Surface Chemistry
Intext, Exercise Questions and PYQs
MCQsClassify the following as amorphous or crystalline solids-
Polyurethane, naphthalene, benzoic acid, Teflon, potassium nitrate, cellophane, polyvinyl chloride, fiberglass, copper.
Answer: Amorphous solids: Polyurethane, teflon, cellophane, polyvinyl chloride, fibre glass.
Crystalline solids: Benzoic acid, potassium nitrate, copper.
Why do solids have a definite volume ?
Answer: In solids, the constituent particles are bonded together by strong attractive forces between them. On changing of pressure, the intermolecular space remains unaffected. Thus, the volume of solids is definite.
Refractive index of a solid is observed to have the same value along with all directions. Comment on the nature of this solid. Would it show cleavage property ?
Answer: Since refractive index of a solid is observed to have the same value along with all directions, the solid must be amorphous. On cutting amorphous solid it will undergo irregular cleavage.
Classify the following solids in different categories based on the nature of intermolecular forces operating in them: Potassium sulphate, tin, benzene, urea, ammonia, water, zinc sulphide, graphite, rubidium, argon, silicon carbide.
Answer:
Ionic Solids: Potassium sulphate, Zinc sulphide
Metallic Solids: Tin, Rubidium
Molecular Solids (non-polar): Benzene, Argon
Molecular Solids (polar): Urea
Molecular Solids (H-bonded): Ammonia, Water
Covalent or network Solids: Graphite, Silicon carbide
Solid A is a very hard electrical insulator in solid as well as in molten state and melts at extremely high temperature. What type of solid is it ?
Answer: Covalent solid.
Ionic solids conduct electricity in molten state but not in solid state. Explain.
Answer: In solid-state, the ions are fixed and the ions cannot move in an applied electric field. But in a molten state, the ions are free to move and hence conduct electricity.
What type of solids are electrical conductors, malleable and ductile ?
Answer: Metallic solids, they are conductor due to the presence of free electron in them.
Give the significance of a lattice point.
Answer: The lattice point represents the position of the constituent particles in the crystal lattice. When lattice points are joined by straight lines, they bring out the geometry of the lattice.
Name the parameters that characterize a unit cell.
Answer: A unit cell is characterized by-
The dimensions along the three edges. These are represented by a, b, and c. The angle between the edges. These are represented by α, β, and γ. The angle α is between b and c, β is between a and c and γ is between a and b.
Distinguish between
a. Hexagonal and monoclinic unit cells
b. Face centred and end centred unit cells
Answer:
a. Hexagonal unit cell:
a = b ≠ c and α = β = 90°, γ = 120°
Monoclinic unit cell:
a ≠ b ≠ c and α = γ = 90°, β ≠ 90°
b. Face-centred unit cell: Particles at the comers as well as at the centre of each face.
Number of particles per unit cell = 8 × 18 + 6 × 12 = 4
End – centred unit cell: Particles at the corners and at the centres of two end faces.
Number of particles per unit cell = 8 × 18 + 2 × 12 = 2
Explain, how many portions of an atom located at corner and body centre of a cubic unit cell is part of its neighbouring unit cell ?
Answer: In a crystal, each comer of a unit cell is common to 8 unit cells and hence each comer atom is shared by 8 unit cells. Thus, 1/8 atom is part of its neighbouring unit cell.
The atom located at the body centre of a cubic unit cell belongs to only one unit cell. Thus it is not shared with the neighbouring unit cell.
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:
The number of octahedral voids = N
The number of tetrahedral voids = 2N
∴ Total number of voids = 3N
The number of close packed atoms = 0.5 mol = 0.5 × 6.022 × 1023
∴ Total number of voids = 3 × 0.5 × 6.022 × 1023 = 9.03 × 1023
The number of tetrahedral voids = 2 × 0.5 × 6.022 × 1023 = 6.022 × 1023
A compound is formed by two elements M and N. The element N forms ccp and atoms of M occupy 1/3rd of tetrahedral voids. What is the formula of the compound ?
Answer:
Since N forms ccp arrangement so, it will have 4 atoms in a unit cell.
