Physical chemistry

1. Physical Properties of liquid Prof. Jadhav Swapnil S.
Introduction:-
The physical properties of liquid are those which could be studied by
determined without causing any chemical change in it. Physical properties of
substance depend upon the intermolecular forces which originate in the internal
structure of their molecule.
Eg. Surface tension, viscosity, Refractive Index etc.
Classification of physical properties:-
1) Additive Properties:-
The properties of substance which depends on sum of corresponding properties
of the constituents are called additive properties.
Eg. Mass, molecular weight, molecular heat, dipole, dipole, radioactivity
2) Constitutive properties:-
The properties which mainly depend on the arrangements of constituents and
to a smaller extent on their number and nature are called constitutive properties.
Eg. Optical activity, surface tension, viscosity
3) Additive and Constitutive Properties:-
An additive property which depends on the intermolecular structure is called
Additive and Constitutive Properties
Eg. Parachor, viscosity, surface tension, molecular refractivity, atomic volume
4) Colligative properties:-
The properties which depend on the number of particles in the solution and
not on their nature are called Colligative properties.
Eg. Vapour pressure, elevation in boiling point, depression in freezing point.
Surface tension:-
This property arises from the
intermolecular forces of attraction.
Consider a molecule A in the bulk of
the liquid which is attracted equally in
all direction by neighboring molecules.
Consider molecule B on the surface
of liquid which is attracted downward by
neighboring molecules. Thus there is tendency of surface molecules to go into the
bulk of the liquid. Thus the liquid surface is under tension and tends to reduce to
minimum. This inward attraction gives rise to a force in the plane of surface called
Surface tension.
Def.:- Surface tension may be defined as “the force in dynes acting along the
surface of liquid at right angle to any line 1 cm in length.”
Unit:- S.I. = N/m or J/ m2
C.G.S = dyn/cm or ergs/ cm2

2. Parachor:-
It is defined as the molecular volume of liquid when its surface tension is unit.
Macleod (1923) gives relation between surface tension and density of liquid as
γ = surface tension C = Macleod constant
--- (1) D = density of liquid.
d = density of sat. vapour of liquid.
Sugden (1924) modified Macleod equation by multiplying both sides by M,
M = molecular weight
--- (2) M & C are constant for given liquid
Hence, MC = constant = P
--- (3) P is called Parachor
Bellow critical temperature ‘d’ is negligible comparison with ‘D’ we get,
--- (4) M/D = molecular volume.
If γ = 1 then, P = M/D ie. P = molecular volume.
For two liquids, we can write,
and
Taking ratio, we get, --- (5)
Eq. (5) we say that, a comparison of parachors means, comparison of molecular
volumes under the same condition of surface tension.
parachor is both an additive and constitutive property. Ie. the parachor of an
individual compound can be expressed as a sum of :
(1) Atomic Parachors which are the contributions of each of the atoms present in the
molecule.
(2) Structural Parachors which are the contributions of the various bonds and rings
present in the molecule.

3. Use of Parachor in Elucidating Molecular Structure:-
(1) Structure of Benzene (Vogel)
The Kekule’s formula for benzene is shown aside, the value of its parachor can
be calculated by using Vogel’s data.
6 Carbon atoms : 6 × 8.6 = 51.6
6 Hydrogen atoms : 6 × 15.7 = 94.2
3 double bonds (=) : 3 × 19.9 = 59.7
6-membered ring : 1 × 1.4 = 1.4
Parachor of benzene = 206.9
The experimental value of the parachor of benzene is 206.2. Since the
calculated parachor tallies with that determined by experiment, the Kekule’s structure
for benzene is supported.
(2) Structure of Quinone (Sugden)
The two possible structural formulas proposed for quinine. The parachors
calculated for the two structures are
Structure A
6 C : 6 × 8.6 = 51.6
4 H : 4 × 15.7 = 62.8
2 O : 2 × 19.8 = 39.6
4 (=) : 4 × 19.9 = 79.6
1 six-membered ring : 1 × 1.4 = 1.4 Total = 235.0
Structure B
6 C : 6 × 8.6 = 51.6
4 H : 4 × 15.7 = 62.8
2 O : 2 × 19.8 = 39.6
3 (=) : 3 × 19.9 = 59.7
2 six-membered rings : 2 × 1.4 = 2.8 Total = 216.5.
The experimental value of parachor for quinone is 236.8 which is near to structure
A. Therefore, the structure A represents quinone correctly.
(3) Structure of Nitro group (Sugden)
The parachor has also been found useful in providing information regarding
the nature of bonds present in certain groups.
The nitro group (–NO2), for example, may be represented in four ways:
The experimental value of parachor for – NO2 group has been found to be 73.0. Hence
structure II is correct.

