CBSE Class 12 Chemistry Solid State VBQs

VALUE BASED QUESTIONS

Chapter-1.SOLID STATE

Very Short Tyoe Questions Answer:

Question. Why is glass considered supercooled liquid?
Answer. Glass is considered as super cooled liquid because glass is an amorphous solid and has tendency to flow very slowly like liquids

Question. Write a feature which will distinguish a metallic solid from an ionic solid. 
Answer. (i) Metallic solids are malleable and ductile whereas ionic solids are hard and brittle. 
(ii) In solid state, ionic solids are electrical insulators because ions are not free to move about, e.g. NaCl, Na2SO4, etc. while metals are good electrical
conductors in solid state because of the presence of free electrons e.g. copper, nickel etc.

Question. Define primitive unit cells.
Answer. A unit cell in which constituent particles (lattice points) are present only at the corner positions is called primitive or simple unit cell.

Question. Why are crystalline solids anisotropic?
Answer. Crystalline solids are anisotropic in nature because of different arrangements of particles in different directions. Some of their physical properties like electrical conductivity, refractive index, etc., shows different values in different directions in the same crystals

Question. Write a distinguishing feature of metallic solids.
Answer. Malleable, ductile, lustrous and conductor of heat and electricity.

Question. Give an example each of a molecular solid and an ionic solid. 
Answer. Molecular solids : HCl, SO₂, H₂O (any one)
Ionic solids : KCl, CuSO₄, ZnS (any one)

Question. How many atoms per unit cell (Z) are present in bcc unit cell?
Answer. In a body centred cubic unit cell.
(i) 8 corners × 1/8 per corner atom = 8 × 1/8 = 1 atom
(ii) 1 body centre atom = 1 × 1 = 1 atom
∴ Total number of atoms per unit cell
= 1 + 1 = 2 atoms

Question. Which one of the following is an example of molecular solid : CO₂ or SiO₂? 
Answer. Carbon dioxide (CO₂) is a molecular solid.
Quartz (SiO₂) is a covalent solid.

Question. “Stability of a crystal is related to the magnitude of its melting point.” How?
Answer. The crystalline solids have sharp melting points. Melting point of a solid is related to the force of attraction between its molecules. Higher the melting point of a crystalline substance, greater is the force between the constituent particles and hence, greater is the stability.

Question. What type of interactions holds the molecules together in a polar molecular solid? 
Answer. Strong dipole-dipole interactions

Question. Some of the glass objects recovered from ancient monuments look milky instead of being transparent. Why?
Answer. Some of the glass objects from ancient monuments look milky instead of being transparent because it undergoes heating during the day and cooling at night i.e., annealing over a number of years. As a result, it acquires some crystalline character. They become a bit opaque

Question. How will you distinguish between the following pair of terms :
Crystal lattice and unit cell?
Answer. Arrangement of lattice points in three dimensional space is called crystal lattice. The smallest repeating unit which represents arrangement of lattice points in a crystal lattice is called unit cell.

Question. How many atoms constitute one unit cell of a face-centred cubic crystal?
Answer. In face centred cubic unit cell
(i) No. of atoms contributed from the corners of the unit cell

Question. How many effective sodium ions are located at the edge centre of a unit cell in a sodium chloride crystal?
Answer. 

Question. What is the formula of a compound in which the element Y forms ccp lattice and atoms of X occupy 1/3^rd of tetrahedral voids?
Answer. 

Question. What is the formula of a compound in which the element Y forms ccp lattice and atoms of X occupy 2/3^rd of octahedral voids?
Answer. No. of Y atoms per unit cell in ccp lattice

Question. How will you distinguish between the following pair of terms :
Tetrahedral and octahedral voids? 
Answer. A void surrounded by four atoms whose centre when joined forms tetrahedron is tetrahedral void.
Number of tetrahedral voids = 2 × Number of atoms.
A void surrounded by six atoms whose centre when joined forms an octahedron is called octahedral voids.
Number of octahedral voids = Number of atoms

Question. An ionic compound AB₂ possesses CaF₂ type crystal structure. Write the coordination number of A²⁺ and B^– ions in crystals AB₂.
Answer. Coordination number of A²⁺ = 8
Coordination number of B^– = 4

Question. What is the coordination number of each type of ions in a rock-salt type crystal structure?
Answer. In a rock salt type crystal structure, coordination number is 6 : 6
⇒ Coordination number of M⁺ = 6
Coordination number of A^– = 6

Question. Express the relationship between atomic radius (r) and the edge length (a) in the bcc unit cell.
Answer. 

Question. Express the relationship between atomic radius (r) and the edge length (a) in the fcc unit cell.
Answer. 