Number of N atoms in unit cell = 4
For each atom, there are two tetrahedral voids,
so, there are 8 tetrahedral voids per unit cell.
No. of M atoms = 1/3 × 8 = 8/3
Formula = M8/3 N4 or M2N3.
Which of the following lattices has the highest packing efficiency
a. simple cubic
b. body-centered cubic and
c. hexagonal close-packed lattice ?
Answer:
Hexagonal close-packed lattice has the highest packing efficiency.
The packing efficiencies are:
Simple cubic = 52.4%
Body centred cubic = 68%
Hexagonal close packed = 74%
What type of defect can arise when a solid is heated ? Which physical property is affected by it and in what way ?
Answer: Vacancy defect. It results in a decrease in the density of the substance since some of the lattice sites are occupied.
What type of stoichiometric defect is shown by-
a. ZnS b. AgBr.
Answer:
a. ZnS, shows Frenkel defect due to large difference in size of ions.
b. AgBr, shows both Frenkel defect and Schottky defect.
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 as an impurity, some of the sites of the original cations are occupied by the cations of higher valence and produce vacant sites.
Ionic solids, which have anionic vacancies due to metal excess defect, develop colour ? Explain with the help of a suitable example.
Answer: The anionic vacancies due to metal excess defect in ionic solids are occupied by free electrons to maintain electrical neutrality. When visible light strike on the crystals, they absorb energy and electrons gets excited and imparts colour. For example : When NaCl is heated in presence of sodium vapours, Na+ ions are in excess, Cl– ions leave their normal site and come to the surface. The vacant site of anion is occupied by electron forming F-centre. They absorb light from visible region and radiate complementary colour.
A group-14 element is to be converted into an n-type semiconductor by doping it with a suitable impurity. To which group should this impurity belong ?
Answer: Group 14 element can be converted to n-type semiconductor by doped with a Group 15 element.
What type of substances would make better permanent magnets, ferromagnetic or ferrimagnetic ? Justify your answer.
Answer: Ferromagnetic materials would make better permanent magnets than ferrimagnetic materials because, in ferromagnetic solids, the magnetic moments of unpaired electrons spontaneously align themselves in the same direction. However, in ferrimagnetic solids the magnetic moments of the domains are aligned in the parallel and anti-parallel direction in unequal numbers. See NCERT for Figure.
What makes a glass different from a solid such as quartz ? Under what conditions could quartz be converted into glass ?
Answer: Glass is an amorphous solid in which the constituent particles have only short-range order. On melting quartz and then cooling it rapidly, it is converted into glass.
A. The basis of similarities and differences between metallic and ionic crystals.
B. Ionic solids are hard and brittle.
Answer:
A. Ionic solids: In ionic solids, the constituent particles are positive and negative ions. The positive ions are surrounded by negative ions and vice versa. The ions are held together by a strong electrostatic force of attraction.
Since the ions are firmly fixed in the crystal lattice, they are hard and brittle with high melting point, high heat of fusion and lattice energy. These are insulators in the solid-state but are good conductors in solution or molten state. e.g. NaCl etc.
b. Molecular solids: Molecular solids are made up of small discrete molecules. They are of two types-
i. Non-polar molecular solids: In these solids, the non-polar molecules like H2, Cl2, O2 etc are held together by weak dispersion forces or London forces. They are soft, non-conductors of electricity and have low melting points.
ii. Polar molecular solids: Molecules having polar covalent bonds are held together by dipole-dipole interactions. Solid SO2, NH3 etc. are examples of this type. They are soft and non-conductors of electricity.
iii. Hydrogen bonded molecular solids: The molecules are held together by strong hydrogen bonds.e.g. Ice.
B. Ionic solids are brittle because when they are subjected to distortion, the ions of the same charge are brought close together and the repulsive forces between them cause the crystal to cracks.
In terms of band theory, what is the difference
a. between a conductor and an insulator ?
b. between a conductor and a semiconductor ?
