procedure and Observations |
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| Add about 2 ml water to a test tube. In a
fume cupboard, carefully add approx. 0.5 ml tetrachlorosilane.
A vigorous reaction occurs at once. The tetrachlorosilane sinks
to the bottom as a droplet, with a whitish skin and gas forming
at the interface with the water. |
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If the tetrachlorosilane was discolored by impurities,
it turns clear very rapidly. The size of the droplet
decreases rapidly as the gas and skin forms, with the
gas rising to the surface and parts of the skin detaching
in “strings.” At the end of the reaction,
the test tube contains a whitish, gel-like substance
and about 2 ml of liquid. Measure the pH of the liquid.
The indicator paper shows that the liquid is highly acidic
(pH = 1). The dried product is a white powder that does
not even appear to change when calcined.
The photo shows one test tube and its contents immediately
after hydrolysis and one after the liquid has evaporated. |
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3
Discussion of Results |
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Tetrachlorosilane hydrolyzes
in water to hydrogen chloride and orthosilicic acid.
Hydrogen chloride reacts with water to form hydrochloric acid (reason
why liquid is acidic), but some of it also escapes from the test tube
in gas form.
The orthosilicic acid formed – Si(OH)4 oder H4SiO4 – condenses
at once to polysilicic acid and is recognisable as “strings” or
skin. The impurities in the tetrachlorosilane, namely AlCl3 and
FeCl3 react with water even faster than tetrachlorosilane
does. The evidence for this is that the silane droplet loses all its
color.
Boiling down and further heating causes progressive dehydration of the
polysilicic acid. The dried, calcined powder ultimately has the composition
of quartz (SiO2), even its structure is not remotely like
the monocrystalline structure of quartz. |
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4
Tips and Comments |
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- The experiment is only recommended for chemistry lessons
in exceptional circumstances, such as when the focus is on
the
chemistry of silicon.
- It may be performed in conjunction with the experiment "Synthesis
of tetrachlorosilane" by pupils with proven experimental
skills under supervision as part of a project or group work
because it provides a deep insight into the chemistry of
silicon and its compounds.
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5
Supplementary Information |
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The hydrolysis of SiCl4 in
this experiment illustrates the special position enjoyed by silicon
between the metals and the nonmetals. Metal chlorides are generally
salts, whereas nonmetal chlorides are generally gases or liquids.
Although SiCl4, like the analogous nonmetal chloride CCl4,
is a liquid, it hydrolyzes very easily and therefore has similarity
to several metal chlorides, such as AlCl3 and FeCl3.
Other properties of silicon and silicon compounds that show silicon to
be a semimetal are presented in the references below, which may be used
for chemistry lessons. |
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References |
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| M. Tausch, M. von Wachtendonk
(editors), CHEMIE S II, STOFF-FORMEL-UMWELT, C.C. Buchner, Bamberg
(1993), (1998), S. 357 - 367 |
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