4. (4) Structure of phosphorus pentachloride (PCl5 ):-
It may have two structural formulae
The experimental value of parachor for PCl5 is 282.5. Hence structure II is correct.
Viscosity:- Viscosity resists the flow of liquid.
“The property of liquid which determine the rate of flow of liquid is
called viscosity of liquid.”
Let us examine a liquid flowing on a glass
surface (Fig. 11.22). The molecular layer in
contact with the stationary surface has zero
velocity. The successive layers above it move
with increasingly higher velocities in the direction
of the flow.
Now consider two adjacent moving layers of a liquid (Fig. 4.2).
Let these be separated by a
X = distance between two layer. v = velocity difference between two layer.
F = force of friction
Then
F α F = η ×
η (eta) is known as the Coefficient of Viscosity.
It may be defined as: the force of resistance per unit area required to maintain
unit velocity difference between two layers of a liquid which are at a unit
distance from each other.
Unit:- CGS = Poise (P) or centipoise (10-2
) and millipoise (10–3
) or dyn/cm2
.
SI = N/m2
.s or Pa.s or Kg/m.s
1 poise = 1 g cm–1
s–1
= 0.1 kg m–1
s–1
The reciprocal of coefficient of viscosity is called Fluidity (φ). φ =
 The Fluidity is a measure of the ease with which a liquid can flow.
 Viscosity of liquid decrease with increase in temperature.

5. Determination of Viscosity by Ostwald’s viscometer.
The coefficient of Viscosity of a liquid can be determined with the help of
Pioseulle’s equation as :
Where,
V = volume of the liquid flowing through capillary in
time t, P = applied pressure,
r = radius of the capillary. l= length of the capillary.
The experimental measurement of P, r, l and V offers considerable difficulty.
Hence, it is difficult to find the absolute coefficient of viscosity (η) from Poiseulle’s
equation. Thus, the viscosity of a liquid is determined with respect to that of water by
Ostwald Viscometer.
Construction:-
It consists of U shaped glass tube having two
bulbs. The left-hand arm is essentially a capillary
with two calibration marks A and B bellow &
above the bulb. The right-hand arm is wide
(pipette) and has a bulb C at the base.
Experimental Procedure:-
1) A definite volume of liquid under examination
is poured into the bulb C of viscometer.
2) This liquid is sucked through the left-arm slightly
above the mark A.
3) The liquid is then allowed to flow back and the
time of flow of liquid from A to B is noted.
4) Then the apparatus is cleaned and the process is
repeated for 2nd
with water whose viscosity is known.
Calculation:-
It is observed that the pressure P depends on
(1) h = height of liquid level in the two arm. (2) d1 = density of liquid.
(3) g = acceleration due to gravity.
Then Pioseulle’s equation becomes
For 1st liquid for 2 nd liquid

6. Tacking ratio of above two equations, we get,
When η2 is known and by measuring densities & times of flow of the two liquid η1
of first liquid can be calculated.
Advantages:-
1) It is very convenient apparatus for determination of viscosity.
2) Viscosity at different temperature can be determined as viscometer can be easily
suspended in thermostat.
Refractive Index (Snell’s Law):-
When a beam of light passes from a less dense (rarer) like air to more dense
medium like liquid, it is refracted (bent) towards normal. This is called refraction of
light.
“The ratio of sine of angle of incidence to the sine of angle of refraction is
constant” This is known as Snell’s Law.
n = n is refractive index of 2nd
medium w.r.t. 1st
medium.
Where i = angle of incidence r = angle of refraction.
The refractive index (n) of a substance is also defined as the ratio of the
velocity of light in vacuum or air to that in the medium.
n =
Velocity of light in vacuum
Velocity of light in medium
If i = 90° then sin i = sin 90° = 1 Then, n =
The refractive index of a liquid can be easily determined to a high degree of
accuracy. It is a characteristic property of a liquid. It increase with temperature and
decrease with wavelength of light used. D-line of sodium is used for standard
measurement. Refractive index is a ratio, it has no units.
If the refractive index of a liquid is measured at 20ºC and using D-line of
sodium, it is represented by the following symbol. n20
D