Answer:
a. The energy gap between the valence band and the conduction band in an insulator is very large whereas in a conductor the energy gap is either very small or there is overlapping between the valence band and conduction band.
b. In a conductor, the energy gap between the valence band and conduction band is very small or there is overlapping between valence band and conduction band. But in a semiconductor, there is always a small energy gap between them.
Why is glass considered as super cooled liquid ?
Answer: Glass is an amorphous solid. Like liquids it has tendency to flow but very slowly. Generally, the glass panes in the windows or doors of old buildings are invariably found to be slightly thicker at the bottom than that at the top after very long time.
Ionic solids conduct electricity in molten state but not in solid state. Explain.
Answer: In the molten state, ionic solids dissociate to give free ions and hence can conduct electricity. However in the solid state, as the ions are not free but remain held together by strong electrostatic forces of attraction, they cannot conduct electricity in the solid state.
With the increase in temperature, electrical conductivity of semiconductors increases. Why ?
Answer: The energy gap between valence band and conduction band is less. So, when increasing the temperature, some electrons from the valence band jump to the conduction band due to which electrical conductivity increases with the increase in temperature.
Write Bragg equation.
Answer:
Bragg equation is- nλ = 2d sin θ
Where, d = Distance between two consecutive planes in a crystal
θ = Incident angle of X-rays
n = Simple whole number
λ = Wavelength of X-rays
By this distance d between the planes of the crystal is determined.
Name the parameters that characterise a unit cell.
Answer: A unit cell is characterised by the following six parameters a, b, c, α, β and γ.
Its dimensions along the three edges a, b and c. These edges may or may not be mutually perpendicular
Angles between the edges, α, (between b and c), β (between a and c) and γ (between a and b).
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 defect is produced in the crystal because on heating, some atoms or ions leave the lattice site completely, some lattice sites become vacant. As a result of this defect the density of the substance decreases as some atoms ions leave the crystal completely.
Name the type of crystal defect which is produced when NaCl crystal is doped with MgCl2
Answer: Cation vacancy defect or Non-stoichiometric defect or impurity defects.
Which point defect in crystals does not alter the density of the relevant solid
Answer: Frenkel defect
Which point defect in its crystal units alters the density of a solid
Answer: Schottky defect
Which point defect in its crystal units increases the density of a solid
Answer: Metal excess defect increases the density of a solid due to presence of extra cations in the interstitial sites
Give the difference between crystalline solid and amorphous solid.
Answer: Differences between Crystalline solid and Amorphous solid are given below-
Crystalline solid:
a. Structure of these solid are of definite geometrical shape.
b. In the internal structure, particle are arranged systematically.
c. Crystalline solids are solids in real sense.
d. Melting points of these compounds are sharp and definite.
e. These solids shows anisotropy property.
f. Cooling curve are not continuous.
g. Crystalline solids possess less energy.
Amorphous solid:
a. No any definite geometrical shape in the structure.
b. No any definite arrangement of particles in the internal structure.
c. Amorphous solids are supercooled liquids.
d. Melting point of these compounds are not definite and sharp.
e. These solids shows isotropy property.
f. Cooling curves are continuous.
g. Amorphous solids possess higher energy.
What is the number of atoms in a unit cell of a face-centred cubic crystal
Answer: The number of atoms in a unit cell of fcc-crystal is 4 atoms
Crystalline solids are anisotropic in nature. What does this statement mean ?
Answer: It means that crystalline solids show different values of their some properties like electrical conductivity, refractive index, thermal expansion etc. in different directions
What type of stoichmetric defect is shown by AgBr and Agl ?
Answer: AgBr shows both Frenkel defect and Schottky defect whereas Agl shows Frenkel defect.
