Chemistry

States of matter – SESSION IV

Session Objectives

Session objectives Vapour pressure of liquids Surface tension Viscosity Solids and their types

Vapour pressure of liquids The pressure exerted by vapours in equilibrium with liquid at a given temperature. Surface tension the units of surface tension are Nm –1 or J m –2 A B molecule in the bulk of the liquid molecule on the surface of the liquid (experiences resultant downward force)

Factors affecting surface tension (i)Temperature: Surface tension decreases with increase in temperature (because increase in temperature decreases the intermolecular forces.) (ii)Effect of surfactants: The surface active agents decreases the surface tension of water For example, soap, detergent, ethanol. Addition of surfactants decreases the surface tension of liquid. Surfactants

Consequences of surface tension Spherical shape of liquid drops Capillary action

Do you know? How does water rises in a capillary? The surface tension pulls the water into the capillary. The surface tension of a fine capillary is very large therefore, it overcomes the attraction of gravity on water.

Viscosity The internal resistance which one liquid layer offers to another layer sliding over it, during its flow is known as its viscosity.

Viscosity Where, f = Force of friction between two layers of liquid  = Coefficient of viscosity a = Area of one layer of liquid in square centimetre x = Distance between the two layers of liquid. v = Difference in velocities of two layers of liquid. Units of viscosity = dynes cm –2 s SI units of viscosity = Nm –2 s

What is solid ? Solids have definite shape, definite volume and strong force of attraction among constituting particles.

Crystalline and amorphous solids Amorphous or pseudo solidsCrystalline or true solids Constituents are arranged in orderly fashion. Constituents are not arranged in orderly fashion. Anisotropic Isotropic Have long range order Have short range order Undergo clear cleavage Undergo irregular cut Sharpe melting pointMelting over range of temperature Have definite heat of fusion e.g., NaCl Do not have definite heat of fusion e.g., Glass

Ionic crystals Positive and negative ions arranged in a definite order Strong electrostatic forces of attraction Brittle, high melting point, good conductors in the aqueous solution or fused state, high heats of fusion Salts like NaCl, KNO 3, LiF, BaSO 4

Molecular crystals Small molecules van der Waal's forces Soft, low melting point, volatile, electrical insulators, poor thermal fusion Solid CO 2 (dry ice), CH 4,wax

Covalent crystals Atoms chemically bounded together in the form of a network Covalent bond forces Very hard, high melting point, poor conductors of heat and electricity high heats of fusion Diamond, silicon, quartz.

Metallic crystals Positive ions and mobile electrons Electrical attractions (metallic bond) Very soft to very hard, low to high melting point, good conductors of electricity and heat,metallic lustre, malleable and ductile, moderate heats of fusion All metals and some alloys

Lattice or space lattice or crystal lattice Regular arrangements of the constituent particles in three dimensional space

Unit cell Is the smallest portion of the space lattice which when repeated again and again in different directions generates the complete space lattice.

Unit cell

Bravais (1848) Possible shapes of unit cell Cubic Tetragonal Orthorhombic Hexagonal Trigonal or Rhombohedral Monoclinic Triclinic

Arrangement of atoms/ ions in unit cell Primitive or Basic Body centered Face centered End centered

Primitive cubic or simple cubic Total no. of constituents per unit cell = Total number of corners x contribution by each atom =8 x 1/8 =1 C a b Simple or primitive Rank

Body centered cubic unit cell Total no. of constituents per unit cell. = Total contribution of constituents at corners + Total contribution of particle at centre =8 x 1/8 + 1 x 1 =1 + 1 =2

Face centered cubic unit cell Total no. of constituents per unit cell. = Total contribution by constituents at corners + Total contribution by constituents on the faces =8 x 1/8 + 1/2 x 6 =1 + 3 =4

End centered cubic unit cell Total no. of constituents per unit cell. = Total contribution by constituents at corners + Total contribution by particles on the faces. =8 x 1/8 + 1/2 x 2 =1 + 1 =2

Co-ordination number Number of spheres which are touching a particle sphere In ionic crystalsNumber of oppositely charged ions surrounding a particular ion e.g., Co-ordination number of Cl – and Na + in NaCl molecule is 6 each.

Question

Illustrative example If three elements P,Q and R crystallize in a cubic solid lattice with P atoms at the corners, Q atoms at the cube centre and R atoms at the centre of the faces of the cube, then write the formula of the compound. Solution:

Packing fraction Is the fraction of total volume of a cube occupied by constituent particles.

Packing fraction of simple cubic crystal For simple cubic crystal(scc) Packing fraction = For bcc, body diagonal, 4r Packing fraction = a A B

Packing fraction of face cubic crystal For fcc, face diagonal, 4r = Packing fraction = A B C a

Density The number of particles present per unit cell. Suppose the edge length of the unit cell = a Number of atoms present in one unit cell = Z Atomic mass of the element = M

Density Density of unit cell (  ) = Mass contained in one unit cell = Number of particles in one unit cell × Mass of one particle Since mass of one particle =

Density Therefore, mass contained in one unit cell = Volume of unit cell = (Edge length) 3 = a 3

Do you know density depends on the type of the crystal structure.

A B C D E F N L M O X-ray reflection from crystals

Class exercise 1 Which of the following liquids is most difficult to suck into pipette? (a) Toluene(b) Water (c) Glycerol(d) Lemon juice Since it has highest viscosity. Solution:- Answer is (c).

Class exercise 2 Which of the following is pseudo solid? (a) Diamond(b) Common salt (c) Graphite(d) Plastic (d) Plastic is pseudo solid or amorphous solid. Solution:

Class exercise 3 Which one of the following is not the property of crystalline solid? (a) Isotropic (b) Sharp melting point (c) Definite regular geometry (d) High intermolecular forces (a) Crystalline solids are an isotropic. Solution:

Class exercise 4 With increase in temperature, the fluidity of liquids (a) increases (b) remains constant (c) Decreases (d) None of these Solution: Hence, the answer is (a)

Class exercise 5 The rise of liquid in a capillary tube is due to (a) viscosity(b) effusion (c) diffusion(d) surface tension The liquid in capillary rises due to surface tension. Solution: Hence, the answer is (d).

Class exercise 6 Which of the following statements is correct? (a) Vapour pressure decreases with increase of temperature (b) Vapour pressure increases with increase of temperature (c) Vapour pressure is independent of temperature (d) None of these Vapour pressure Temperature Solution: Hence, the answer is (b).

Class exercise 7 Particles of quartz are packed by (a) van der Waals’ forces (b) covalentely bonded forces (c) electrical attraction forces (d) None of these Quartz is a covalent solid. Solution: Hence, the answer is (b).

Class exercise 8 Packing fraction of a scc unit cell is (a) 52%(b) 74% (c) 68%(d) 92% Solution: Hence, the answer is (a).

Class exercise 9 Calculate the packing fraction of a fcc unit cell if two face-centred atom have removed from the unit cell. For fcc, effective number of atom = Since two face-centred atoms have lost. Solution:

Class exercise 10 Metallic gold crystallizes in fcc lattice with edge length 4.07 Å. Find its density. (Au = 197). = 19.4 g/cm 3 Solution:

Thank you