Answers of Questions from Previous notes A] Multiple ...

XII SCIENCE WORK FROM HOME - 2020 CHEMISTRY
Date: 12/06/2020 Page 1 of 20
Answers of Questions from Previous notes
A] Multiple choice questions [1 Mark]
1. c) 32 % 2. a) 12 (in Q.B. this option is not given)
3. a) 3.011 x 1023,6.022 x 1023 4. d) 175 pm
5. a) 0.5 6. c) 8
7. a) 1:1 8. a) 1
9. b) 2 10.c) 1:1.3:1.4
11. c) ABCABC 12. b) 74%
13. a) r = √3
15. a) 8 16. c) 3.82 Ao
17. c) hcp 18. a) ccp & hcp
19. c) 552 pm 20. b)
6
1) 1:2 2) 0.5236
3) 74 % 4) 12
7) 7 8) Refer page No.6
9) Refer page No. 3 10) Refer page No.5
11) Ionic solid
C] Answer the following. [2M/3M]
1) Two properties that are common to both hcp and ccp lattices:-
i) The co-ordination number of any sphere in hcp or ccp structure is 12.
ii) The octahedral voids and tetrahedral voids occur in both.
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2) Distinguish between tetrahedral voids and octahedral voids.
Tetrahedral voids Octahedral voids
connected to form
connected to form octahedral
number of close packed
close packed spheres.
is ABAB type.
ABCABC type.
structure is formed.
3) Distinguish between Hexagonal close packing and square close packing.
Hexagonal close packing square close packing
1. The co-ordination number of
spheres is 6.
spheres is 4.
2. Void space is less. 2. Void space is more.
3. Spheres arrangement
each sphere of 2nd row are in
depression of 1st layer.
other.
5) a) scc packing fraction:-0.524, Packing efficiency: - 52.4 %
and void space: - 47.6 %
6) Refer page No. 6
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7) Refer page No.11
8) a) scc crystal: - Refer page No.6 b) bcc structure: - Refer page No.7
c) fcc structure: Refer page No.8
9) Refer page No. 6
D] Numerical problems:-
1) a) edge length of unit cell = r/0.3535 = 4.085 x10-8 cm
b) volume of Ag atom = 4
3 3 = 1.261 x 10-23 cm3
c) volume of a unit cell = a3
volume occupied by 4 Ag atoms =
percent volume = 74 % The percent of empty space = 26 %
2) Solution: - Number of tetrahedral voids = 2B
Number of A atoms = 2B x 1/3
Ratio of A and B atoms is 2/3 B:1B = 2/3:1=2:3
Formula of compound is A2B3
3) Number of atoms in ‘x’ g metal =
ρa3 = 1.618 x 1021
ρa3 = 8.09 x 1020
4) Number of atoms in 0.8 mol = 0.8 x NA
Number of octahedral voids = number of atoms
Number of tetrahedral voids = 2 x number of atoms
Total voids = 14.45 x 1023
5) =
ρ r =
*****
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Solid State
(Next Part)
§§ Crystal defects or imperfections: - The irregularities in the arrangement of
constituent particles of a solid crystal are called defects.
Defects are created during the process of crystallization if it occurs at faster rate.
Ideal crystals with no defects are possible only at the absolute zero temperature.
Electrical neutrality is independent of crystal defects.
Sometimes defects are to be intentionally created for manipulating the desired
properties in crystalline solid.
There are are three types of defects: - i) Point defect ii) Line defects
iii) Plain defects
§§ Point Defects: - These defects are irregularities produced in the arrangement
of basis at lattice points in crystalline solids.
There are major three classes of point defects :- a) Stoichiometric point defects
b) Impurity defects c) Non Stoichiometric point defects
§§ a) Stoichiometric point defects: - In Stoichiometric point defects, the
stoichiometry (the fixed ratio of atoms or number of cations and anions) remains
same as represented by its chemical formula.
There are four types of Stoichiometric point defects:-
i) Vacancy defects ii) Self-interstitial defects
iii) Schottky defect iv) Frenkel defect
§ i) Vacancy defect :- During crystallization of a solid a particle is missing from its
regular site in the crystal lattice and creates a vacancy in the lattice structure . The
crystal is said to have a vacancy defect. See fig 1.12
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It can also be developed when the substance is heated.