7. Specific Refraction or Specific Refractivity:-
Lorenz and Lorenz (1880), on the basis of electromagnetic theory of light derived
the following relation for the refractive power of substance.
Where, n = refractive index.
d = density
R = Specific Refraction.
Molar (Molecular) Refraction or Molecular Refractivity (RM):-
The product of molecular weight (M) and specific refractivity (R) is called molar
or Molecular Refractivity (RM).
RM = R × M Ie.
The value of molar refraction is characteristic of a substance. It is Temperature-
independent. Since the value of refractive index (n) is dimensionless, from equation it
is evident that RM α M/d. ie. molar volume. Hence molar refractivity is expressed in
cm3
/mol.
Measurement of refractive index by Abbe’s Refractometer:-
Principle:-
The refractive index of liquid is measure on the basis of critical angle principle.
As ζi increases, ζr also increases, but to certain limit. When ζi > 900
, ray suffers
total internal reflection.
When ζi < 900
(rays a, b)we observed light band.
When ζi > 900
(ray d) we observed dark band.
When ζi = 900
(ray c) we observed sharp edge which gives angle of refraction r’ and
we get condition,

8. n = = =
r’ is called critical angle & this phenomenon is known as critical angle
phenomenon.
Note:- If angle of incidence (i) is greater than critical angle (r’), the light is reflected
instead of refracted. This is called “total internal reflection.”
Abbe’s Refractometer:-
Construction and working:-
Abbe’s Refractometer is as shown in fig. The optical system consist following
part- (1) A mirror M. (2) A fixed telescope T.
(3)The prism box PQ which can be rotate by adjusting screw R.
A thin film of the liquid is placed between the two prisms P & Q. The surface of
prism ‘P’ is polished white, while that of ‘Q’ is finely ground. P & Q are held in contact
with each other in a metal box.
Light from source ‘S’ is made to fall on prism ‘Q’ with the help of a mirror. The
light enters the liquid at all angles of incidence. However, no ray can enter the upper
prism with greater angle of refraction than the grazing incidence (i.e., at an angle)
slightly less than 90º. Thus, the view in the telescope through eye-piece ‘O’ appears to
be divided into two bands, one bright (light) and one dark. The prism box is rotated
till the cross wire of the telescope coincides with the edge of the bright band. A
pointer attached to the prism assembly indicates the refractive index directly on an
engraved scale ‘Z’.
When white light is employed, sharp edge is made by compensator ‘W’.
Note:- In order to maintain temperature of liquid to be examine, two prism are
enclosed in a water jacket.

9. Advantages:-
1) The instrument is easy to handle. 2) Only few drops of liquid are required.
3) Refractive Index can be measured directly and quickly. 4) Temperature can be
controlled. 5) The Refractive Index range is from 1.3 to 1.7 with an accuracy of ±
0.0002 unit.
Molecular Refractivity and Chemical Constitution:-
Molecular Refractivity is proportional
to molecular volume. Molecular Refractivity
is also an additive and partly constitutive
property.
Hence it may be used to study the structure
of molecules. Here calculated values of
molecular refractivities are compared with the
observed or experimental values.
b) For allyl alcohol:-
3 C : 3 × 2.42
6 H : 6 × 1.10
1 = bond : 1 × 1.73
1 O in OH : 1 × 1.53
-----------------------------------
Total RM = 17.12
Since, calculated
molecular refractivity of
allyl alcohol (17.12) is
close to experimental
values (16.974).
Hence the compound
must be allyl alcohol
rather than acetone.