SILICON

CHAPTER-4

SILICON

Text Book Questions and Answers

I. Four alternatives are given to each of the following incomplete statement / question. Choose the right answer
1. The acid used to remove unreacted silica in extraction of silicon is
(a) Hydrochloric acid (b) hydrofluoric acid (c) nitric acid (d) sulphuric acid.
Answer: (b) hydrofluoric acid


2. The other product in the following chemical equation is, Si + 2H₂O → SiO₂ +__________
3. Which silicon compound among these is used as insulating material?
II. Fill in the blanks with suitable words.
III. Match the following

Amorphous Silicon	Crystalline Silicon
1. It is a brown powder	1. It is a dark grey, crystalline solid. It roughly resembles the structure of diamond
2. It does not conduct electricity at low temperature.	2. Slightly conducts electricity.
3. It is more reactive than the crystalline variety	3. It is less reactive than the amorphous variety.
4. When heated in the air, it oxidizes at the surface level and it catches fire.	4. It does not burn in the air even when heated.

(a) H₂ (b) O₂ (c) Si (d) O₃
Answer: (a) H₂

(a) Silicon dioxide (b) silicon carbide (c) silicones (d) zeolite.
Answer: (c) silicones

1. Two allotropic forms of silicon are amorphous and crystalline.
2. Crystalline silicon is obtained by reducing silica with coke.
3. When a mixture of silicon and coke is heated in an electric furnace, the product obtained is silicon carbide

1. Water softeners                                   a) calcium silicate
2. Abrasive material                                b) permutit (1)
3. Constituent of concrete mixture c) silicone
                                                                         d) Silica (3)

                                                           e) carborundum (2)

IV. Answer the following
1. What is the role of hydrochloric acid and hydrofluoric acid in the extraction of silicon?
Hydrochloric acid is used to dissolve the magnesium oxide and hydrofluoric acid used to remove unreacted silica.
2. Name the element which is most abundant in earth’s crust.
Oxygen
3. Write the chemical equations of the reaction taking place when silicon reacts with a. oxygen b. steam
a. Oxygen
Si + O₂ → SiO₂
b. Steam
Si + 2H₂O → SiO₂ + 2H₂↑
4. How is silicon carbide obtained?
When a mixture of silicon and coke is heated in an electric furnace, silicon carbide is formed.
Si + C → SiC
5. Describe the methods of extraction of amorphous silicon and crystalline silicon.
a. Extraction of amorphous silicon.
Finely powdered silica (sand or quartz) is mixed with magnesium powder and heated in a fire- clay crucible. Magnesium oxide and silicon are formed. The product is washed with dilute hydrochloric acid to dissolve magnesium oxide, a basic impurity. Then it is washed with hydrofluoric acid to remove unreacted silica. The powder left behind is silicon its amorphous form.
SiO₂ + 2Mg → Si + 2MgO
b. Extraction of crystalline silicon.
Crystalline silicon is obtained by reducing silica with coke. When excess of silica is heated with coke in the electric furnace in the absence of air dark grey coloured silicon is obtained in its crystalline form.
SiO₂ + 2C → Si + 2CO↑

