Silicon is the most important element used in semiconductors such as transistors and diodes. Without a supply of pure silicon it would be impossible to make the range of high quality computers, calculators, telephones, radios, etc., upon which modem industrial societies have come to rely. Crude silicon can be obtained by reducing silica (sand) with coke:
SiO2 (s) + 2C(s) à Si(s) + 2CO(g)
The product is contaminated by silicon carbide, SiC2 This carbide is also known as carborundum. It is extremely hard. The bits of steel drills are often impregnated with carborundum in order to reduce the wear on them.
Much better quality silicon is produced by reducing silicon tetrachloride with hydrogen:
SiCl4(l) + 2H2(g) à Si(s) + 4HCl(g)
IMPORTANT USES OF SILICON
1. Silicon is one of the most abundant of elements and an essential ingredient of large number of minerals that make up Earth’s surface. Sand, clay and stuff we call ‘earth’ is a mixture of silicon and oxygen.
2. Pure silicon is a semiconductor. When small amounts of other elements (such as boron) are added to silicon, its resistance is reduced and it becomes a good conductor. Tiny pieces of specially treated silicon are the basis of microchips. Transistors and other electronic components are built into the microchip. Microchips are found in electronic devices such as radios, television sets, computers, calculators. Microchips contain complete electronic circuits in these devices. Computers contain many microchips.
The main advantage of microchips is that they occupy little space and act as effective conductors in many different ways. The microchip industry is one of the fastest growing industries worldwide. Microchips have revolutionized our way of life.
3. Silicon and oxygen make giant three dimensional structures called as silicates. Silicates are also used as the basis of ceramics. Ceramics are much better electrical and thermal insulators than metals, and have greater rigidity, hardness and temperature stability than organic polymers. Following a great deal of research, new ceramic materials have been developed that contain, for example, borides, carbides and nitrides. They have found an increasing number of uses ,for example, glasses for covering solar panels, parts of turbines and internal combustion engines, and refractory brick linings for high temperature furnaces.
4. Silicon is also used in photoelectric cells in solar panels to convert solar energy to electrical energy for use in space-craft, artificial satellites, electronic watches and pocket size solar calculators, domestic heating, lighting and the provision of other sources of energy.
5. Silicon is widely distributed as silica or sand (SiO2) and tetraoxosilicates(IV), SiO44- . Glass is made from tetraoxosilicate(IV) materials.
STRUCTURE OF CO2 AND SiO2
There is a marked difference in the structure and properties of CO2 and other dioxides. CO2 is a gas at room temperature. On the other hand, silicon dioxide or silica is solid at room temperature. CO2 is a monomeric, linear, nonpolar molecule in which carbon forms pπ-pπ bonds with oxygen.
It may be considered to be a resonance hybrid of the following structures.
Silicon is unable to form pπ-pπ bond with oxygen atom due to its relatively large size. Thus, it satisfies its tetra covalency with four oxygen atoms and constitutes three dimensional network structure in which each silicon atom forms four Si-0 bonds which are tetrahedrally disposed.
In turn each oxygen atom of SiO2 is shared by two silicon atoms. Three crystalline modifications of SiO2 are quartz, cristobalite and tridymite. In addition to these several amorphous forms of silica such as silica gel and .fumed silica are known.
Since Si-O bonds are very strong (368 kJ mol-1), therefore, silica is relatively inert and has a very high m.p.
COMPRISON OF PROPERTIES CO2 AND SiO2
1. It exists as gas. 1. It is a solid.
2. It is a linear, monomeric 2. It is a three dimensional
non-polar molecule. network solid.
3. Appreciably soluble in 3. In soluble in water.
4. On reduction with coke 4. On reduction with coke
gives carbon monoxide in electrical furnace gives
CO2 + C à 2CO silicon carbide
SiO2 + 3C à SiC + 2CO