Due to vacancy defect, the mass of substance decreases however the volume
remains unchanged as a result density of the substance decreases.
§ ii) Self interstitial defect: - When some particles of a crystalline elemental solid
occupy interstitial sites (space or voids in between the particles) in the crystal
structure, it is called Self interstitial defect.
This effect occurs in the following two ways:
a) An extra particle occupies an empty interstitial space in the crystal structure.
See fig 1.13
The extra particles increase the total mass of substance without increasing
volume, hence its density increases.
b) An elemental solid particle gets shifted from its original lattice point and
occupies an interstitial space in the crystal. See fig 1.14
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In this because of the displacement of particle a vacancy defect is created.
This defect preserves the density of the substance because there is neither loss
nor gain in mass of a substance.
§ iii) Schottky defect: - When there exist two holes per ion pair lost, one created
by missing cation and the other by missing an anion . Such paired cation anion
vacancy defect is a Schottky defect. Fig.1.15
Conditions for the formation of Schottky defect: - It is found in ionic compounds.
a) Ionic compound should have high degree of ionic character.
b) Ionic compound should have high co-ordination number of anion.
c) Ionic compound should have small difference between size of cation and anion
i.e. ratio
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Consequences of Schottky defect:-
a) As the number of ions decreases, mass decreases however volume remains
unchanged. Hence density of a substance decreases.
b) The number of missing cation and anions is equal, the electrical neutrality of
the compound is preserved. This defect is observed in ionic crystals such as
NaCl , AgBr and KCl.
§ iv) Frenkel defect: - It arises when an ion of an ionic compound is missing from
its regular lattice site and occupies interstitial site between lattice points.
More commonly cations occupies interstitial site as being small in size than
anion and it is easier to accommodate the interstitial space. See fig. 1.16
Here also vacancy defect is created so it can be regarded as the combination of
vacancy defect and interstitial defect.
Conditions for the formation of Frenkel defect: - It is found in ionic compounds.
a) Ionic compound should have large difference between sizes of cation and
anion.
b) Ionic compound should have low co-ordination number of ions.
Consequences of Frenkel defect:-
a) As no ions are missing from the crystal lattice as a whole the density of a
substance and its chemical properties remain unchanged.
b) The number of cation and anions is equal, the crystal as a whole remains
electrically neutral. This defect is observed in ionic crystals such as ZnS, AgBr,
AgCl, AgI and CaF2.
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§§ b) Impurity Defect: - It arises when foreign atoms (atoms different from the host
atoms) are present in the crystal lattice.
There are two kinds of Impurity Defects:-
i) Substitutional Impurity Defect ii) Interstitial impurity defect
§ i) Substitutional Impurity Defect: - In this defect, the foreign atoms (impurity
atoms) are found at the lattice sites in place of host atom.
For example: - a) Solid solutions of metals (alloys):- In Brass alloy, impurity atom
is Zn and host atom is Cu. Fig 1.17
b) During the crystallization of NaCl, small amount of SrCl2 impurity is added. The
impurity Sr2+ ions (aliovalent ions that is ions with oxidation state different from
that of host ions) occupy some of the regular sites of Na+ host ions. Every ion Sr2+
removes two Na+ ions. Fig 1.18
§ ii) Interstitial impurity defect: - In this defect, the impurity atoms occupy
interstitial spaces of lattice structure.
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For example: - In steel carbon atoms are present at interstitial spaces .Fig 1.19
§§ C] Nonstoichiometric defects: - It arises when the ratio of number of atoms of
one kind to that of other kind or the ratio of number of cations to anions
becomes different from that of indicated by its chemical formula. In short,
stoichiometry of the compound is changed. There are two types of
Nonstoichiometric defects,
§ i) Metal deficiency defect: - This defect is possible only in compounds of metals
that show variable oxidation states.
In some crystals, positive metal ions are missing from their original lattice sites.
The extra negative is balanced by the presence of cation of the same metal with
higher oxidation state than that of missing cation.