Number of atoms per unit cell in Simple Cubic, BCC and FCC crystal
Answer: Simple Cubic: 1, BCC: 2, FCC: 4
Which type of ionic substances show Schottky defect in solids
Answer: Highly ionic compounds with high coordination number and small difference in size of cations and anions show schottky defect
What is the formula of a compound in which the element Y forms ccp lattice and atoms of X occupy 1/3rd of tetrahedral voids
Answer: Formula is X2Y3
Coordination number of Ca+2 and S-2 ions in CaS crystal
Answer: CaS is a rock salt type crystal so Coordination number of both the Ca+2 and S-2 ions are 6
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: No. of atoms in the HCP = 0.5 × 6.022 × 1023 = 3.011 × 1023
No. of octahedral voids = No. of atoms in packing = 3.011 × 1023
No. of tetrahedral voids = 2 × No. of atoms in packing = 2 × 3.011 × 1023 = 6.022 × 1023
So, Total no. of voids = 3.011 × 1023 + 6.022 × 1023 = 9.033 × 1023
What is the formula of a compound in which the element Y forms ccp lattice and atoms of X occupy 2/3rd of tetrahedral voids
Answer: Y forms ccp so, Y = 8 X (1/8) = 1
X occupies 2/3rd of the tetrahedral voids
so, no. of X = 2 X (2/3) = 4/3
So, Ratio = X : Y
4/3 : 1
so, formula is X4Y3
How will you distinguish between the following pairs of terms-
a. Tetrahedral and octahedral voids
b. Crystal lattice and unit cell
Answer:
a. Tetrahedral void: The void between three spheres in direct contact and capped by fourth sphere is called tetrahedral void. Its coordination number is 4.
Octahedral void: The void between three spheres in direct contact capped by three more spheres is called octahedral void. Its coordination number is 6.
b. Crystal lattice: It is geometrical arrangement of the atoms, ions or molecules of the crystalline solid as points in space.
Unit cell: It is a portion of crystal lattice which on repetition gives crystal lattice.
What change occurs when AgCl is doped with CdCl2
Answer: Its conductance increases due to movement of Ag+ ions.
Which stoichiometric defect in crystals increases the density of a solid ?
Answer: Interstitial defect increases the density of a solid.
Crystalline solids are anisotropic in nature.’ What does this statement mean ?
Answer: It means crystalline substances show different optical and electrical properties in different directions, i.e. anisotropic in nature.
In terms of band theory, what is the difference
a. between a conductor and a semiconductor ?
b. between a conductor and an insulator ?
Answer:
a. In a semiconductor, there is small energy gap between valence band and conduction band, whereas in a conductor, either there is partially filled valence band or there is overlapping between valence band and conduction band.
b. In a conductor, there is overlapping between conduction band and valence band. In an insulator, there is a large energy gap between conduction band and valence band.
>What type of interactions hold the molecules together in a polar molecular solid ?
Answer: Dipole-dipole interactions, dispersion forces and van der Waals’ forces of attraction hold the molecules together in a polar molecular solid.
What type of semiconductor is obtained when silicon is doped with arsenic ?
Answer: n-type semiconductor is obtained when silicon is doped with arsenic.
What is the number of atoms in a unit cell of a simple cubic crystal ?
Answer: The number of atoms in a unit cell of a simple cubic crystal is one (8 ☓ 1/8 = 1).
What is meant by an intrinsic semiconductor ?
Answer: The semiconductor which is used in pure form is called intrinsic semiconductor.
What change occurs when AgCl is doped with CdCl2
Answer: p-type semiconductor.
What type of defect can arise when a solid is heated ?
Answer: When a solid is heated, it can have vacancy defect (Schottky defect).
What is meant by ‘doping’ in a semiconductor ?
Answer: The introduction of a small amount of impurity in a semiconductor is called doping.
How are the following properties of crystals affected by Schottky and Frenkel defects ?
a. Density
b. Electrical conductivity
Answer:
a. Density remains the same in Frenkel defect, whereas Schottky defect leads to lowering of density. b. Electrical conductivity increases in both Frenkel and Schottky defects.
Write a point of distinction between a metallic solid and an ionic solid other than metallic lustre.
Answer: Metallic solids can conduct electricity due to movement of electrons in solid state, whereas ionic solids conduct electricity in molten state or in aqueous solution only due to the movement of ions.
How may the conductivity of an intrinsic semiconductor be increased ?
Answer: It can be increased by adding appropriate amount of suitable impurity. This process is called doping.
What is meant by the term ‘forbidden zone’ in reference to band theory of solids ?
