Microscale Gas Chemistry
 Experiments with Silane

Link to SiH4 data page including physical properties.

General Safety Precautions.
    Always wear safety glasses.  Gases in syringes may be under pressure and could spray liquid chemicals.  Follow the instructions and only use the quantities suggested.

Use a fume hood if available.
    The gas-generation and gas-washing steps should be carried out inside a working fume hood.

Toxicity.
     Silane is spontaneously flammable in air and produces SiO2 when it burns.  Do not breath airborne SiO2 fumes which are known to cause silicosis disease in the lungs.  These experiments must be done in a fume hood.

Suitability.
     All of these experiments are suited for use as classroom demonstrations.  The techniques described herein are more advanced than those used in the first ten parts of this series.  Individuals attempting these experiments should be experienced with the simpler syringe/gas techniques.  These experiments are not generally advised for use as laboratory experiments conducted by typical high school students.  Advanced students or students with special laboratory skills could be allowed to perform these experiments under close supervision by the instructor.

Syringe Lubrication.
    We recommend lubricating the black rubber diaphragm of the plunger with silicone spray (available from hardware stores) or medium-grade silicone oil (Educational Innovations, $5.95 Part #GAS-150; Fisher Catalog Number S159-500; $34/500 mL.)

Equipment. (This equipment can be ordered from a variety of vendors including Educational Innovations, Flinn Scientific (US sales only), Micro Mole, and Fisher Scientific.  Part numbers and links to their websites are provided.)

Chemicals. Silane and Disilane.
     The gas prepared by the following instructions is mostly SiH4(g) with small amounts of Si2H6(g) and possibly higher silanes as well.  For our purposes, this simple method of preparation outweighs the fact that the product is a mixture of silanes.  Very pure SiH4(g) can be prepared by one of several more difficult approaches described in the literature. (Finholt et. al., J. Am. Chem. Soc. 69, 2692 (1947); Cotton, F. A.; Wilkinson, G.; Murillo, C. A.;  and Bochmann, M.; Advanced Inorganic  Chemistry, Sixth Ed., Wiley, 1999; Weiss, H. G. and Fisher, H. D. Inorg. Chem. 1963, 2 880.)

    All of these experiments can be done with one or two syringefuls of silane.

Preparation of Silanes.
     Silane is generated by the reaction of magnesium silicide, Mg2Si(s)4, with 1 M HCl(aq) by the In-Syringe Method.  Thoroughly lubricate the diaphragms on two plungers with silicone oil or spray so that the plungers move readily in the syringe barrels.  Place 0.20 g Mg2Si(s) in a vial cap and then lower the cap into the syringe barrel by water flotation.  The cap should rest upright on the bottom of a syringe with all of the reagent still in the cap.  Install the plunger while maintaining the syringe in a vertical position.  The plunger should fit snugly against the rim of the vial cap.  Pour approximately 15 mL 1 M HCl(aq) into a small weighing boat.  Draw 10-mL of the HCl into the syringe and then cap the LuerLOK fitting.  Shake the syringe in order to react the chemicals.  Note!  You will notice a few flashes of fire inside the syringe as silane is generated and reacts with the small amount of air originally present in the syringe.  The theoretical yield for this reaction is 64 mL, however, no more than 50-mL SiH4(g) are typically produced.  After the reaction has stopped, it is necessary to transfer this highly flammable gas to a clean, air-free syringe.
 

Transferring Silane to a Clean Syringe.
       Silane is a pyrophoric gas; exposure to air will result in instant fire!  Without air present, silane will not burn.  Equip a clean syringe with a 15-cm length of latex tubing.  Suction up 15-mL water and invert the syringe so that the tubing is facing upward.  Discharge 5-mL water to remove bubbles of air.  Use a hemostat (or clamp) to close the latex tubing near the end so that the tubing is filled with water as shown in Figure 1.