For Example, in the compound NiO one Ni2+ ion is missing creating a vacancy at
its lattice site. The deficiency of two positive charges is made by presence of of
two Ni3+ ions at the other sites of Ni2+ ions. The composition of NiO then becomes
Ni0.97O1.0. See fig 1.20
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§ ii) Metal excess defect:-
a) A neutral atom occupies interstitial position: In ZnO lattice one neutral Zn atom
is present in the interstitial space. See fig 1.21 (a)
b) An extra positive ion occupies interstitial position: When ZnO is heated it
decomposes as,
ZnO Zn2+ + ½ O2 + 2e-
The excess Zn2+ ions trapped in the interstitial site in the lattice and electrons
also diffuse there. See fig 1.21 (b)
In both cases, nonstoichiometric formula of ZnO is Zn1+xO1.0.
c) By anion vacancies (F-centres or colour):- This defect imparts colour to the
colourless crystal. For example: - When NaCl crystals are heated in presence
of Na vapours, Na atoms are deposited on the crystal surface. Cl- ions diffuse
to the crystal surface creating vacancies at their regular sites. These Cl- ions
combine with Na atoms on the surface to form NaCl, by releasing electron from
sodium atom.
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These electrons occupy vacant sites of anions and called as F-centres or colour
centres. NaCl shows yellow colour due to the formation of F-centre.
The Nonstoichiometric formula of NaCl is the Na1+xCl1.0. See fig 1.22
§§ Electrical properties of solids:-
i) Conductors ii) Insulators iii) Semiconductors
§ i) Conductors :-Solids having electrical conductivities in the range 104 to 107 ohm-
1m-1 are called Conductors. e.g. Metals conduct electricity by movement of
electron and ionic solids (electrolytes) conduct electricity by movement of
ions.
§ ii) Insulators :- Solids having low electrical conductivities in the range 10-20 to 10-
10 ohm-1m-1 are called Insulators. e.g. Non-metals and Molecular solids.
§ iii) Semiconductors :- Solids having electrical conductivities in the range 10-6 to
104 ohm-1m-1 are called Semiconductors. e.g. Metalloids like Silicon,
Germanium.
metalloids are explained in terms of band theory.
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A band is made of closely spaced electronic energy levels. Band formation can be
correlated to formation of molecular orbitals (MO’s) by interaction of atomic
orbitals.
Band theory involves following concepts.
§ a) Conduction band: - The highest energy band formed by interaction of the
outermost energy levels of closely spaced atoms in solids is conduction band.
It may be partially filled or vacant. Electrons in conduction band are mobile
and delocalized over the entire solid. They conduct electricity when electrical
potential is applied.
§ b) Valence band: - The band having lower energy than conduction band is the
valence band is called valence band. The electrons in valence band are not
free to move because they are tightly bound to the respective nuclei.
§ c) Band gap:-The energy difference between valence band and conduction band
is called band gap.
When band gap is too large to promote electrons from valence band to vacant
conduction band by thermal energy, is called Forbidden zone.
When band gap is small, electrons from valence band can be promoted to
conduction band by absorption of energy such as thermal energy,
electromagnetic energy.
§§ Electrical property of Conductors on the basis of Band Theory: - The number of
electrons in conduction band of metals is large. Hence metals are good
Conductors of electricity. The conduction band and valence band in metals
overlap each other .See fig 1.23
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The band formation in metallic Conductors results in delocalization of the
outermost electrons leaving behind metal ions. It is described as ‘cations of metal are immersed in the sea of electrons.’ Conductivity of metals decreases with
increase in temperature: - The cations of metal atoms occupying lattice sites
vibrate about their mean positions. At high temperature, metal cation undergoes
increased vibrational motion. The flow of electrons is interrupted by increased
vibrational motion. So conductivity of metals decreases with increase in
temperature.
Metallic Sodium is Conductor of electricity:-
The electronic configuration of Na = [Ar] 3s1. Partially filled 3s AOs (Atomic
orbitals) interact to give rise same number of of MOs (Molecular orbitals). All
these closely spaced MOs form 3s continuous band i.e. conduction band. There is
no band gap. So it shows conductivity.