Question. Define the following term : Schottky defect
Answer. Schottky defect : The defect in which cations and anions are missing in the stoichiometric ratio of compound is called Schottky defect. Schottky defect is actually vacancy defect in ionic solids. In this defect, electrical neutrality is maintained. 
In this defect, density of solid decreases. Schottky defect is shown by ionic solids in which the cation and anion are of almost similar sizes.
Examples : NaCl, KCl, CsCl, AgBr, etc.

Question. Which stoichiometric defect does not change the density of the crystal?
Answer. Frenkel defect does not change the density of the solid.

Question. What type of point defect is produced when  AgCl is doped with CdCl₂?
Answer. Addition of CdCl₂ to AgCl crystal causes impurity defect.
In this defect also one Cd²⁺ ion replaces two Ag⁺ ions from the crystal. One site is occupied by one Cd²⁺ ion and other site remains vacant.

Question. Account for the following :
Schottky defects lower the density of related solids.
Answer. Schottky defects occur when cations and anions are missing from their lattice site. Mass of unit cell decreases which decreases the density of the solid.

Question. Which stoichiometric defect increases the density of a solid?
Answer. Interstitial defect

Question. What type of semiconductor is formed when silicon is doped with As?
Answer. n-type semiconductor is formed when silicon is doped with arsenic.

Question. What type of semiconductor is obtained when silicon is doped with boron?
Answer. p-type semiconductor is obtained when silicon is doped with boron.

Question. What type of defect can arise when a solid is heated?
Answer. On heating a solid vacancy defect is produced in the crystal. This is because on heating, some lattice sites become vacant.

Question. How may the conductivity of an intrinsic semiconductor be increased?
Answer. The conductivity of intrinsic semiconductor like silicon is too low to be of practical use. This conductivity is increased by adding an appropriate
amount of suitable impurity like Al or As which is electron deficient or electron rich. So, the electrical conductivity of silicon is increased.

Question. What are n-type semiconductors?
Answer. A semiconductor in which doped impurity has more valence electrons than the pure element is called n-type semiconductor. e.g., Ge or Si (Group-14) doped with P or As (Group-15).

Question. What is meant by an ‘intrinsic semiconductor’?
Answer. Pure substance which acts as semiconductor is called intrinsic semiconductor. e.g., Si and Ge at high temperature.

Question. Account for the following :
Conductivity of silicon increases on doping it with phosphorus.
Answer. 
Silicon when doped with phosphorus, which contains five valence electrons, they occupy some of the lattice sites in silicon crystal. Four out of five electrons are used in the formation of four covalent bonds with the four neighboring silicon atoms. The fifth electron is extra and becomes delocalised.
These delocalised electrons increases the conductivity of doped silicon.

Question. What is meant by ‘doping’ in semiconductor?
Answer. Addition of an appropriate amount of suitable impurity in a crystalline solid is called doping.
Doping is done to increase the conductivity of intrinsic semiconductors.

Question. What type of stoichiometric defect is shown by AgBr and AgI?
Answer. Frenkel defect is shown by ionic solids in which diference in size of cations and anions is large.
Examples : ZnS, AgI and AgBr

Question. Why is Frenkel defects not found in pure alkali metal halides?
Answer. Frenkel defects is not found in alkali metal halides because the ions cannot get into the interstitial sites due to their larger size.

Question. Which point defect of its crystals decreases the density of a solid?
Answer. Schottky defect.

Question. Assign reason for the following :
Phosphorus doped silicon is a semiconductor.
Answer. Phosphorus is pentavalent. When phosphorus is doped in silicon it increases the number of electrons which increases the conductivity of doped silicon and makes it semiconductor

Question. Explain the following with suitable examples :
Piezoelectric effect
Answer. Piezoelectric effect : When the electricity is produced by applying mechanical stress on some polar crystals, it is known as piezoelectric effect. Quartz shows this property

Question. What type of magnetism is shown by a substance if magnetic moments of domains are arranged in same direction? 
Answer. Ferromagnetism

Question. Define the following term :
Ferromagnetism 
Answer. Ferromagnetism : Materials which are strongly attracted by magnetic field are called ferromagnetic materials and the property thus exhibited is caused ferromagnetism.
e.g., Fe, Co, Ni show ferromagnetism at room temperature.

Question. Write the type of magnetism observed when the magnetic moments are oppositely aligned and cancel out each other.
Answer. Antiferromagnetism

Question. Write the type of magnetism observed when the magnetic moments are aligned in parallel and anti-parallel directions in unequal numbers.
Answer. Ferrimagnetism

Question. What is meant by ‘antiferromagnetism’?
Answer. Antiferromagnetism : Paramagnetic substances which show very poor magnetic effect are antiferromagnetic and this property is called
antiferromagnetism.
Examples : MnO, Fe₂O₃, Cr₂O₃

Question. What type of substances would make better magnets, ferromagnetic or ferrimagnetic?
Answer. Ferromagnetic substances because these substances become a permanent magnet.

