Experiments - Synthesis of tetrachlorosilane

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: Slide B4

Synthesis of Tetrachlorosilane

1 Materials, Chemicals, Time Needed

2 retort stands with connecting rod
Retort clamps and sleeves
Thick-walled reaction tube
Dropping funnel with pressure compensation, stopper, small two-neck round-bottom flask
3 test tubes with side arms
2 absorption bottles
Liebig condenser (with readily cleaned inner tube) and tubing
Tall, large glass beaker
Three-way valve
Perforated rubber stoppers
Angled glass tubes (see sketch)
Straight glass connecting tubes
Tubing connectors
Magnesia or porcelain boat
Teclu burner

Hydrochloric acid, conc, w = 37%, C
Sulfuric acid, conc, w = 98 %, C
Sodium hydroxide, conc, w = 30 %, C
Potassium permanganate, O, Xn
Silicon powder
Sodium chloride for freezing mixture
Sodium hydrogen sulfite, Xi

This experiment requires a lot of time. Overall, one repetition will take from 1.5 to 2 hours. It takes 30 to 60 minutes just for set-up, the precise time depending on experience and how well the glassware fits together. The experiment itself takes 20 to 30 minutes. Allow an additional 30 minutes for dismantling and cleaning the apparatus.

2 Procedure and Observations

As chlorine will be used, conduct the experiment in a fume cupboard. Also use the fume cupboard when flushing the apparatus for the first time.
Wear safety glasses, rubber gloves and laboratory coat.

Set up the apparatus as shown in the diagram.

Place about 5 g potassium permanganate in the round-bottom flask and add 40 ml concentrated hydrochloric acid to the dropping funnel. To the drying and absorption train, add the sulfuric acid for drying the chlorine gas, and the sodium hydroxide solution for absorbing it. Place the amorphous silicon powder in a porcelain or magnesia boat in the center of the reaction tube.
Assemble the complete apparatus and test it for leaks by connecting the gas generator to the reaction tube via the three-way valve and warming the flask with your hand. You should notice a pressure difference in the second absorption bottle.
Cool the receiving vessel with the freezing mixture and turn on the cooling water. Now open the three-way valve of the dropping funnel in the direction of the first absorption bottle and allow hydrochloric acid to drip slowly onto the potassium permanganate. When the chlorine gas is being generated at a uniform rate, switch over the three-way valve and flush the rest of the apparatus with chlorine before using the Teclu burner to heat the silicon in the reaction tube, gently at first and then strongly. When about 0.5 to 1.0 ml product has collected in the receiving vessel, cool the reaction tube in the stream of chlorine initially. Then switch the three-way valve over to the first absorption bottle. The receiving vessel can now be removed and the product used for other purposes.
When the hydrochloric acid drips onto the potassium permanganate, puffs of greenish chlorine gas are initially evolved with every drop. After about 1 minute, the gas is evolved at a uniform rate; this can be seen from the bubbles rising in the sulfuric acid.
When heated in the stream of chlorine gas, the silicon powder in the boat lights up brightly in a small line following prolonged strong heating on the side nearer the dropping funnel. The glow front gradually follows the direction of the gas stream. White smoke forms behind the silicon and precipitates out on the rear section of the tube in the form of a yellow to orange coating. Clear condensation is observed in the upper section of the Liebig condenser. A yellow-orange liquid slowly collects in the receiving vessel. During this reaction, no bubbles are seen in the absorption bottle at the end of the apparatus.
Stop heat input after about 0.5 ml product has formed. Smoke formation ceases very quickly. After a short interval, small bubbles may be seen forming again in the absorption bottle. When the reaction tube is cool enough to be handled, turn off the supply of hydrochloric acid and direct the chlorine gas into the first absorption bottle. When no more chlorine gas is being generated, open the apparatus inside the fume cupboard and leave for one hour. Then clean it.
Keep the tetrachlorosilane for another experiment (see "Hydrolysis of tetrachlorosilane”).

3 Discussion of Results

At elevated temperatures, silicon reacts with chlorine to form tetrachlorosilane.

This is a redox reaction in which the silicon is oxidized by the chlorine.
Although the reaction is exothermic, it only proceeds quickly at elevated temperatures. Since only a relatively small amount of silicon is used in the experiment, the heat of reaction is not enough to keep the reaction going. For this reason, continued heating is needed while the chlorine is reacting with the silicon.
The silicon sample is not fully consumed in the reaction, but its appearance is substantially altered by the reaction. The following photomicrographs show the sample before and after use in this experiment:

The mass of the solid reagent decreases by several hundred milligrams during the reaction with chlorine. There is little point in trying to quantify the reaction exactly because, firstly, the change in weight is too slight and, secondly, the silicon sample used generally still contains traces of aluminum and iron, which also react with chlorine.

The AlCl₃ and FeCl₃ formed sublime relatively easily and are deposited as a yellow (pure aluminum chloride), but more often orange (aluminum chloride contaminated with iron chloride) solid on the wall of the reaction tube or the condenser.
Tetrachlorosilane is a clear, water-white liquid when pure, but it is obtained in this experiment as a yellow to orange liquid due to the contaminants just mentioned. The reaction product shown in the photo is a fairly heavily contaminated sample of tetrachlorosilane.

4 Tips and Comments

If a Liebig condenser is not used, it could easily happen that the freezing mixture is not cold enough to cool the product and that the product will be driven into the absorption bottle. There, it forms a layer of foam on the surface of the sodium hydroxide. This will not occur if the stream of chlorine gas is very weak, but that is very hard to achieve.
Try to prevent too much deposition on the reaction tube because that only further contaminates the product.
A Liebig condenser with a larger internal diameter will make it easier to brush clean since some of the deposit there is not immediately soluble in water. A condenser of this kind provides adequate cooling since no observations to the contrary are made in the absorption bottle.
The pressure equalizer can be easily cleaned of manganese dioxide with a weakly acidic, highly dilute solution of sodium hydrogen sulfite. Since this generates sulfur dioxide, it is imperative that cleaning be performed in the fume cupboard.
In the References quoted below, this experiment is included as a teacher’s experiment in the lesson entitled manufacture and properties of silicon.

5 Supplementary Information

The example of tetrachlorosilane clearly shows that silicon occupies an intermediate position between metals and non-metals.
On one hand, tetrachlorosilane is not a typical salt like the metal chlorides, but rather a liquid like carbon tetrachloride, i.e. a chloride of carbon, a typical non-metal. On the other hand, the reaction between tetrachlorosilane and water is different from that of carbon tetrachloride and water. Its reaction with water resembles those of the metal chlorides SnCl₄ and AlCl₃ (see also the experiment “Hydrolysis of tetrachlorosilane”.

6 References

M. Tausch, M. von Wachtendonk (editors), CHEMIE S II, STOFF-FORMEL-UMWELT, C.C. Buchner, Bamberg (1993), (1998), S. 357f