Answer: The gap between valence band and conduction band in the case of insulator is very large. This zone is called forbidden zone.
Polyurethane, naphthalene, benzoic acid, Teflon, potassium nitrate, cellophane, polyvinyl chloride, fiberglass, copper.
Answer: Amorphous solids: Polyurethane, teflon, cellophane, polyvinyl chloride, fibre glass.
Crystalline solids: Benzoic acid, potassium nitrate, copper.
Why do solids have a definite volume ?
Answer: In solids, the constituent particles are bonded together by strong attractive forces between them. On changing of pressure, the intermolecular space remains unaffected. Thus, the volume of solids is definite.
Refractive index of a solid is observed to have the same value along with all directions. Comment on the nature of this solid. Would it show cleavage property ?
Answer: Since refractive index of a solid is observed to have the same value along with all directions, the solid must be amorphous. On cutting amorphous solid it will undergo irregular cleavage.
Classify the following solids in different categories based on the nature of intermolecular forces operating in them: Potassium sulphate, tin, benzene, urea, ammonia, water, zinc sulphide, graphite, rubidium, argon, silicon carbide.
Answer:
Ionic Solids: Potassium sulphate, Zinc sulphide
Metallic Solids: Tin, Rubidium
Molecular Solids (non-polar): Benzene, Argon
Molecular Solids (polar): Urea
Molecular Solids (H-bonded): Ammonia, Water
Covalent or network Solids: Graphite, Silicon carbide
Solid A is a very hard electrical insulator in solid as well as in molten state and melts at extremely high temperature. What type of solid is it ?
Answer: Covalent solid.
Ionic solids conduct electricity in molten state but not in solid state. Explain.
Answer: In solid-state, the ions are fixed and the ions cannot move in an applied electric field. But in a molten state, the ions are free to move and hence conduct electricity.
What type of solids are electrical conductors, malleable and ductile ?
Answer: Metallic solids, they are conductor due to the presence of free electron in them.
Give the significance of a lattice point.
Answer: The lattice point represents the position of the constituent particles in the crystal lattice. When lattice points are joined by straight lines, they bring out the geometry of the lattice.
Name the parameters that characterize a unit cell.
Answer: A unit cell is characterized by-
The dimensions along the three edges. These are represented by a, b, and c. The angle between the edges. These are represented by α, β, and γ. The angle α is between b and c, β is between a and c and γ is between a and b.
Distinguish between
a. Hexagonal and monoclinic unit cells
b. Face centred and end centred unit cells
Answer:
a. Hexagonal unit cell:
a = b ≠ c and α = β = 90°, γ = 120°
Monoclinic unit cell:
a ≠ b ≠ c and α = γ = 90°, β ≠ 90°
b. Face-centred unit cell: Particles at the comers as well as at the centre of each face.
Number of particles per unit cell = 8 × 18 + 6 × 12 = 4
End – centred unit cell: Particles at the corners and at the centres of two end faces.
Number of particles per unit cell = 8 × 18 + 2 × 12 = 2
Explain, how many portions of an atom located at corner and body centre of a cubic unit cell is part of its neighbouring unit cell ?
Answer: In a crystal, each comer of a unit cell is common to 8 unit cells and hence each comer atom is shared by 8 unit cells. Thus, 1/8 atom is part of its neighbouring unit cell.
The atom located at the body centre of a cubic unit cell belongs to only one unit cell. Thus it is not shared with the neighbouring unit cell.
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:
The number of octahedral voids = N
The number of tetrahedral voids = 2N
∴ Total number of voids = 3N
The number of close packed atoms = 0.5 mol = 0.5 × 6.022 × 1023
∴ Total number of voids = 3 × 0.5 × 6.022 × 1023 = 9.03 × 1023
The number of tetrahedral voids = 2 × 0.5 × 6.022 × 1023 = 6.022 × 1023
A compound is formed by two elements M and N. The element N forms ccp and atoms of M occupy 1/3rd of tetrahedral voids. What is the formula of the compound ?
Answer:
Since N forms ccp arrangement so, it will have 4 atoms in a unit cell.
