Chemistry of Silicon (Si) - Practice Questions

Multiple Choice Questions (MCQs)

Question 1

Which of the following statements about silicones is incorrect?

A) They are polymers containing R₂SiO repeating units. B) They are generally hydrophobic in nature. C) They are thermally stable and resistant to chemical attack. D) They contain strong Si=Si double bonds in their backbone.

Solution:

Correct Answer: D)

Explanation: Silicones are organosilicon polymers with the general formula (R₂SiO)n, where R is an alkyl or aryl group. Their backbone consists of Si-O-Si linkages, not Si=Si double bonds. The Si-O bond is very strong, contributing to their thermal stability and chemical inertness. The presence of non-polar alkyl or aryl groups makes them hydrophobic.

Question 2

When silica (SiO₂) reacts with hydrogen fluoride (HF), the product formed is:

A) SiF₄ B) H₂SiF₆ C) SiF₄ and H₂O D) SiF₄ and H₂SiF₆

Solution:

Correct Answer: C)

Explanation: Silica (SiO₂) is a highly stable compound. It reacts with hydrogen fluoride to form silicon tetrafluoride and water. SiO₂(s) + 4HF(aq) → SiF₄(g) + 2H₂O(l) SiF₄ can further react with HF in the presence of water to form fluorosilicic acid (H₂SiF₆), but the primary and initial product from the reaction of SiO₂ with HF is SiF₄ and water. The question asks for “the product formed,” implying the direct reaction.

Question 3

Among the given compounds, which one is known as carborundum?

A) SiC B) SiO₂ C) SiH₄ D) SiF₄

Solution:

Correct Answer: A)

Explanation: Carborundum is the common name for silicon carbide (SiC). It is an extremely hard material, often used as an abrasive and in refractory materials due to its high thermal stability. SiO₂ is silica, SiH₄ is silane, and SiF₄ is silicon tetrafluoride.

Assertion-Reason Questions

Question 4

Assertion (A): Silicon does not form stable pπ-pπ multiple bonds readily. Reason (R): The atomic orbitals of silicon are much larger and more diffused than those of carbon, making pπ-pπ overlap ineffective.

A) Both A and R are true, and R is the correct explanation of A. B) Both A and R are true, but R is not the correct explanation of A. C) A is true, but R is false. D) A is false, but R is true. E) Both A and R are false.

Solution:

Correct Answer: A)

Explanation: Assertion (A) is true. Unlike carbon, which readily forms stable C=C and C=O pπ-pπ bonds, silicon typically forms single bonds. Reason (R) is also true and is the correct explanation for Assertion (A). Due to silicon’s larger atomic size (period 3 element) compared to carbon (period 2 element), its 3p orbitals are more diffused. This leads to poor and ineffective lateral overlap necessary for strong pπ-pπ bonding, making multiple bonds with silicon much less stable. Instead, silicon prefers to form strong Si-O single bonds or extended Si-Si networks.

Question 5

Assertion (A): Silicones are water-repellent (hydrophobic) in nature. Reason (R): The alkyl groups attached to the silicon atoms in silicones are non-polar and project outwards, making the surface non-polar.

A) Both A and R are true, and R is the correct explanation of A. B) Both A and R are true, but R is not the correct explanation of A. C) A is true, but R is false. D) A is false, but R is true. E) Both A and R are false.

Solution:

Correct Answer: A)

Explanation: Assertion (A) is true. Silicones are indeed hydrophobic, meaning they repel water. This property makes them useful in various applications like waterproofing agents and sealants. Reason (R) is also true and provides the correct explanation for Assertion (A). Silicones have a Si-O-Si backbone, but the silicon atoms are also bonded to organic alkyl (R) or aryl groups. These organic groups are non-polar. When silicones are formed into a bulk material, these non-polar R groups effectively surround the polar Si-O core, presenting a non-polar, water-repelling surface.

Short Answer Questions

Question 6

Explain the preparation and two important uses of silicones.

Model Answer:

Preparation of Silicones: Silicones are organosilicon polymers prepared from alkyl or aryl substituted chlorosilanes, such as RSiCl₃, R₂SiCl₂, R₃SiCl, or mixtures thereof.

1. Direct Synthesis: Alkyl halides (e.g., CH₃Cl) react with silicon at high temperatures (around 570 K) in the presence of copper powder as a catalyst to form various methyl-substituted chlorosilanes. 2CH₃Cl + Si \xrightarrow\{Cu, 570K\} (CH₃)₂SiCl₂ (dimethyl dichlorosilane) Other products like CH₃SiCl₃ and (CH₃)₃SiCl are also formed.
2. Hydrolysis: The chlorosilanes undergo hydrolysis to form silanols, which are unstable and readily undergo condensation polymerization. (CH₃)₂SiCl₂ + 2H₂O → (CH₃)₂Si(OH)₂ + 2HCl (dimethyl silanol)
3. Polymerization: The silanols condense by eliminating water molecules to form linear, cyclic, or cross-linked polymeric silicones. n (CH₃)₂Si(OH)₂ → [-O-Si(CH₃)₂-]n + nH₂O (Linear silicone)

Uses of Silicones:

1. Waterproofing Agents: Due to their hydrophobic nature and low surface tension, silicones are used in waterproofing fabrics, paper, and masonry.
2. Sealants and Adhesives: They are excellent sealants, used in construction, automotive, and electronic industries, because of their flexibility, thermal stability, and adhesive properties.
3. Lubricants: Silicone oils are used as high-temperature lubricants due to their thermal stability and low viscosity changes with temperature.
4. Cosmetics and Medical Implants: Their biocompatibility and inertness make them suitable for use in cosmetics, personal care products, and various medical implants (e.g., breast implants, pacemakers).

Question 7

Silicon dioxide (SiO₂) is a covalent network solid, while carbon dioxide (CO₂) is a discrete molecular solid. Account for this difference in structure and properties.

Model Answer:

The fundamental difference in the structures of SiO₂ and CO₂ arises from the distinct nature of pπ-pπ bonding involving carbon and silicon.

1. Carbon Dioxide (CO₂):

   • Carbon is a small atom in Period 2. It can form strong and stable pπ-pπ bonds with oxygen.
   • In CO₂, carbon forms two double bonds with two oxygen atoms (O=C=O). This results in a discrete, linear, triatomic molecule.
   • These CO₂ molecules are held together by weak intermolecular van der Waals forces.
   • Consequently, CO₂ is a gas at room temperature and is a molecular solid at low temperatures, with a very low melting and boiling point.

2. Silicon Dioxide (SiO₂):

   • Silicon is a larger atom in Period 3. Due to its larger atomic size and more diffuse 3p orbitals, pπ-pπ overlap with oxygen is much less effective and less stable compared to carbon.
   • Instead of forming double bonds, silicon prefers to form four strong Si-O single bonds.
   • In SiO₂ (e.g., quartz), each silicon atom is tetrahedrally bonded to four oxygen atoms, and each oxygen atom is shared by two silicon atoms. This forms a giant, three-dimensional covalent network structure (or macromolecule).
   • Breaking this network requires a significant amount of energy to overcome the strong covalent bonds.
   • Therefore, SiO₂ is a hard, high-melting point solid that is insoluble in water and generally unreactive.

In summary, the ability of carbon to form stable pπ-pπ bonds leads to discrete CO₂ molecules, while the inability of silicon to do so effectively results in an extended covalent network structure for SiO₂.

High-Order Thinking Skills (HOTS) Question

Question 8

Silicon is a semiconductor and is extensively used in electronic devices. Explain how the electrical conductivity of pure silicon can be drastically altered by doping, describing both n-type and p-type semiconductors.

Detailed Chemical Explanation:

Pure silicon is an intrinsic semiconductor, meaning it has very low electrical conductivity at room temperature. Its conductivity increases with temperature as more electrons gain enough energy to break free from covalent bonds and move into the conduction band, creating electron-hole pairs. However, for practical electronic applications, its conductivity needs to be precisely controlled, which is achieved through a process called doping.

Doping involves adding a small amount of an impurity element to pure silicon. These impurities create either an excess of electrons or an excess of “holes” (vacant electron sites), thereby increasing conductivity.

1. n-Type Semiconductor (Negative-type):

   • Dopant: Pure silicon (Group 14 element, 4 valence electrons) is doped with a pentavalent impurity from Group 15, such as Phosphorus (P), Arsenic (As), or Antimony (Sb).
   • Mechanism: When a pentavalent atom (e.g., P) replaces a silicon atom in the crystal lattice, four of its five valence electrons form covalent bonds with the four surrounding silicon atoms. The fifth valence electron is loosely held by the phosphorus atom.
   • Charge Carriers: This extra electron is not involved in bonding and requires very little energy to move into the conduction band. It becomes a free electron, which is a negative charge carrier. The impurity atoms are called donor impurities because they “donate” an electron.
   • Conductivity: In n-type semiconductors, electrons are the majority charge carriers, and holes are the minority charge carriers. The conductivity is primarily due to the movement of these excess free electrons.

2. p-Type Semiconductor (Positive-type):

   • Dopant: Pure silicon is doped with a trivalent impurity from Group 13, such as Boron (B), Aluminum (Al), or Gallium (Ga).
   • Mechanism: When a trivalent atom (e.g., B) replaces a silicon atom in the crystal lattice, it uses its three valence electrons to form covalent bonds with three of the four surrounding silicon atoms. To complete the fourth covalent bond, the boron atom “accepts” an electron from an adjacent silicon-silicon bond.
   • Charge Carriers: This acceptance of an electron creates a hole (a positive vacancy) in the covalent bond network. This hole can then move through the crystal lattice as an electron from another bond jumps into it. The impurity atoms are called acceptor impurities because they “accept” an electron, creating a hole.
   • Conductivity: In p-type semiconductors, holes are the majority charge carriers, and electrons are the minority charge carriers. The conductivity is primarily due to the movement of these holes.

By doping, the concentration of charge carriers (electrons or holes) can be increased by several orders of magnitude, making silicon a highly effective and controllable semiconductor, crucial for diodes, transistors, integrated circuits, and other electronic components.