     Connect the latex tubing to the silane-preparation syringe as follows.  Position the two syringes with the water-filled syringe on top and the silane preparation syringe on the bottom as shown in Figure 2.  Use a ring stand and two clamps (not shown) to hold the syringes in position.  CAUTION!  Remove the syringe cap from the silane-preparation syringe with a pliers; the contents are likely to be under positive pressure and some flashes of fire with popping noises may be noted.  The pliers serves to keep hands a safe distance away.  Caution: Remove the syringe cap with the cap end of the syringe directed upward so that the reagents will not spray out of the syringe.

     After the two syringes are connected as per Figure 2, transfer the silane to the water-filled syringe by pushing inward on the lower plunger at the same time as pulling outward on the top plunger at the same rate.  When silane transfer is complete, transfer about half of the water (5-mL) from the upper syringe into the lower syringe.  This fills the latex tubing with water.  Clamp the tubing shut near the lower end (nearest the silane preparation syringe) so that the tubing remains filled with water.  Remove the silane preparation syringe from the latex tubing and set it aside.

Figure 1

Figure 2

      "Store" silane as shown in Figure 3.  Connect the latex tubing to a glass pipet (the diameter of the tubing is such that it fits snugly inside the glass pipet.)  The lower elbow of the latex tubing is filled with water and serves as an "air-lock" for the silane.  Use a ring stand and two clamps (not shown) to hold the syringe and the glass pipet in this position.  Remove the hemostat after the apparatus has been clamped in this position.


Figure 3

Disposal.
     Unwanted samples of SiH4(g) should be discarded by burning in a fume hood as per Experiment 1A or by reaction with KOH(aq) as per Experiment 5.
 
 

Experiments with Silane.

Experiment 1. Silanes React with Air.
Equipment:

Chemicals: Part A.
    Slowly discharge 5 - 10 mL silane into the air.  The gas will bubble through the water and burn with a bright yellow light.  The first few mL will burn inside the pipet at the water's surface, but after the oxygen in the pipet has been consumed, the flame will appear at the mouth of the pipet.

Part B.
    Place a clean, dry medium-size test tube over the end of the pipet.  Discharge about 10 mL of silane into the test tube .  A white coating of SiO2(s) will be deposited on the walls of the test tube as the gas burns.  As the oxygen becomes depleted, the silane will dissipate to the mouth of the test tube and form a white cloud without flames.

Part C.
    Discharge some silane through water in a beaker or plastic cup.  It is necessary to temporarily reposition the pipet for this procedure.  Silane does not dissolve in water and the bubbles rise to the surface where they burst into flames upon contact with the air.  Suction 5-mL water back into the latex tubing in order to re-establish the airlock.  Return the pipet to its "stored" position as shown in Figure 3.







Experiment 2. Silane Reacts with Oxygen.
Equipment:

Chemicals:       Fill a medium test tube with water and clamp it in position upside down in a beaker of water.  There should be no bubbles of air in the test tube.  Deliver 10-mL silane to the test tube via the glass pipet as shown in Figure 4.  Suction 5-mL water back into the latex tubing in order to re-establish the airlock.  Return the pipet to its "stored" position as shown in Figure 3.  Next, bubble 5-mL bursts of O2(g) into the test tube.  Bright flashes of fire inside the test tube will accompany the reaction between silane and O2(g).  The test tube will become full of smoke.  The reaction is:

SiH4(g) + 2 O2(g)  SiO2(s) + 2 H2O(l)


Figure 4






Experiment 3. Silane Reacts with Chlorine.
Equipment:

Chemicals:    Fill a medium test tube with water and clamp it in position upside down in a beaker of water.  There should be no bubble of air in the test tube.  Deliver 10-mL silane to the test tube via the glass pipet as shown in Figure 4.  Suction 5-mL water back into the latex tubing in order to re-establish the airlock.  Return the pipet to its "stored" position as shown in Figure 3.  Before adding the Cl2 to the silane, discharge about 5-mL Cl2(g) into a fume hood or into 1 M NaOH(aq) in order to purge the latex tubing of air.  Next, bubble 5-mL bursts of Cl2(g) into the test tube.  Bright flashes of fire and audible pops will accompany the reaction.  The test tube will become full of smoke.  The reaction produces HCl which can be tested for with Universal indicator and/or Ag+(aq) solution:

SiH4(g) + 4 Cl2(g) -----> SiCl4(l) + 4 HCl(aq)

A more impressive reaction occurs if the Cl2(g) is delivered directly into the silane gas with a 25-cm length of latex tubing that has been worked up to the gas phase of the silane-containing test tube.  Purge the latex tubing of air before use.