Number of N atoms in unit cell = 4
For each atom, there are two tetrahedral voids,
so, there are 8 tetrahedral voids per unit cell.
No. of M atoms = 1/3 × 8 = 8/3
Formula = M8/3 N4 or M2N3.
Which of the following lattices has the highest packing efficiency
a. simple cubic
b. body-centered cubic and
c. hexagonal close-packed lattice ?
Answer:
Hexagonal close-packed lattice has the highest packing efficiency.
The packing efficiencies are:
Simple cubic = 52.4%
Body centred cubic = 68%
Hexagonal close packed = 74%
What type of defect can arise when a solid is heated ? Which physical property is affected by it and in what way ?
Answer: Vacancy defect. It results in a decrease in the density of the substance since some of the lattice sites are occupied.
What type of stoichiometric defect is shown by-
a. ZnS b. AgBr.
Answer:
a. ZnS, shows Frenkel defect due to large difference in size of ions.
b. AgBr, shows both Frenkel defect and Schottky defect.
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 as an impurity, some of the sites of the original cations are occupied by the cations of higher valence and produce vacant sites.
Ionic solids, which have anionic vacancies due to metal excess defect, develop colour ? Explain with the help of a suitable example.
Answer: The anionic vacancies due to metal excess defect in ionic solids are occupied by free electrons to maintain electrical neutrality. When visible light strike on the crystals, they absorb energy and electrons gets excited and imparts colour. For example : When NaCl is heated in presence of sodium vapours, Na+ ions are in excess, Cl– ions leave their normal site and come to the surface. The vacant site of anion is occupied by electron forming F-centre. They absorb light from visible region and radiate complementary colour.
A group-14 element is to be converted into an n-type semiconductor by doping it with a suitable impurity. To which group should this impurity belong ?
Answer: Group 14 element can be converted to n-type semiconductor by doped with a Group 15 element.
What type of substances would make better permanent magnets, ferromagnetic or ferrimagnetic ? Justify your answer.
Answer: Ferromagnetic materials would make better permanent magnets than ferrimagnetic materials because, in ferromagnetic solids, the magnetic moments of unpaired electrons spontaneously align themselves in the same direction. However, in ferrimagnetic solids the magnetic moments of the domains are aligned in the parallel and anti-parallel direction in unequal numbers. See NCERT for Figure.
What makes a glass different from a solid such as quartz ? Under what conditions could quartz be converted into glass ?
Answer: Glass is an amorphous solid in which the constituent particles have only short-range order. On melting quartz and then cooling it rapidly, it is converted into glass.
A. The basis of similarities and differences between metallic and ionic crystals.
B. Ionic solids are hard and brittle.
Answer:
A. Ionic solids: In ionic solids, the constituent particles are positive and negative ions. The positive ions are surrounded by negative ions and vice versa. The ions are held together by a strong electrostatic force of attraction.
Since the ions are firmly fixed in the crystal lattice, they are hard and brittle with high melting point, high heat of fusion and lattice energy. These are insulators in the solid-state but are good conductors in solution or molten state. e.g. NaCl etc.
b. Molecular solids: Molecular solids are made up of small discrete molecules. They are of two types-
i. Non-polar molecular solids: In these solids, the non-polar molecules like H2, Cl2, O2 etc are held together by weak dispersion forces or London forces. They are soft, non-conductors of electricity and have low melting points.
ii. Polar molecular solids: Molecules having polar covalent bonds are held together by dipole-dipole interactions. Solid SO2, NH3 etc. are examples of this type. They are soft and non-conductors of electricity.
iii. Hydrogen bonded molecular solids: The molecules are held together by strong hydrogen bonds.e.g. Ice.
B. Ionic solids are brittle because when they are subjected to distortion, the ions of the same charge are brought close together and the repulsive forces between them cause the crystal to cracks.
In terms of band theory, what is the difference
a. between a conductor and an insulator ?
b. between a conductor and a semiconductor ?
Answer:
a. The energy gap between the valence band and the conduction band in an insulator is very large whereas in a conductor the energy gap is either very small or there is overlapping between the valence band and conduction band.
b. In a conductor, the energy gap between the valence band and conduction band is very small or there is overlapping between valence band and conduction band. But in a semiconductor, there is always a small energy gap between them.
