chemistry. states of matter – session iv session objectives
TRANSCRIPT
Vapour pressure of liquids
The pressure exerted by vapours in equilibrium with liquid at a giventemperature.
Surface tension
the units of surface tension are Nm–1 or J m–2
A
Bmolecule 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
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 largetherefore, 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.
av
fx
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 solids Crystalline or true solids
Constituents are arranged in orderly fashion.
Constituents are not arranged in orderly fashion.
AnisotropicIsotropic
Have long range order Have short range order
Undergo clear cleavage Undergo irregular cut
Sharpe melting point Melting over range of temperature
Have definite heat of fusione.g., NaCl
Do not have definite heat of fusione.g., Glass
Ionic crystals
Positive and negative ions arranged in a definite order
Strong electrostaticforces of attraction
Brittle, high melting point,good conductors in the aqueous solutionor fused state, highheats of fusion
Salts like NaCl,KNO3, LiF, BaSO4
Molecular crystals
Small molecules
van der Waal'sforces
Soft, low melting point,volatile, electricalinsulators, poor thermalfusion
Solid CO2 (dry ice),CH4,wax
Covalent crystals
Atoms chemically bounded together in the form of anetwork
Covalentbond forces
Very hard, high meltingpoint, poor conductors of heat and electricity high heats of fusion
Diamond, silicon,quartz.
Metallic crystals
Positive ions andmobile electrons
Electrical attractions(metallic bond)
Very soft to very hard, low to high melting point, good conductors ofelectricity and heat,metallic lustre, malleable and ductile, moderateheats 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 directionsgenerates the complete space lattice.
Bravais (1848)
Possible shapes of unit cell
CubicTetragonal
Orthorhombic
Hexagonal
Trigonal or Rhombohedral
Monoclinic
Triclinic
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
=1C
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 crystals Number of oppositely charged ions surrounding a particular ione.g., Co-ordination number of Cl– and Na+ in NaCl molecule is 6each.
Illustrative example
If three elements P,Q and R crystallizein 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:1
Atoms of elements P per unit cell 8 18
Atoms of elements Q per unit cell 1
1Atoms of elements R per unit cell 6 3
2
3The formula of compund PQR
Packing fraction
• Is the fraction of total volume of a cube occupied by constituent particles.
Packing fraction(PF) =
Volume occupied by effective number of particlesVolume of the unit cell
Packing fraction of simple cubic crystalFor simple cubic crystal(scc)
Packing fraction = 3
3
41 r
3
a
3
3
4r
3 a 2r 0.5232r
For bcc, body diagonal, 4r 3a
Packing fraction =
3
3
42 r
3 0.68 i.e., 68%4r
3
aA B
Packing fraction of face cubic crystal
For fcc, face diagonal, 4r = 2a
Packing fraction =
3
3
44 r
3 0.74 i.e., 74%4r
2
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
Volume of the unit cell
Mass contained in one unit cell = Number of particles in one unit cell × Mass of one particle
Since mass of one particle = Atomic mass
Avogadro ' s number
Density
Therefore, mass contained inone unit cell =
A
MZ
N
Volume of unit cell = (Edge length)3 = a3
A3
MZ
N
a
3
A
Z M
a N
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
1Fluidity
Coefficient of viscosity
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%
3
3
41 r
3For scc, PF 0.52 since (a 2r)(2r)
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 = 1 18 4 3
8 2
Since two face-centred atoms have lost.
3
3
43 r
3PF 0.555, i.e., 55.5%4r
2
Solution:
Class exercise 10
Metallic gold crystallizes in fcc lattice with edge length 4.07 Å. Find its density. (Au = 197).
3AV
Z MDensity,
N a
3
323 8
4 197g / cm
6.023 10 4.07 10
= 19.4 g/cm3
Solution: