Questions - Answers

Very Important Questions:   (2 Marks)

1. SiO₂ is a solid while CO₂ is gas at ordinary temperature. Why?
A: Due to small atomic size of Carbon, it can form 2 double bonds and 2 p orbitals of it can overlap with p orbitals of oxygen. The shape of the molecule is linear. CO₂ molecules are held together by weak vander waal forces and exists as a gas.
                                            O = C = O
              Whereas Si can form 4 strong single covalent bonds due to its large atomic size. Three dimensional polymeric tetrahedron network makes SiO₂ solid.
 

2. Why does EN Value remain constant in Si, Ge, Sn & Pb?
A: Due to filling of d-orbitals in Ge & Sn, f-orbitals in Pb in their inner shells EN value remain constant in Si, Ge, Sn and Pb.
 

3. Explain why diamond is very hard?
A: As all the 4 valance electrons of carbon are forming 4 strong covalent bonds and forms a three dimensional giant polymer with tetrahedrons. So it is very hard.

4. What is allotropy? Name 2 crystalline allotropes of Carbon.
A: The existence of an element in different physical forms but possessing same chemical properties. eg: Graphite and diamond.
 

5. What is Catenation?
A: The phenomenon of formation of long chains and rings due to self linkage of atoms of an element. eg: Carbon, Silicon.
 

6. CCl₄ is not effected by H₂O while SiCl₄ is rapidly changed why?
A: Carbon has no d-orbitals in its valency shell and can't expand its octet SOCCl₄ is not hydrolysed by water. Whereas Si has vacant d-orbitals in its valency shell, can coordinate with water molecules. So SiCl₄ can be hydrolysed by water.
 

7. How does graphite function as a lubricant?
A: C undergoes sp² hybridisation and forms 3 covalent bonds with 3 carbon atoms in the same plane to form 2 dimensional hexagonal rings. These layers can slide easily one over the other due to weak vander waals forces. So it function as a lubricant.
 

8. Graphite is a good conductor. Explain.
A: 4th valence electron in each carbon atom remains free (Unpaired) as three -

- electrons are taking part in sp² hybridisation. All these unpaired electrons freely move throughout the crystal lattice and makes graphite a good conductor.
 

9. Give the hybridization of carbon in
a) CO₃^-2   b) Diamond  c) Graphite   d) Fullerene
A: a) sp³   b) sp³   c) sp²   d) sp²
 

10. What do you understand by inert pair effect?
A: The reluctance of pair of s electrons present in valency shell to take part in bonding.
A: Pb⁺² is more stable than Pb⁺⁴ ion.

(4 Marks Questions)

1. Graphite is a good conductor of electricity, while diamond is not. Why?
A: Each carbon atom in graphite undergoes sp² hybridisation and forms 3 Covalent bonds with three other carbon atoms. 4th Valence electron remains free and can move throughout the crystal lattice and makes graphite at good conductor of electricity. Whereas each carbon atom in diamond. Undergoes sp³ hybridisation by utilising all the 4 valence electrons to form 4 strong covalent bonds. As no free electrons are available, diamond is a bad conductor of electricity.
 

2. Write a short note on fullerenes?
A: Third important crystalline allotrope of carbon is fullerene. It is formed by heating of graphite in an electric are in presence of inert gas like He or Ar. It has cage like structure. C₆₀ is called Buckminister fullerene and having Soccer ball shape. Carbon undergoes sp² hybridization. It is the purest form of carbon. It consists 20 six membered rings and 12 five membered rings. Delocalised electrons make it aromatic. ΔfH(-) value of fullerene is highest among crystalline allotropes of carbon.
C - C distance is 143.5 pm, where as C = C distance is 138.3 pm.