Why is glass considered as super cooled liquid ?
Answer: Glass is an amorphous solid. Like liquids it has tendency to flow but very slowly. Generally, the glass panes in the windows or doors of old buildings are invariably found to be slightly thicker at the bottom than that at the top after very long time.
Ionic solids conduct electricity in molten state but not in solid state. Explain.
Answer: In the molten state, ionic solids dissociate to give free ions and hence can conduct electricity. However in the solid state, as the ions are not free but remain held together by strong electrostatic forces of attraction, they cannot conduct electricity in the solid state.
With the increase in temperature, electrical conductivity of semiconductors increases. Why ?
Answer: The energy gap between valence band and conduction band is less. So, when increasing the temperature, some electrons from the valence band jump to the conduction band due to which electrical conductivity increases with the increase in temperature.
Write Bragg equation.
Answer:
Bragg equation is- nλ = 2d sin θ
Where, d = Distance between two consecutive planes in a crystal
θ = Incident angle of X-rays
n = Simple whole number
λ = Wavelength of X-rays
By this distance d between the planes of the crystal is determined.
Name the parameters that characterise a unit cell.
Answer: A unit cell is characterised by the following six parameters a, b, c, α, β and γ.
Its dimensions along the three edges a, b and c. These edges may or may not be mutually perpendicular
Angles between the edges, α, (between b and c), β (between a and c) and γ (between a and b).
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 defect is produced in the crystal because on heating, some atoms or ions leave the lattice site completely, some lattice sites become vacant. As a result of this defect the density of the substance decreases as some atoms ions leave the crystal completely.
Name the type of crystal defect which is produced when NaCl crystal is doped with MgCl2
Answer: Cation vacancy defect or Non-stoichiometric defect or impurity defects.
Which point defect in crystals does not alter the density of the relevant solid
Answer: Frenkel defect
Which point defect in its crystal units alters the density of a solid
Answer: Schottky defect
Which point defect in its crystal units increases the density of a solid
Answer: Metal excess defect increases the density of a solid due to presence of extra cations in the interstitial sites
Give the difference between crystalline solid and amorphous solid.
Answer: Differences between Crystalline solid and Amorphous solid are given below-
Crystalline solid:
a. Structure of these solid are of definite geometrical shape.
b. In the internal structure, particle are arranged systematically.
c. Crystalline solids are solids in real sense.
d. Melting points of these compounds are sharp and definite.
e. These solids shows anisotropy property.
f. Cooling curve are not continuous.
g. Crystalline solids possess less energy.
Amorphous solid:
a. No any definite geometrical shape in the structure.
b. No any definite arrangement of particles in the internal structure.
c. Amorphous solids are supercooled liquids.
d. Melting point of these compounds are not definite and sharp.
e. These solids shows isotropy property.
f. Cooling curves are continuous.
g. Amorphous solids possess higher energy.
What is the number of atoms in a unit cell of a face-centred cubic crystal
Answer: The number of atoms in a unit cell of fcc-crystal is 4 atoms
Crystalline solids are anisotropic in nature. What does this statement mean ?
Answer: It means that crystalline solids show different values of their some properties like electrical conductivity, refractive index, thermal expansion etc. in different directions
What type of stoichmetric defect is shown by AgBr and Agl ?
Answer: AgBr shows both Frenkel defect and Schottky defect whereas Agl shows Frenkel defect.
