Question

Explain the method of extraction of amorphous silicon using silica.

Answer 1

Hint: For extracting a pure metal from its oxide we can always go for a displacement reaction where a more reactive metal displaces a less reactive metal from its metal oxide. A displacement reaction is the one wherein the atom or a set of atoms is displaced by another atom in a molecule. \[A+B-C\to A-C+B\]. The above equation exists when A is more reactive than B.

Complete answer:
To extract silicon metal from its oxide we have to react it with a more reactive metal from the reactivity series. In this case we use Magnesium metal Mg. When we give high temperatures to the crucible having Mg metal and silica the reaction will take place between them.
When finely powdered silica is mixed with Mg powder and the mixture is heated in a fire clay crucible we obtain magnesium oxide and silicon as the products. These products are then washed with dilute HCl to dissolve by-product magnesium oxide which is present as a basic impurity in the crucible. Then we have to remove the unreacted silica, and to remove the unreacted silica it is washed with hydrofluoric acid. The powder which is then left behind is silicon in its amorphous form.

$\text{Si}{{\text{O}}_{2}}~+~2\text{Mg}\to 2\text{MgO}~+~\text{Si}$

Note: The reactivity series of metals, also known as the activity series, refers to the arrangement of metals in the descending order of their reactiveness. The data provided by the reactivity series of metals can be used to predict whether a metal can displace another in a single displacement reaction. Those at the top and bottom are keenest to react with other elements to form compounds. More reactive metals will displace other metals from their compounds. A metal or element above in an activity series can displace all the elements below it.