Ncert solution for class 12 chemistry (States of Matter)

Class 12 ncert solutions


Ques 1:- What is space lattice?

Ans:- Crystal lattice or space lattice represents a three dimensional arrangement of constituent particles of a crystal.

Ques 2:- Explain Frenkel defect with an example.

Ans. Frenkel defect :This defect is shown by ionic solids. This defect was pointed out by Frenkel, a

Russian scientist in 1926. In this defect, an ion is missing from its normal position and occupies an interstitial site between the lattice points.

Since, the number of cations and anions in the crystal remains the same, therefore, the crystal remains electrically neutral. This defect generally occurs in those compounds in which:

  1. coordination number is low and
  2. anions are much larger in size than cations. it creates a vacant defect at its original site and an interstitial defect at its new location. Frenkel defect is also known as dislocation defect. Common examples of crystals showing Frenkel defect are AgL, AgBr and ZnS. Since, there is no absence of ions from the lattice, the density remains the same. lonic solid AgBr shows both, Schottky and Frenkel defects. However, this defect increases the dielectric constant of the medium.

Ques 3:- What are body centred unit cells? Calculate the number of atoms in a body centred cubic unit cell. 

Ans. The unit cell in which the particles are present at all the corners and at the body centre are called body centred unit cells.

Number of atoms in a body centred cubic unit cell:

In a body centred cubic  unit cell of a monoatomic substance, the points are present at all the eight corners as well as at the centre of the cube. The contribution due to each atom at corner of the unit cell is 1/8. Hence, the contribution due to all the atoms at 8 corners is 8x1/8 =1 

Thus, total number of atoms in a bcc unit cell = 1due to  corners+ 1 at body centre = 2 atoms.

Ques 4:- What do you understand by imperfections in ionic crystals? Name the types of imperfections which occurs in ionic crystals.

Ans:- Any deviation from the completely ordered arrangement of constituent particles in a crystal is called imperfections. In crystalline solids, there are three types of atomic imperfections:

  1. Stoichiometric imperfections,
  2. Non-stoichiometc imperfections,
  3. Impurity defects.

Ques 5:- Calculate number of particles per unit cell in a fcc crystal.

Ans:- (a) In the fcc unit cell, there are eight atoms at corners and each contributes 1/8 of the unit cell. Thus, contribution of atoms present at corners per unit cell = 8×1/8 =1.

(b) There are six faces in the unit cell and atom on each face is shared by two unit cells therefore, contribution made by the atoms at the faces =6×1/2 =3 . Thus, total number of atoms per unit cell = 1+ 3= 4 atoms. 

Ques 6:- What are crystalline solids ? Give two examples.

Ans:- A solid is said to be crystalline, if the various constituent structural units (atoms, ions or molecules) of which the solid is made up are arranged in a definite geometrical pattern in three dimensional space and there is short range as well as long range within the solid. A crystalline solid usually consists of a large number of small crystals and each such crystal has a definite characteristic geometrical shape. In a crystalline solid, constituent particles are systematically arranged and this arrangement  repeats itself periodically, over the entire crystal. Sodium chloride, sulphur, diamond, quartz and iodine are some examples of crystalline solids.

Ques 7:-  Why do solids have a definite volume?

Ans:- The intermolecular forces between the particles in the solid state are very strong and as a result of this, the constituent particles are very closely packed and are not free to move. Due to this reason, solids have a definite volume.

Ques 8:- Why are solids rigid?

Ans:- It is because  the constituent particles in solids are closely packed and the forces of attraction  among these particles are very strong. They have fixed positions and can only oscillate about their mean position.

Ques 9:- Why is glass considered a super cooled liquid?

Ans:- It is because, like liquids, amorphous solids (like glass) have a tendency to flow, though very slowly and therefore, these are called pseudo solids or super cooled liquids. Glass panes fixed in the windows or doors of old buildings are found to be slightly thicker at bottom than at the top. This is because, glass is a super cooled liquid which flows down very slowly and makes the bottom portion slightly thicker than the top.

Ques 10:- Classify the following as amorphous or crystalline solids : Polyurethane, naphthalene, benzoic acid, teflon, potassium nitrate, cellophane, polyvinyl chloride, fibre glass, copper.
Ans. Crystalline solids: Benzoic acid, potassium nitrate, copper, naphthalene.
Amorphous solids : Polyurethane, terlon, cellophane, polyvinyl chloride, fibre glass.

Ques 11:- Ionic solids conduct electricity in molten state but not in solid state. Explain.
Ans. lonic solids do not conduct electricity in solid state because the ions are not free to move about in the crystal lattice of ionic solids as these ions are held together by strong electrostatic forces ot attraction. However, in the molten state, the ions become free to move about and therefore, ionic solids conduct electricity  on
molten state. 

Ques 12:- Solid A is very hard electrical insulator in solid as well as in molten state and melts at extremely  high temperature What type of solid it is? 
Ans. It is a covalent or network solid like C (diamond), Si02 (quartz) or SiC(silicon carbide,i.e.
carborundum) as all these are  hard and melt at high temperature. Due to non availability of free electrons , they act as insulator.

Ques 13:- What type of solids are electrical conductors, malleable and ductile? 
Ans. Metallic solids are electrical conductors and are highly malleable and ductile.

Ques 14:- What is bravais lattices? 
Ans. Based on the various combinations of  the lattice spacings along the three axes, i.e., length of intersecting edges and the three angles between edges of unit cell we can define seven crystal systems. The simplest amongst these systems is the  while triclinic is the most complex.

Q.15 :- Give the significance of "lattice point".

Ans. (i) Lattice point in the crystal lattice gives the location of one particle in space.
(ii) Lattice points help to predict the geometry of the lattice.

Q.17. Explain how much portion of an atom located at (i) corner and (ii) body-centre of a cubic unit cell is part of its neighbouring unit cell.
Ans. (i) In a cubic unit cell, each atom at corner is shared by eight unit cells and hence each atom located at the corner of a cubic unit cell contributes 1/8th to the neighbouring unit cell.
ii) An atom present at the body-centre is not shared by any other unit cell so it does not contribute to any of the neighbouring unit cell. Thus, it belongs fully to the unit cell.

Post a Comment