Metallic Magnesium is Conductor of electricity:-
The electronic configuration of Mg = [Ar] 3s23p0. Completely filled 3s AOs
(Atomic orbitals) interact to give rise same number of MOs (Molecular
orbitals).All these closely spaced MOs form 3s continuous band i.e. conduction
band. Interaction of vacant 3p AOs of all Mg atoms give rise same number of MOs
together called 3p band. The filled 3s band and vacant 3p band overlap each other.
As a result electrons move from 3s to 3p band and conduct electricity.
§§ Electrical property of Insulators on the basis of Band Theory:-
In Insulators valence band is completely filled with electrons and the conduction
band is empty. The conduction band and valence band in Insulators are separated
by a large energy gap called forbidden zone. Thermal energy is insufficient to
promote electrons from valence band to conduction band .As a result conduction
band remains vacant. The material is therefore an Insulator. See fig 1.24
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§§ Electrical property of Semiconductors on the basis of Band Theory:-
Electrical conductivity of Semiconductors is intermediate between metals and
Insulators. In Semiconductors valence band is completely filled with electrons
and the conduction band is empty. However the energy gap between two bands
is smaller than that in an Insulators. See fig 1.25
At a temperature above absolute zero few electrons in the valence band have
enough thermal energy to jump through the small band gap and occupy higher
energy conduction band. The electrons in conduction band are free to move.
When electric potential is applied to a Semiconductor, it conducts a small amount
of electricity. Such a pure Semiconductor material which has a very low but finite
electrical conductivity is called Intrinsic Semiconductors.
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The electrical conductivity of a Semiconductor increases with increasing
temperature because number of electrons get promoted conduction band with
thermal energy and vacancies are created in valence band.
It is said that Semiconductors are Insulators at low temperature and conductors at high temperature.
A doped Semiconductor, having higher conductivity than pure intrinsic
semiconductor is an Extrinsic Semiconductor.
The addition of minute quantity of impurities to a Semiconductor to increase its
conductivity is called doping .The added impurity is called Dopant.
There are two types of Extrinsic Semiconductor: - namely n-type semiconductor
and p-type Semiconductors
§ i) n-type semiconductor: - It can be obtained by adding group 15 element to
intrinsic semiconductor which belongs to group 14.
Example: - Doping of Si with Phosphorous. ‘P’ atoms occupy some vacant sites in
the lattice in place of Si atoms, See fig. 1.26
Four of the five electrons of P are utilized in bonding with four Si atoms. Since P
has an extra electron, in this doping more electrons are there in conduction band
than conduction band in pure Si. Thus conductivity of Si doped with P is higher
than that of Pure Si. The electrons in conduction band move under influence of an
applied potential and conduct electricity.
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The charge carriers are the increased number of electrons, it is said to be n-type
semiconductors.
e.g. Si or Ge doped with group 15 elements such as P, As, Sb (Antimony), Bismuth.
§ ii) p-type Semiconductor:- It is produced by doping a pure Semiconductor
material ( Si or Ge) with an impurity of group 13 elements which contain less
number of valence electrons than of the pure semiconductor. See fig. 1.28
Example: - Consider pure Si doped with Boron. B atom forms bonds with three Si
atoms only as it has only three valence electrons. The missing fourth electron
creates an electron vacancy, called as hole which behaves as positive charge. A
hole has tendency to accept electron from its close vicinity. Here charge carriers
are holes (positive charge) hence are known as p-type Semiconductor.
§§ Magnetic properties of solids: - In solids magnetic property is observed due to
unbalanced spin of unpaired electron in an orbital. When an electrons are paired
their spin is balanced and no magnetic property is observed. On the basis of
magnetic properties solids are classified as,
§ i) Diamagnetic solids: - The substances with all electrons paired are weakly
repelled by magnetic field. These substances are said to be Diamagnetic
substances. Pairing of electrons balances the spins and hence cancels their
magnetic moment.
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§ ii) Paramagnetic solids: - The substances with unpaired electrons are weakly
attracted by magnetic field. These substances are said to be Paramagnetic
substances. The spinning of unpaired electron gives rise to a magnetic moment
due to which sub is attracted in external magnetic field.
e.g. oxygen, Cu2+ , Fe3+ , Cr3+
§ iii) Ferromagnetic solids: - The substances containing large number of unpaired
Electrons are attracted strongly by magnetic field. These substances are said to
be ferromagnetic.
