1. Experiment is contained in the WACKER's Experimental Kit.

No

 2. Experimental procedure has been modified

/

 3. A separate experimental procedure has been devised

Yes

 4. Video clip available

No

 5. Flash animation available

No

 6. Other materials: Worksheet 6

Hydrolysis of Tetrachlorosilane

TopDown 1 Materials, Chemicals, Time Needed
  • 2 test tubes
  • Test-tube rack
  • Glass rod

The experiment takes about 3 minutes.

TopDown 2 Procedure and Observations
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.

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.

TopDown 3 Discussion of Results

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.

TopDown 4 Tips and Comments

  • 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.

TopDown  5 Supplementary Information

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.
TopBottom  6 References
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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