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Solid State Chemistry Class 12 Important Questions with Answers Free pdf
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1. Classify the following solids based on intermolecular forces. sodium chloride, urea, glucose, zinc, ice, Diamond, Gold, Potassium sulphate, silicon carbide.
Ans. Molecular solid urea, glucose, ice Ionic solid sodium chloride, potassium sulphate Covalent solid Diamond, silicon carbide Metallic solid – Gold, zinc.
2. Classify the solids on the basis of intermolecular force of attraction.
Ans. On the basis of intermolecular force of attraction the solids are classified as- ionic solid, covalent solid, molecular solid and metallic solid. In ionic solid interparticle forces are ionic bonds (NaCl).
In covalent solid interparticle forces are covalent bonds (diamond).
In molecular solid interparticle forces are Van Der Waals force (iodine) or hydrogen bond (ice).
In metallic solid interparticle forces are metallic bonds (all metals).
3 What is the difference between crystalline solid and amorphous solid ?
– Have sharp melting point
-These are anisotropic. The physical properties, i.e,
refractive index, conductivity etc are different in different direction
-These are called true solids and give a regular cut when cut with a knife. e.g. rocksalt, sugar, etc.
– The particles are not arranged in a regular order. Have no sharp melting point.
– These are isotropic. The physical properties are the same in all directions.
– These are supercooled Liquids or pseudo solids, and give irregular cuts.
e.g. glass, plastic, rubber etc.
4. Explain why ionic solids do not conduct electricity in solid state but in molten state,
Ans. In solid state the oppositely charged ions are held together strongly by ionic bonds. So ions are not free to move. Hence do not conduct electricity in solid state. But in molten state the 1ons are free to move and become good conductors of electricity.
5. A solid has a cubic structure in which X atoms are located at corners of the cube, Y atoms are at the centres and 0′ atoms are at the edge centres. Find the formula of the compound.
Ans. Since there are 8 corners and each corner atom shares with 8 unit cells, so the number of atoms of ‘X
per unit cell = 1/8 x8=1
Y atoms are present at the centre of the unit cell. So
number of Y atoms per unit cell = 1
There are 12 edges and each ‘0’ atom at the edge
shares with four unit cells.
So the number of ‘O’ atoms per unit cell = 1/4 x12=3.
Now formula of the compound is XYO,
5. Lithium crystallizes in the bcc lattice. Find the number of unit cells in 14g of lithium.
Ans. In bcc crystal number of “Li’ atoms per unit
cell =8x 1/8 +1 =2
Number of atoms in 14g ‘Li’ = 14/7 x N, =2N⁰
So number of unit cells in 14g L1
=2N⁰/2 = N⁰ = 6.023 x 1023
6. Distinguish between cubic and orthorhombic unit cells.
Ans. In cubic unit cell, all sides are of same length
and the angles between all the faces are 90°. Cubic unit cells are of three types, such as simple cubic, body centred cubic and face centred cubic.
In orthorhombic unit cell, all the sides are unequal
(a*b#c) but angles between all the faces are
90⁰ (a B⁰=y⁰= 90″ ).Orthorhombic unit cells are of four types, such as simple, body centred, face centred, end centred.
7. Find the packing efiīciency in fcc arrangement.
Ans. In foc arrangement the number of atoms per
unit cell =4.
So volume of four spheres = 4x 4/3 πr³ = 16/3 πr³
In fcc the spheres at the corners are in contact with spheres at the face centres. Therefore face diagonal = 4r
If the edge length is ‘a’, then
a +a = (4r) =2a² = 16r²
Now volume of unit cell = a =16√2r³
volume of spheres/volume of cube × 100
= ——-. ×100
= 1/3√2 × 22/7 x100
= 11√2/21 × 100 = 74 %
8. What is the difference between Schottky defect and Frenkel defect ?
Ans. Schottky defect
The +ve and -ve ions are missing completely from their lattice positions in the crystal.
This type of defect decreases the density of the crystal
It occurs in ionic compounds with higher co- ordination number and with similar cationic and
e.g. NaCl, CsC1, KBr etc.
The ion occupies an interstitial position in the crystal lattice.
This type of defect does not change the density of the crystal
It occurs in ionic compounds with low coordination number and large difference in size of the cations
e.g. AgCl, ZnS, AgBr, etc.
9. What is difference between p-type and n-type Semiconductor ?
Ans. p-type semiconductor
These are obtained due to metal deficiency defects.
Can obtained by adding a little Gr-T3 element to
Gr-14 (Ge) element.
Electron vacancy or positive holes are formed which
are responsible for electric conduction.
Ge doped with (Ga)or In acts as p-type semiconductor, where p -means(+ve holes are created responsible For conduction.
These are obtained due to metal(excess defect.
Can be obtained by adding a little Gr-15 plement to the Gr-14 (Si, Ge) element.
One electron of Gr-15 element remains free and
serves to conduct electricity.
Ge doped with(A) acts as n-type semiconductor,
where ‘n’ means(-ve and electrons used for conduction.
10. What are point defects ? Describe two types of point defects.
Ans. Point defects are the irregularities from ideal arrangement of 1ons in a crystalline substance. The two types of point defects are Stoichiometric defects – These are the point defects that do not disturb the stoichiometry of cations and anions as per the chemical formula of the solīd.
Non-stoichiometric defects These defects arise when the stoichiometry of a substance is disturbed. i.e cations and anions are not in the same ratio as rated by their chemical formula.