Number of atoms per unit cell in Simple Cubic, BCC and FCC crystal
Answer: Simple Cubic: 1, BCC: 2, FCC: 4
Which type of ionic substances show Schottky defect in solids
Answer: Highly ionic compounds with high coordination number and small difference in size of cations and anions show schottky defect
What is the formula of a compound in which the element Y forms ccp lattice and atoms of X occupy 1/3rd of tetrahedral voids
Answer: Formula is X2Y3
Coordination number of Ca+2 and S-2 ions in CaS crystal
Answer: CaS is a rock salt type crystal so Coordination number of both the Ca+2 and S-2 ions are 6
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: No. of atoms in the HCP = 0.5 × 6.022 × 1023 = 3.011 × 1023
No. of octahedral voids = No. of atoms in packing = 3.011 × 1023
No. of tetrahedral voids = 2 × No. of atoms in packing = 2 × 3.011 × 1023 = 6.022 × 1023
So, Total no. of voids = 3.011 × 1023 + 6.022 × 1023 = 9.033 × 1023
What is the formula of a compound in which the element Y forms ccp lattice and atoms of X occupy 2/3rd of tetrahedral voids
Answer: Y forms ccp so, Y = 8 X (1/8) = 1
X occupies 2/3rd of the tetrahedral voids
so, no. of X = 2 X (2/3) = 4/3
So, Ratio = X : Y
4/3 : 1
so, formula is X4Y3
How will you distinguish between the following pairs of terms-
a. Tetrahedral and octahedral voids
b. Crystal lattice and unit cell
Answer:
a. Tetrahedral void: The void between three spheres in direct contact and capped by fourth sphere is called tetrahedral void. Its coordination number is 4.
Octahedral void: The void between three spheres in direct contact capped by three more spheres is called octahedral void. Its coordination number is 6.
b. Crystal lattice: It is geometrical arrangement of the atoms, ions or molecules of the crystalline solid as points in space.
Unit cell: It is a portion of crystal lattice which on repetition gives crystal lattice.
What change occurs when AgCl is doped with CdCl2
Answer: Its conductance increases due to movement of Ag+ ions.
Which stoichiometric defect in crystals increases the density of a solid ?
Answer: Interstitial defect increases the density of a solid.
Crystalline solids are anisotropic in nature.’ What does this statement mean ?
Answer: It means crystalline substances show different optical and electrical properties in different directions, i.e. anisotropic in nature.
In terms of band theory, what is the difference
a. between a conductor and a semiconductor ?
b. between a conductor and an insulator ?
Answer:
a. In a semiconductor, there is small energy gap between valence band and conduction band, whereas in a conductor, either there is partially filled valence band or there is overlapping between valence band and conduction band.
b. In a conductor, there is overlapping between conduction band and valence band. In an insulator, there is a large energy gap between conduction band and valence band.
>What type of interactions hold the molecules together in a polar molecular solid ?
Answer: Dipole-dipole interactions, dispersion forces and van der Waals’ forces of attraction hold the molecules together in a polar molecular solid.
What type of semiconductor is obtained when silicon is doped with arsenic ?
Answer: n-type semiconductor is obtained when silicon is doped with arsenic.
What is the number of atoms in a unit cell of a simple cubic crystal ?
Answer: The number of atoms in a unit cell of a simple cubic crystal is one (8 ☓ 1/8 = 1).
What is meant by an intrinsic semiconductor ?
Answer: The semiconductor which is used in pure form is called intrinsic semiconductor.
What change occurs when AgCl is doped with CdCl2
Answer: p-type semiconductor.
What type of defect can arise when a solid is heated ?
Answer: When a solid is heated, it can have vacancy defect (Schottky defect).
What is meant by ‘doping’ in a semiconductor ?
Answer: The introduction of a small amount of impurity in a semiconductor is called doping.
How are the following properties of crystals affected by Schottky and Frenkel defects ?
a. Density
b. Electrical conductivity
Answer:
a. Density remains the same in Frenkel defect, whereas Schottky defect leads to lowering of density. b. Electrical conductivity increases in both Frenkel and Schottky defects.
Write a point of distinction between a metallic solid and an ionic solid other than metallic lustre.
Answer: Metallic solids can conduct electricity due to movement of electrons in solid state, whereas ionic solids conduct electricity in molten state or in aqueous solution only due to the movement of ions.
How may the conductivity of an intrinsic semiconductor be increased ?
Answer: It can be increased by adding appropriate amount of suitable impurity. This process is called doping.
What is meant by the term ‘forbidden zone’ in reference to band theory of solids ?
Answer: The gap between valence band and conduction band in the case of insulator is very large. This zone is called forbidden zone.
