Case study based questions from Solid state

 1. Read the passage given below and answer the following questions: 5 × 1 = 5 marks

  The adjective “Crystalline” when applied to solids, implies an ideal crystal in which the structural units, termed as unit cells, are repeated regularly and indefinitely in three dimensions in space. The unit cell, containing at least one molecule, has a definite orientation and shape defined by the translational vectors a, b and c. The unit cell therefore has a definite volume, V that contains the atoms and molecules necessary for generating the crystal. Every crystal can be classified as a member of one of the seven possible crystal systems or crystal classes that are defined by the relationship between the individual dimensions a, b and c of the unit cell and between the individual angles, α, β and γ of the unit cell. The structure of the given crystal may be assigned to one of the 7 crystal systems, to one of the 14 Bravais lattices. Based on these observations, seven crystal systems were identified: triclinic, monoclinic, trigonal or rhombohedral, tetragonal, hexagonal, rhombic or orthorhombic and cubic.

The following questions are multiple choice questions. Choose the most appropriate answer:

(i)The crystal system of a compound with unit cell dimensions, 

a = 0.387 nm, b = 0.387 nm and c = 0.504 nm 

and α = β = 90⁰ and γ = 120⁰ is

(a)    Cubic                                

(b)     hexagonal                                     

(c)     orthorhombic                 

(d)    rhombohedral

Ans- b

(ii)In a triclinic crystal

(a)    a = b = c, α = β = γ = 90⁰                                                          

(b)     a ≠ b = c, α = β = γ = 90⁰

(c) a ≠ b ≠ c, α ≠ β ≠ γ ≠ 90⁰                                                           

(d) a ≠ b ≠ c, α = γ = 90⁰, β≠ 90⁰    

Ans- c

 (iii) The unit cell with dimensional  a = b ≠ c, α = β = γ = 90⁰  is

(a)    Cubic                                

(b)    triclinic                             

(c)      hexagonal                                     

(d)    tetragonal

Ans- d

(iv). An example of orthorhombic crystal system is

(a)    SnO₂                   

(b)    KNO₃    

(c)     ZnO                    

(d)     K₂Cr₂O₇

Ans- b

(v) The number of atoms per unit cell of bcc structure is

 (a) 1                    

 (b) 2                         

 (c) 4                  

 (d) 6

Ans-b

2. Read the passage given below and answer the following questions:5 × 1 = 5 marks.

A Schottky defect is an excitation of the site occupations in a crystal lattice leading to point defects named after Walter H. Schottky. In elemental crystals it refers to simple lattice vacancies, also called valency defects. In ionic crystals, this defect forms when oppositely charged ions leave their lattice sites and become incorporated for instance at the surface, creating oppositely charged vacancies. These vacancies are formed in stoichiometric units, to maintain an overall neutral charge in the ionic solid.

A Frenkel defect is a type of point defect in crystalline solids, named after its discoverer Yakov Frenkel. The defect forms when an atom or smaller ion (usually cation) leaves its place in the lattice, creating a vacancy and becomes an interstitial by lodging in a nearby location.[2] In elemental systems, they are primarily generated during particle irradiation, as their formation enthalpy is typically much higher than for other point defects, such as vacancies, and thus their equilibrium concentration according to the Boltzmann distribution is below the detection limit. In ionic crystals, which usually possess low coordination number or a considerable disparity in the sizes of the ions, this defect can be generated also spontaneously, where the smaller ion (usually the cation) is dislocated

(i) Schottky defect in crystals is observed when

  (a) an ion leaves its normal site and occupies an interstitial site

  (b) unequal number of cations and anions are missing from the lattice

  (c) density of the crystal increases

  (d) equal number of cations and anions are missing from the lattice

Ans- d

(ii) Schottky defect generally appears in :

 (a) NaCl       

 (b) KCl   

 (c) CsCl  

 (d) all of these

Ans-d

(iii).  Which of the following crystals does not exhibit Frenkel defect?

(a) AgBr         

(b) AgCl             

(c) KBr               

(d) ZnS

Ans- c

(iv). When electrons are trapped into the crystal in anion vacancy, the defect is known as :

  (a) Schottky defect

  (b) Frenkel defect

  (c) Stoichiometric defect

  (d) F-centre

Ans-d

(v) The appearance of colour in solid alkali metal halides is generally due to

 (a) Schottky defect

(b) Frenkel defect

(c) Interstitial positions

 (d) F-centre

Ans-d

3.Read the passage and answer the questions given. 

Solid is that form of matter which possess rigidity and hence a definite shape and a definite volume. If intermolecular forces are greater than thermal energy, substance exists as solid. Solids can be classified into two types. Crystalline solids and amorphous solids. Crystalline solids have regular arrangement of particles, definite geometrical shapes, sharp melting points and definite heat of fusion. On the other hand, amorphous solids have no regular arrangement of particles, irregular shapes, melt over a range of temperature and no definite heat of fusion. They are isotropic and under go irregular cleavage on cutting. Ionic solids, Molecular solids, Network covalent solids and metallic solids are other types of metallic solids.

 i). A Solid is very hard, electrical insulator in solid as well as molten states and melts at high temperature. What type of solid is it?

a. Ionic

Network Covalent

Metalic

Molecular