Pyrophorisity of Silane

 

Magnesium silicide in contact with hydrochloric acid produces silane which in contact with air ignites spontaneously.

Materials
Test tube (16 mm x 160 mm), laboratory beaker of 100 mL, small spoon, metal tray with sand, burner, metal stand with clamp.

Chemicals
Magnesium powder, silicon dioxide powder (silica), 1:1 hydrochloric acid

Fig. 1. Apparatus

Fig. 2. Burning of Silane

Experimental

  • Make a mixture of 2 g magnesium and 2 g of silica.
  • Add the mixture in a test tube.
  • Assemble apparatus as it is shown on the Fig.1 in a fume hood.
  • Heat the test tube with a burner.
  • When the whole amount of the substance turns black stop the heating.
  • After cooling, if necessary, break the test tube in a mortar and remove the pieces of glass.
  • In a fume hood, add some of the content of the test tube in small portions in a beaker with hydrochloric acid as shown on Fig. 2.

Observation and Discussion

Magnesium silicide is prepared by heating mixture consisted of magnesium and silicon dioxide.


2Mg(s) + SiO2(s) = Si(s) + 2MgO(s)

When it is added to the hydrochloric acid a gas is evolved.

Mg2Si(s) + 4HCl(aq) = SiH4(g) + 2MgCl2(aq)

This gas is silane and ignites in contact with oxygen to produce silicon dioxide.


SiH4(g) + 2O2(g) = SiO2(s) + 2H2O(g)


Safety Tips
The experiment must be performed in a fume hood. In the beginning, the test tube with the mixture should not be heated at the bottom of the test tube. Due to the possibility of reaction:


Mg(g) + H2O(l) = H2(g) + MgO(s)

which can take place in presence of moisture in the mixture, the content can be thrown out of the test tube. When the reaction starts one can notice burning as a glow in the test tube and the heating should be stopped.