Question. What type of magnetism is shown in the following alignment of magnetic moments?
↑ ↑ ↑ ↑ ↑ ↑
Answer. Ferromagnetism

Short Type Questions Answer:

(i) Dimensions along the three edges represented as a, b and c.

(ii) Angles between edges a, b and g.

Hence, a unit cell is represented by six parameters a, b, c, a, b and g.

 

Question. A compound forms hcp structure. What is the total number of voids in 0.5 mol of it?
How many of these are tetrahedral voids?
Answer. Number of atoms in 0.5 mol hcp lattice
= 0.5 × 6.022 × 10²³ = 3.011 × 10²³
∴ Number of octahedral voids = Number of atoms
= 3.011 × 10²³
Number of tetrahedral voids = 2 × Number of atoms
= 2 × 3.011 × 10²³ = 6.022 × 10²³
Total number of voids = Number of octahedral voids
+ Number of tetrahedral voids
= 3.011 × 10²³ + 6.022 × 10²³
= 9.033 × 10²³ voids

Question. An alloy of gold and cadmium crystallises with a cubic structure in which gold atoms occupy the corners and cadmium atoms fit into the face centres. Assign formula for this alloy.
Answer. 
When the particles are present not only corners but also at the centre of each face of the unit cell. It is called face-centred unit cell.
Number of atoms in a unit cell of the face-centredcubic (fcc) ⇒ 8 atoms on the corners (gold) and 6 atoms of cadmium on the face (one of each face). Contribution by atoms (gold) on the corners

Question. Aluminium crystallises in a fcc structure. Atomic radius of the metal is 125 pm. What is the length of the side of the unit cell of the metal?
Answer. For fcc (or ccp), a = 2√2r
= 2 × 1.414 × 125 pm = 354 pm

36. Calculate the packing eficiency of a metal for a simple cubic lattice. 
Answer. Packing eficiency

Question. Chromium metal crystallises in a body centred cubic lattice. The length of the unit cell edge is found to be 287 pm. Calculate the atomic radius of chromium. 
Answer. 

Question. The edge of the face-centred cubic unit cell of aluminium is 404 pm. Calculate the radius of aluminium atom.
Answer. Edge of the face-centred cubic unit cell, a = 404 pm

Question. An element with density 11.2 g cm^–3 forms a fcc lattice with edge length of 4 × 10^–8 cm. Calculate the atomic mass of the element.
(Given : NA = 6.022 × 10²³ mol^–1)
Answer. Given : d = 11.2 g cm^–3, Z = 4, a = 4 × 10^–8 cm,
M = ?, N_A = 6.022 × 10²³ mol^–1

Question. An element with density 2.8 g cm^–3 forms a fcc unit cell with edge length 4 × 10^–8 cm. Calculate the molar mass of the element.
(Given : NA = 6.022 × 10²³ mol^–1)
Answer. Given, Density of solid, d = 2.8 g cm^–3
For fcc unit cell, Z = 4
Edge length, a = 4 × 10^–8 cm,
Molar mass, M = ?
N_A = 6.022 × 10²³ mol^–1

Question. An element crystallises in structure having fcc unit cell of an edge 200 pm. Calculate the density if 200 g of this element contains 24 × 10²³ atoms.
Answer. Edge length = 200 pm
Volume of the unit cell = (200 × 10^–10 cm)³ 
= 8 × 10^–24 cm³
In a fcc unit cell there are four atoms per unit cell

Question. The unit cell of an element of atomic mass 108 u and density 10.5 g cm^–3 is a cube with edge length, 409 pm. Find the type of unit cell of the crystal.
[Given : Avogadro’s constant = 6.023 × 10²³ mol^–1]
Answer. Atomic mass, M = 108 u, d = 10.5 g/cm³, a = 409 pm

Question. An element X crystallises in fcc structure. 208 g of it has 4.2832 × 10²⁴ atoms. Calculate the edge of the unit cell, if density of X is 7.2 g cm^–3.
Answer. For fcc structure Z = 4, d = 7.2 g/cm³
Let a be the edge length in pm

Question. Explain how we can determine the atomic mass of an unknown metal if you know its density and the dimension of unit cell of its crystal.
Answer. We can determine the atomic mass of an unknown metal by using the formula of density ofits unit cell.
d (Density)

By knowing density d, dimension of unit cell we can calculate M, the atomic mass of metal as for a given unit cell, Z is fixed and NA is a universal constant.

Question. A copper crystal has a face-centred cubic lattice structure. Atomic radius of the copper atom is 128 pm. Calculate the density of copper.
Atomic mass of copper = 63.5
Answer. For fcc, Z = 4

Question. Examine the given defective crystal.