These substances can be permanently magnetised. They retain magnetism even
after the removal of external magnetic field.
e.g. Fe, Co, Ni, Gd, CrO2.
&
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Question Bank
1) Which of the following will be paramagnetic?
a) Zn2+ b) Na+ c) Cd d) K
2) When a paramagnetic substance is placed in the magnetic field, its mass is
apparently.....
3) Among the following substance which cannot be solidified is...
a) Hydrogen b) Helium c) Radon d) Nitrogen
4) Among the following Diamagnetic and Paramagnetic species are...
a) Ti3+ , Zn2+ b) Fe2+ , Fe3+ c) Sc3+ , Cr3+ d) Cd2+ , Hg2+
5) The substances which can be permanently magnetised are....
a) Diamagnetic b) Paramagnetic c) ferromagnetic d) Non-magnetic
6) Paramagnetism arises due to presence of..... electrons.
a) unpaired b) small number of c) paired d) large number of
7) p-type semiconductor is made by mixing Silicon with impurities of....
a) germanium b) Boron c) Arsenic d) Antimony
8) p-type Semiconductor is formed when trace amount of impurity is added to
silicon. The number of valence electrons in the impurity atom must be...
a) 1 b) 2 c) 3 d) 5
9) Semiconductors are manufactured by addition of impurities of...
a) actinoids b) Lanthanides
c) s-block elements d) p-block elements
10) In crystalline solid, few of the cations moved from their positions into the
interstitial position. The defect is called....
a) Line b) Schottky c) Frenkel d) Interstitial
11) Due to Frenkel defect the density of the ionic solid.......
a) increases b) decreases c) fluctuates d) remains same
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12) n-type Semiconductor is formed when trace amount of impurity is added to
silicon. The number of valence electrons in the impurity atom must be...
a) 4 b) 2 c) 3 d) 5
13) Semiconductor obtained by doping Germanium with Boron is of....
a) p-type b) n-type c) intrinsic d) non-conducting type
14) Semiconductor obtained by doping Si with elements of group 13 and 15
respectively is called...
a) p-type b) n-type c) n-type, p-type d) p-type, n-type
15) Schottky defects are observed in which solid among the following?
a) Brass b) ZnS c) Steel d) Cesium chloride
16) Electrical property of a metal is due to....
a) defect in crystal b) presence of unpaired electron
c) large atomic size d) presence of free electron
17) In Frenkel defect....
b) density of substance is changed
c) both cation and anions are missing
d) overall electrical neutrality is preserved.
18) Water exhibits ...... property.
19) Frenkel defect belongs to.... defect.
a) point b) Interstitial c) vacancy d) impurity
20) The type of defect in NaCl crystal will be...
a) point b) Interstitial c) vacancy d) impurity
B] Answer in one sentence. [1Mark]
1) Due to which defect density of a crystal is lowered?
2) Which type of defect is observed in AgCl?
3) By which defect alloys are formed?
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4) Which type of Semiconductor is formed by addition of arsenic to silicon?
5) Which type of magnetism is observed in CrO2?
6) Explain, ‘Iron (Z = 26) is strongly ferromagnetic’.
7) What is meant by Paramagnetic solids?
8) What is meant by Conduction band?
9) What are Intrinsic Semiconductors?
10) What are Schottky defects?
C] Answer the following .[2M/3M/4M]
1) Mention the magnetic properties of......
a) Benzene b) oxygen c) Cu+ d) Cu2+ e) Gadolinium
f) Cr3+ g) 11Na h) 12Mg i) 17Cl– j) 20Ca2+ k) 27Co
2) Distinguish metal conductors, Insulators, Semiconductors with the help of
diagrams.
4) How does electrical conductivity of a Semiconductor change with
temperature? Why?
5) What are Diamagnetic substances? Give examples.
6) What are n-type semiconductors? Why is the conductivity of doped n-type
semiconductor higher than that of pure Semiconductor? Explain with diagram.
7) What is impurity defect? What are its types? Explain the formation of vacancies
through aliovalent impurity with example.
8) The following picture represents bands of MO’s for Si. Label valence band,
conduction band and band gap.

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