4.2 Peak Surface Correlation
4.2.2 Peak Surface Data
1.0 Introduction 2 0 ObJeCtives
3.1 Conccptofa Liquid
3 2 Companson ofL <]UJds wJth Gases and Sol1ds
Page 95 95
_, _, Struduruf LH.JUH.b
3.3 I Volume Change During Fusion and Vaporization 3.3 .2 Molar Enthalpics of Fus1on and Vaponzalion 3.3_3 X-raY Diffraction by L1quids
3.) Surtacc Tcns10n and ViscosJt,· <;7
4.11 ( UJH.:Iu JOn 11!11
S o Summar I0 I
6 I! References and Other Resource'S 7 0 Tutor-Marked Assignment
101 I0I
1.0 Introduction
In Module 2, Unit 2, we discussed the characteristics of ideal gases. We assumed that there is no attractive or repulsive interaction between the individual molecules. In Module 2. Unit 5, this treatment was modified to account for the behaviour of real gases at low temperatures and high pressures and to explain the liquefaction of gases. Finite sizes of the gaseous molecules and their weak interaction were recognized. In Module 3. Units I and 2. we studied the strong interactionsa solid crystal and the order!,· arrangement of particles in it. In this Unit and in Unit 5 of this Module, we will discuss the characteristics of liquids in contrast to those of gases and solids. Our aim in this unit is not to list the properties of liquids but to correlate these to the intermolecular interactions.
We will describe the features of a model proposed for the structure of liquids. We shall explain the correlation bdwcen the intennolecular forces and the properties of liquids such as surface tension.
viscosity. vapour pressure. boiling point and molar enthalpy of vaporization.
2.0 Objectives
By the end of this Unit. you should be able to
• Explain the concept of a liquid
• Compare distinctly liquids with gases and solids in Module I
• Explain accurately the structure of liquids
• State the significance of surface tension and viscosity of liquids.
3.1 Concept of a Liquid
Milk, kerosene, petroL water. salt solution. alcohol and sugar solution arc all liquids. Mercury is even a heavy liquid. Why do we say these are all liquids despite the fact that:
(i) They are different substances
(ii) Some contain a mixture of solid and liquid substances (iii) Some are organic and some inorganic
(iv) One of them 1s even metallic
(v) Some are compounds while some are elements.
What then makes a liquid? There is no doubt you have interacted with each of the samples listed as liquids. Pcrharps a close examination of these substances will enable you to have the right description of what a liqmd is.
Activity I
Apparatus: Test tubes. balance. water. milk. kerosene. petrol. salt solution.
Procedure:
Weigh each of the 5 test tubes.
Collect 5cm' of each of the samples separately mto the weighed five test tubes Re-weigh each of the test tubes with content inside.
Determine the dens1ty of each liquid.
Shake the content in the test tubes.
Examme the ;urfacc of each liquid in the test tubes.
Table 4.2: Viscosity (l]) of some liquids at 298 K
Liquid 11 /Pa s
,,
Water 8.YO X 10
1
Benzene 6.0 X 10
Glycerol 0.9945
Chloroform 4.7 X 10
'
Liquids with larger intermolecular forces tlow slowly and arc called viscous liquids. Hydrogen bonding is particularly important in this respect because it can bind neighbouring molecules together much strongly This accounts for the fact that water has higher viscosity than benzene and chloroform. which have no hydrogen bonding. Glvcerol has very high viscosity. mainly due to numerous bonds it can form.
Molecular arrrangement also could cause high viscosity Heavy hydrocarbon oils and grease are not hydrogen bonded but are highly v1scous. Their viscosity arises partly from London forces between molecules and partly because the long cham-like molecules become entangled with each other (Fig 4.4) like cooked noodles served 111 a plate.
Fig. 4.4: '/'he molecules m the heavy hydrocarhon oil entangled together Use the above discussion on surface tension and viscositv to answer the following exercises.
Exercise 2
For waterproof coating of wood paraffin wax is used. Explain the reason. (Hint: paraffin wax is a mixture of solid hydrocarbons)
Exercise 3
Among the alkanes -octane (C,H,) nonane (C,H,,.) and decane (C10
t\,)-
which is expected to have the highest viscosit ··>4.0 Conclusion
I\ this Unit. we studied the characteristics of liquids. The model proposed for the structure of liquid was discussed. Surface tension and viscosity ofliquids were explained and the dependence of these
5.0 Summary
At the end of this Unit. you have learnt that:
• A liquid is a substance that flows and has no definite shape of its own.
• The characteristics of a liquid lie between the extremes of a gas and a solid.
• Three evidences of volume change during fusion and vaporization. molar enthalpies of fusion and vaporization and X-ray diffusion by liquids show that a liquid has considerable measure of orderly arrangement as compared to a gas.
6.0 References and Other Resources
IGNOU. ( 1997). States of Matter: Physical Chemistry CHE-04 New Delhi.
7.0 Tutor-Marked Assignment
(I) Comment on the fact that the densities of solid, liquid and gaseous nitrogen are 1.026, 0.8081 and 125lx 103 kg
ctm·'.
respectively.(2) In a polythene tube. water meniscus is convex. Explain.
(3) Molar enthalpies of vaporization of benzene and naphthalene are 3 I. I and 44 kJ mot·'. Explain.
(4) At room temperature, among water, methyl cyanide and methanol, which is expected to have the highest tension? State the reason.
(5) Why the viscosity of water at 373 K is one-sixth of its viscosity at 273 K'?