NCERT Solutions for Class 12 Chemistry Part 2
Categories: NCERT Solutions
NCERT Solutions for Class 12 Chemistry Part 2
The Solid-State Exercise Solutions
Q1: Name the parameters that characterize a unit cell.
Answer: The six parameters that characterise a unit cell are as follows.
(i) Its dimensions along the three edges, a, b, and c
These edges may or may not be equal.
(ii) Angles between the edges
These are the angle ∝ (between edges b and c), β (between edges and c), and γ (between edges and b).
Q2: What is the two-dimensional coordination number of a molecule in a square close-packed layer?
Answer: In a square close-packed layer, a molecule is in contact with four of its neighbours. Therefore, the two-dimensional coordination number of a molecule in the square close-packed layer is 4.
Q3: A compound forms a hexagonal close-packed structure. What is the total number of voids in 0.5 mol of it? How many of these are tetrahedral voids?
Answer:
Number of close-packed particles = 0.5 x 6.022 x 10
23 = 3.011 x 10
23
Therefore, number of octahedral voids = 3.011 x 10
23
And, number of tetrahedral voids = 2 x 3.011 x 10
23 = 6.022 x 10
23
Therefore, total number of voids = 3.011 x 10
23 + 6.022 x 10
23 = 9.033 x 10
23
Q4: Which of the following lattices has the highest packing efficiency (i) simple cubic (ii) body-centred cubic and (iii) hexagonal close-packed lattice?
Answer: Hexagonal close-packed lattice has the highest packing efficiency of 74%. The packing efficiencies of simple cubic and body-centred cubic lattices are 52.4% and 68% respectively.
Q5: What type of defect can arise when a solid is heated? Which physical property is affected by it and in what way?
Answer: When a solid is heated, vacancy defects can arise. A solid crystal is said to have a vacancy defect when some of the lattice sites are vacant. Vacancy defect leads to a decrease in the density of the solid.
Q6: What type of stoichiometric defect is shown by: (i) ZnS (ii) AgBr
Answer:
(i) ZnS shows Frenkel defect.
(ii) AgBr shows Frenkel defect as well as Schottky defect
Q7: Explain how vacancies are introduced in an ionic solid when a cation of higher valence is added as an impurity in it.
Answer: When a cation of higher valence is added to an ionic solid as an impurity to it, the cation of higher valence replaces more than one cation of lower valence so as to keep the crystal electrically neutral. As a result, some sites become vacant. For example, when Sr
2+
is added to NaCl, each Sr
2+
ion replaces two Na
+
ions. However, one Sr
2+
ion occupies
the site of one Na
+
ion and the other site remains vacant. Hence, vacancies are introduced.
Q8: Ionic solids, which have anionic vacancies due to metal excess defect, develop colour. Explain with the help of a suitable example.
Answer: The colour develops because of the presence of electrons in the anionic sites. These electrons absorb energy from the visible part of radiation and get excited.
For example, when crystals of NaCl are heated in an atmosphere of sodium vapours, the sodium atoms get deposited on the surface of the crystal and the chloride ions from the crystal diffuse to the surface to form NaCl with the deposited Na atoms. During this process, the Na atoms on the surface lose electrons to form Na + ions and the released electrons diffuse into the crystal to occupy the vacant anionic sites. These electrons get excited by absorbing energy from the visible light and impart yellow colour to the crystals.
Q9: A group 14 element is to be converted into n-type semiconductor by doping it with a suitable impurity. To which group should this impurity belong?
Answer : Ann-type semiconductor conducts because of the presence of extra electrons. Therefore, a group 14 element can be converted to n-type semiconductor by doping it with a group 15 element.
Q10: What type of substances would make better permanent magnets, ferromagnetic or ferrimagnetic. Justify your answer.
Answer : Ferromagnetic substances would make better permanent magnets. In solid state, the metal ions of ferromagnetic substances are grouped together into small regions. These regions are called domains and each domain acts as a tiny magnet. In an unmagnetised piece of a ferromagnetic ubstance, the domains are randomly oriented. As a result, the magnetic moments of the domains get cancelled. However, when the substance is placed in a magnetic field, all the domains get oriented in the direction of the magnetic field and a strong magnetic effect is produced.
The ordering of the domains persists even after the removal of the magnetic field. Thus, the ferromagnetic substance becomes a permanent magnet.
Q11: Define the term 'amorphous'. Give a few examples of amorphous solids.
Answer: Amorphous solids are the solids whose constituent particles are of irregular shapes and have short-range order.
These solids are isotropic in nature and melt over a range of temperature. Therefore, amorphous solids are sometimes called pseudo solids or super cooled liquids. They do not have definite heat of fusion. When cut with a sharp-edged tool, they cut into two pieces with irregular surfaces. Examples of amorphous solids include glass, rubber, and plastic.