Answer the following questions :
(i) What type of stoichiometric defect is shown by the crystal?
(ii) How is the density of the crystal afected by this defect?
(iii) What type of ionic substances show such defect?
Answer. (i) Schottky defect
(ii) Density of the crystal decreases.
(iii) This defect is shown by ionic substances in which the cation and anion are of almost similar sizes.

Question. (i) What type of non-stoichiometric point defect is responsible for the pink colour of LiCl?
(ii) What type of stoichiometric defect is shown by NaCl?
Answer. (i) Metal excess defect is responsible for pink colour of LiCl. It is also known as anion vacancy defect.
(ii) NaCl crystal shows Schottky defect.

Question. If NaCl is doped with 10^–3 mole percen SrCl2, what will be the concentration of cation vacancies? (NA = 6.02 × 10²³ mol^–1)
Answer. The number of cation vacancies created in the lattice of NaCl is equal to the number of divalent Sr²⁺ ions added.
Concentration of Sr²⁺ = 10^–3 mol%
= 10^-3/100 = 10^-5 mol
1 mole of Sr²⁺ = 6.023 × 10²³Sr²⁺ ions
10^–5 mol of Sr²⁺ = 6.023 × 10²³ × 10^–5
= 6.023 × 10¹⁸ Sr²⁺ ions.
Hence, the concentration of cation vacancies is 6.023 × 10¹⁸.

Question. Account for following :

(i) Zinc oxide on heating becomes yellow.

(ii) Frenkel defect does not change the density of AgCl crystals.

Answer. (i) Zinc oxide is white in colour at room temperature. On heating, it loses oxygen and turns yellow.

Now, there is excess of zinc in the crystal and excess Zn²⁺ ions move to interstitial sites and the electrons to neighbouring interstitial sites.

(ii) Due to Frenkel defect, no ions are missing from the crystal as a whole. Thus, there is no change in density.

 

Question. What is a semiconductor? Describe the two main types of semiconductors and contrast their conduction mechanism.
Answer. The solids which have conductivities between 10^–6 to 104 ohm^–1 m^–1 are called semiconductors. e.g., Germanium and silicon. The two main types of semiconductors are as follows :
(i) n-type semiconductor : When a silicon crystal is doped with atoms of group-15 elements, such as P, As, Sb or Bi then only four of the five valence
electrons of each impurity atom participate in forming covalent bonds and fifth electron is almost free to conduct electricity. Group-14 elements doped with a group-15 elements are called n-type semiconductors.
(ii) p-type semiconductor : When a silicon crystal is doped with atoms of group-13 elements, such as B, Al, Ga or In. Each impurity atoms form only three covalent bonds with the host atom. The place where

Question. Explain the following terms with suitable examples :
Ferromagnetism and ferrimagnetism
Answer. Ferromagnetism : Substances which are very strongly attracted by the magnetic field are called ferromagnetic and this property is called ferromagnetism.
Examples : Iron, cobalt, nickel, gadolinium and CrO₂.
These substances can be permanently magnetised i.e., they retain magnetic property even in absence of applied magnetic field.
In ferromagnetic substance all the domains (tiny magnet formed by grouping of metal ions in small region) are oriented in the direction of the applied magnetic field and produce a strong magnetic effect.
This ordered orientation of domain persists even when the magnetic field is removed.
Ferrimagnetism : Substances which are weakly attracted by the magnetic field as compared to ferromagnetic substances are called ferrimagnetic and this property is called ferrimagnetism.
Examples : Fe₃O₄ (Magnetite, Ferrites - MgFe₂O₄, ZnFe₂O₄, Ferrimagnetism is observed when the magnetic domains are aligned in parallel and antiparallel directions in unequal numbers. Hence, net magnetic moment is never zero. These substances lose ferrimagnetism and become paramagnetic on heating.

More Important Questions For CBSE Class 12 Chemistry VBQs Solid State........

Question. Solids have fixed shape and volume but experimentally it has been observed that solids are also defective. Two types of defects are found in solids, namely, point defect and line defect.
After reading the above passage, answer the following questions-
(i) What type of defect can arise when a solid is heated?
(ii) How many types of point defects are there in ionic solids?
(iii) Mention the values associated with the above information.
Answer. (i) Vacancy defect.
(ii) 2 types – Schottky and Frenkel defect.
(iii) self-analysis and critical thinking or any related answer.

Question. Rajendra and Manoj went to market to buy study lamp. Shopkeeper showed them one electric rechargeable lamp and one solar lamp. Rajendra wanted to buy electric rechargeable lamp but Manoj advised him to buy the solar lamp.
(a) What is the application of solar cell?
(b) What is the role of semiconductors in solar cells?
Answer. (a) Solar cells convert solar energy into electrical energy. It helps us in saving nonrenewable sources of energy.
(b) The n-type semiconductors is used in making solar cells which stores solar energy in the cell.

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