     The tetrachlorosilane produced by the reaction reacts with water to produce Si(OH)4 and HCl(aq).


Reaction between chlorine and silane.








Experiment 4. Thermal Decomposition.
Equipment:

Chemicals:    Silane decomposes to its elements above 400 oC.   Fill a medium test tube with water and clamp it in position upside down in a beaker of water.  There should be no bubble of air in the test tube.  Deliver 20-mL silane to the test tube via the glass pipet as shown in Figure 4.  Suction 5-mL water back into the latex tubing in order to re-establish the airlock.  Return the pipet to its "stored" position as shown in Figure 3.  Heat the test tube with the torch in the region that contains the silane gas.  Heat gently at first and then strongly in one spot until the glass glows red.  A mirror of silicon with brownish edges will be deposited there according to the reaction:

SiH4(g) -----> Si(s) + 2 H2(g)




Experiment 5. Reaction with KOH(aq).
Equipment:

Chemicals:    Silane hydrolyzes quickly in the presence of base to produce hydrogen:

SiH4(g) + (2+n) H2O(g) -----> SiO2.n H2O(s)+ 4 H2(g)

The reaction is not spectacular as the previous reactions.

     This experiment should be done last because it will destroy all of the silane remaining in the syringe.  Using the silane syringe/pipet-device shown in Figure 3, suction 1-mL 6 M KOH(aq) into the silane-containing syringe for every 2-mL silane.  Next, suction about 10-mL water up through the pipet in order to remove KOH from the pipet and latex tubing.  Close the latex tubing with the hemostat.  Vigorously shake the contents for about one minute.  Discharge the liquid solution.  The remaining gas can be tested for silane by discharging some into the air.  It should not spontaneously burn upon contact with air as it did in Experiment 1.  Discharge some of the gas into a candle flame and it will burn as is expected for hydrogen.

     The gas can be further tested for hydrogen by pulling an equal volume of air into the syringe (no fire) and then discharging the mixture onto the surface of a small piece of activated platinum sponge (Aldrich 26715-5).  (To activate the platinum sponge, heat the sponge to red heat with a burner flame.  Allow the platinum to cool to room temperature before using in the reaction.)  The platinum will glow red as it catalyzes the combustion of hydrogen to make water vapor.
 
 

Clean-up and Storage.

  At the end of the experiments, wipe excess lubricant off of rubber diaphragm. Clean all syringe parts (including the diaphragm), caps and tubing with soap and water.  Use plenty of soap to remove oil from the rubber seal.  This extends the life of the plunger.  It may be necessary to use a 3-cm diameter brush to clean the inside of the barrel.  Rinse all parts with distilled water.  Be careful with the small parts because they can easily be lost down the drain. Important: Store plunger out of barrel.


 
 
This article first appeared in Chem13 News in January, 2000.  The authors of the original Chem13 article are: 

From the Department of Chemistry, Creighton University, Omaha, Nebraska 68178 USA:

Bruce Mattson*, faculty member, principal investigator
Michael Anderson, faculty member, co-principal investigator
Joseph Nguyen, Creighton graduate (chemistry major), May, 2000, now working in Residence Life at Idaho State
Also from Creighton University:
Maneesh Bansal, undergraduate student,  health administration major, plans to graduate in May, 2001 and attend medical school 
*Author to whom correspondence should be addressed.  E-Mail: xenon@creighton.edu

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(This page last updated 29 January 2002)