NCERT Solutions for Class 12 Chemistry Part 3
Categories: NCERT Solutions
NCERT Solutions for Class 12 Chemistry Part 3
The Solid-State Exercise Solutions
Q1 : What makes a glass different from a solid such as quartz? Under what conditions could quartz be converted into glass?
Answer : The arrangement of the constituent particles makes glass different from quartz. In glass, the constituent particles have short range order, but in quartz, the constituent particles have both long range and short range orders. Quartz can be converted into glass by heating and then cooling it rapidly.
Q2 : Classify each of the following solids as ionic, metallic, molecular, network (covalent) or amorphous.
(i) Tetra phosphorus decoxide (P4O10) (vii) Graphite
(ii) Ammonium phosphate (NH4)3PO4 (viii) Brass
(iii) SiC (ix) Rb
(iv) I2 (x) LiBr
(v) P4 (xi) Si
Answer :
Ionic → (ii)Ammonium phosphate (NH4)3PO4, (x) LiBr
Metallic → (viii)Brass, (ix)Rb
Molecular → (i) Tetra phosphorus decoxide (P4O10), (iv) I2, (v) P4.
Covalent (network) → (iii) SiC, (vii)Graphite, (xi) Si
Amorphous → (vi)Plastic
Q3 : (i) What is meant by the term 'coordination number'?
(ii) What is the coordination number of atoms:
(a) in a cubic close-packed structure?
(b) in a body-centred cubic structure?
Answer:
(i) The number of nearest neighbours of any constituent particle present in the crystal lattice is called its coordination number.
(ii) The coordination number of atoms
(a)in a cubic close-packed structure is 12, and
(b) in a body-centred cubic structure is 8
Q4 : 'Stability of a crystal is reflected in themagnitude of its melting point'. Comment. Collect melting points of solid water, ethyl alcohol, diethyl ether and methane from a data book. What can you say about the intermolecular forces between these molecules?
Answer : Higher the melting point, greater is the intermolecular force of attraction and greater is the stability. A substance with higher melting point is more stable than a substance with lower melting point.
The melting points of the given substances are:
Solid water → 273 K
Ethyl alcohol → 158.8 K
Diethyl ether → 156.85 K
Methane → 89.34 K
Now, on observing the values of the melting points, it can be said that among the given substances, the intermolecular force in solid water is the strongest and that in methane is the weakest.
Q5 : How many lattice points are there in one unit cell of each of the following lattice?
(i) Face-centred cubic
(ii) Face-centred tetragonal
(iii) Body-centred
Answer :
(i) There are 14 (8 from the corners + 6 from the faces) lattice points in face-centred cubic.
(ii) There are 14 (8 from the corners + 6 from the faces) lattice points in face-centred tetragonal.
(iii) There are 9 (1 from the centre + 8 from the corners) lattice points in body-centred cubic.
Q6 : Explain
(i) The basis of similarities and differences between metallic and ionic crystals.
(ii) Ionic solids are hard and brittle.
Answer : (i) The basis of similarities between metallic and ionic crystals is that both these crystal types are held by the electrostatic force of attraction. In metallic crystals, the electrostatic force acts between the positive ions and the electrons. In ionic crystals, it acts between the oppositely-charged ions. Hence, both have high melting points.
The basis of differences between metallic and ionic crystals is that in metallic crystals, the electrons are free to move and so, metallic crystals can conduct electricity. However, in ionic crystals, the ions are not free to move. As a result, they cannot conduct electricity. However, in a molten state or in an aqueous solution, they do conduct electricity.
(ii) The constituent particles of ionic crystals are ions. These ions are held together in three-dimensional arrangements by the electrostatic force of attraction. Since the electrostatic force of attraction is very strong, the charged ions are held in fixed positions. This is the reason why ionic crystals are hard and brittle.
Q7 : Non-stoichiometric cuprous oxide, Cu2O can be prepared in laboratory. In this oxide, copper to oxygen ratio is slightly less than 2:1. Can you account for the fact that this substance is a p-type semiconductor?
Answer: In the cuprous oxide (Cu2O) prepared in the laboratory, copper to oxygen ratio is slightly less than 2:1. This means that the number of Cu
+
ions is slightly less than twice the number of O
2-
ions. This is because some Cu
+
ions have been
replaced by Cu
2+
ions. Every Cu
2+
ion replaces two Cu
+
ions, thereby creating holes. As a result, the substance conducts electricity with the help of these positive holes. Hence, the substance is a p-type semiconductor.
Q8: Classify each of the following as being either a p-type or an n-type semiconductor: (i) Ge doped with in (ii) B doped with Si.
Answer: (i) Ge (a group 14 element) is doped with in (a group 13 element). Therefore, a hole will be created and the semiconductor generated will be a p-type semiconductor. (ii) B (a group 13 element) is doped with Si (a group 14 element). Thus, a hole will be created and the semiconductor generated will be a p-type semiconductor.