Additional Question and Answers

John Berzelius first isolated silicon and found it to be an element.
1. Write the atomic number, mass number, symbol and electronic configuration, of a silicon atom.
Atomic number- 14
Mass number – 28
Symbol – Si
Electronic configuration – 1s²2s²2p⁶3s²3p²
2. How abundant is silicon in the earth’s crust?
Silicon is the second most abundant element after oxygen.
3. Name the compounds of silicon.
Sand, Clay. Rocks etc.
4. Write examples of precious stones containing silicon.
Garnet, zircon, topaz and opal.
5. Who was the first to produce silicon from sand? What did this prove?
Berzelius produced crude silicon from sand and proved that sand is a chemical compound and not an element. He also showed that silicon is an element
6. Why was silicon not studied for a long time even after its discovery?
Silicon was not studied for a long time due to difficulty in its isolation, ignorance of its properties and lack of suitable equipment and technique.
7. Why silicon is considered a non-metal?
Silicon does not possess metallic properties. Hence it is considered a non-metal
8. Why silicon is considered a metalloid?
Silicon is a non-metal with some metallic properties. Since silicon has both metallic and non- metallic properties, it is considered a metalloid.
9. What is allotropy?
The existence of a chemical element in more than one physical form is known as allotropy. The various physical forms of an elements are called allotropes.
10. What is the place of silicon in the periodic table?
Silicon is placed in the 14th group 3rd period in the carbon family. It is a p-block element
11. Write a note on the occurrence of silicon.
Silicon is the second most abundant element in the earth's crust. It is never found in nature in elemental form. It is usually found as oxides and silicates. Silicon in oxide form is found in sand, quartz, flint etc, Silicon is found in the form of silicates in rocks and clay.
12. What is the role of magnesium and hydrochloric acid in the extraction of amorphous silicon?
In the process of extraction of silicon, magnesium acts as a reducing agent. It reduces silicon dioxide to silicon. Hydrochloric acid helps to separate silicon from magnesium oxide.
13. What is the role of hydrofluoric acid in the extraction of amorphous silicon from silica?
Hydrofluoric acid is used to remove unspent silica which remains in the mixture of products (magnesium oxide and silicon)
14. Which chemical element can be extracted by reducing sand with coke?
Crystalline silicon can be obtained by reducing sand (Si0₂) with coke.
15. What is the role of coke in the extraction of crystalline silicon from silica?
Coke reduces silica (silicon dioxide) to silicon. Thus it acts as a reducing agent
16. List the important physical properties of silicon
a) Silicon is a semiconductor or non-metal.
b) Amorphous silicon is a dark brown powder insoluble in water. Crystalline silicon forms dark grey, crystalline solid. It roughly resembles the structure of diamond.
c) It has two isotopes namely Si²⁸ and Si³⁰.
d) Silicon crystals are hard enough to scratch glass.
e) Crystalline silicon melting point is 1683K

and boiling point is 2628K.
17. List some of the chemical properties of silicon.
Following are some of the chemical properties of silicon:
a) Silicon is a non-metal.
b) Amorphous silicon is chemically more active than crystalline silicon.
c) Silicon burns brilliantly in air and vigorously in oxygen to form silicon dioxide.
d) Red hot silicon decomposes steam to yield silicon dioxide and hydrogen gas.
e) When a mixture of coke and silicon is heated in an electric furnace, silicon carbide is formed.
18. What are silicones? Mention some of their uses.
Any of the polymeric synthetic materials that usually have resistance to temperature, water, and chemicals, and good insulating and lubricating properties, making them suitable for wide use as oils, water- repellents, resins, etc., are known as silicones.
Silicones are used in water-proofing treatments, moulding compounds, insulating material for electric motors and other electrical appliances. .
19. Why are silicone sockets and switches preferred in heating circuits?
Silicones are highly resistant to heat. They do not soften on heating. Hence they preferred materials for sockets and switches in heating circuits.
20. What is silicon carbide? Mention some of its uses.
A synthetic compound, having the molecular formula SiC, made usually in electric furnaces by fusing silicon and coke is called silicon carbide. It is also known as carborundum.
Silicon carbide is used to sharpen knives, polish granite, to make emery paper and sharpen pencils. It is also used in high power electronic equipment.
21. What is silica gel? Mention some of its uses.
A granular, vitreous, porous form of silicon dioxide made synthetically from sodium silicate is called silica gel.
Silica gel is used in chromatography. It is also used to absorb or remove moisture from camera, shoes and other materials.
22. Write a note on biological significance of silicon.
Silicon is an essential element in biology, although it is required only in tiny traces by animals. However, various sea sponges as well as microorganisms like diatoms, radiolaria and siliceous sponges need silicon to build their body's skeletal structure. It is much more important to the metabolism of plants, particularly many grasses. Some plants like paddy need silicon for their growth.
23. What is silicosis?
A chronic, progressive respiratory disease caused by inhaling silica dust is called silicosis.
24. Why are mine workers provided with a gas mask?
Mining workers are prone to inhaling silica particles and other types of dust. This can cause a chronic ailment called silicosis. In order to protect the workers from such diseases which affect their respiratory system, they are provided with a gas mask.
25. What is silicon carbide commonly called?
Silicon carbide is commonly called carborundum.
26. Which compound of silicon is used in calico printing?
A compound of silicon called sodium silicate is used in calico printing.
27. What is zeolite? Mention its use
A naturally occurring compound of silicon called sodium aluminium silicate is called zeolite. It is used to soften hard water and in chromatography.
28. Write the chemical properties of Silicon.
Amorphous silicon is more active than the crystalline form.
1) Silicon burns brilliantly in air and vigorously in oxygen to form silicon dioxide. (Exothermic reaction)
Si + O₂------> SiO₂
2) Silicon does not react with water. But red hot silicon decomposes steam at red heat liberating hydrogen. (Endothermic reaction)
Si + 2H₂O-------->SiO₂ + 2H₂­
3) When a mixture of silicon and coke is heated in an electric furnace to about 3073K, silicon carbide is formed.(Endothermic reaction)
Si + C--------> SiC
29. Write the uses of silicon compounds.
a) Quartz glass is used for the manufacture of threads which are highly elastic
b) Which are used in electrical instruments (mirror galvanometers) Quartz is used in making chemical apparatus and optical instruments.
c) Sand is used in the manufacture of glass and porcelain.
d) Sand and stone are used as building materials.
e) Sand is used in building to make concrete mix.
f) Sodium silicate is called water glass. Chemically water glass is sodium
g) Silicate with excess of silica. It is used in calico printing. Silicon carbide (SiC) is used as an abrasive for cutting and grinding glass.
h) Silicones are excellent insulating material for electric motors and other electrical appliances.
i) Sodium aluminium silicates (Zeolite) are used in softening hard water.
j) Quartz is used in modern clocks.
k) Silica gel is used in chromatography.
l) Silica of different size embedded on paper is called emery paper or sand paper.
m) Many of the plug points of heating circuits are made from silicones.
n) Silicones are used in water proofing treatments, moulding compounds, insulating material for electric motors and other electrical appliances.
30. Write one use of each of the following: (a) Silicon carbide (b) Sodium aluminium silicate (c) SiO₂
(a) Silicon carbide (Carborundum): Silicon carbide (Sic) is used as an abrasive for cutting and grinding glass.
(b) Sodium aluminium silicate (Zeolite): Sodium aluminium silicates are used in softening hard water.
(c) SiO₂ (Silica): Sand is used in the manufacture of glass and porcelain.
31. Name the allotropic forms of silicon.
a) amorphous and
b) Crystalline.
32. Distinguish between Amorphous Silicon and Crystalline Silicon.
33. Sand is used in sand bath, why?
Sand has the property of high thermal capacity and is non-combustible; hence it is used in sand baths.
34. What does diatoms and radiolarian use as structural material to construct skeletons?
Diatoms and radiolarian use biogenic silica as a structural material to construct skeletons.
35. What is an exothermic reaction?
Chemical reactions which liberate heat energy are called exothermic reaction.
36. What is an endothermic reaction?
Chemical reactions which absorb heat energy are called endothermic reaction.
37. What is emery paper or sand paper?
Silica of different size embedded on paper is called emery paper or sand paper..
38. Write the hazards of silicon.
Miners and stone breakers, in asbestos factory, glass factory cement factory sometimes suffer from a professional hazard is called silicosis.
This is because of the entry of silica into lungs. Silica particles are suspended in the air in the mining area; hence they enter lungs because of long exposure to pollution. The workers must be provided with a gas mask to protect their health.