Intermolecular Forces Explaining Liquid Properties

In the preceding section, we described a number of properties of liquids. Now we want to explain these properties. Many of the physical properties of liquids (and cct-tain solids too) can be explained in terms of intermolecular forces, the forces of interaction between molecules. These forces are normally weakly attractive. 

One of the most direct indications of the attraction between molecules is the heat of vaporization of liquids. Consider a substance like neon, which consists of molecules that are single atoms. (There is no tendency for these atoms to bond chemically.) Neon is normally a gas, but it liquefies when the temperature is low- 

Van der Waals (dipole-dipole, London) Hydrogen bonding Chemical bonding Ionic Covalent 

The Dutch physicist Johannes van der Waals (1837-1923) was the first to suggest the importance of intermolecular forces and used the concept to derive his equation for gases. (See van der Waals equation. Section 5.8.) 

Attractive intermolecular forces can be larger than those in neon. For example, chlorine, CI2, and bromine, Br2, have intermolecular attractive energies of 3.0 kJ/mol and 4.3 kJ/mol, respectively. But even these values are much smaller than bond enCTgies. 

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Species (atoms, molecules, and ions) that are not chemically bonded to each other may interact with one another through intermolecular forces. The strength of intermolecular forces dictates the inherent properties of solids, liquids, and gases. Compounds with very strong intermolecular forces are normally solids at room temperature, whereas compounds with intermediate intermolecular forces are liquids, and those with extremely weak intermolecular forces are gases. The strength of these intermolecular forces explains why a solid such as sucrose melts at 185 °C, whereas ice water melts at 0 °C and molecular nitrogen boils at -196 °C. 

Using PCA, Cramer found that more than 95% of the variances in six physical properties (activity coefficient, partition coefficient, boiling point, molar refractivity, molar volume, and molar vaporization enthalpy) of 114 pure liquids can be explained in terms of only two parameters which are characteristic of the solvent molecule (Cramer 111, 1980). These two factors are correlated to the molecular bulk and cohesiveness of the individual solvent molecules, the interaction of which depends mainly upon nonspecific, weak intermolecular forces. 

Many familiar and observable properties of liquids can be explained by the intermolecular forces just discussed. We all know, for instance, that some liquids, such as water or gasoline, flow easily when poured, whereas others, such as motor oil or maple syrup, flow sluggishly. 

With the exception of gas/gas mixtures, such as air, the different kinds of solutions listed in Table 11.1 involve condensed phases, either liquid or solid. Thus, all the intermolecular forces described in Chapter 10 to explain the properties of pure... 

The existence of strong intermolecular forces of attraction in liquids gives rise to another important property known as surface tension. The phenomenon of surface tension maybe explained by reference in the following figure. 

Learning Goal Physical properties of liquids, such as those discussed in the previous section, can be explained in terms of their intermolecular forces. We have seen (see Section 4.5) that attractive forces between polar molecules, dipole-dipole interactions, significantly decrease vapor pressure and increase the boiling point. However, nonpolar substances can exist as liquids as well many are liquids and even solids at room temperature. What is the nature of the attractive forces in these nonpolar compoimds ... 

Which of the following properties indicates very strong intermolecular forces in a liquid (a) very low surface tension, (b) very low critical temperature, (c) very low boiling point, (d) very low vapor pressure At — 35°C, liquid HI has a higher vapor pressure than liquid HF. Explain. 

Now we will look at some physical properties of liquids. After that, we will explain the experimental values of these properties in terms of intermolecular forces. 

Like vapor pressure and boiling point, surface tension and viscosity are important properties of liquids. These properties can be explained in terms of intermolecular forces. The three kinds of attractive intermolecular forces are dipole-dipole forces, London forces, and hydrogen bonding. London... 

This book gives an account of the bulk properties of solids and liquids (and, particularly, their response to external forces) and an attempt is made to show how many of these properties can be explained in terms of the intermolecular forces and the internal energy. In this chapter a simple account is given of the most important properties of solids and liquids in terms of intermolecular forces. No detailed account of the origin of these forces is given, but the basic features are described and characterised. 

The properties listed in Table 13-1 can be quahtatively explained in terms of the kinetic-molecular theory of Chapter 12. We saw in Section 12-13 that the average kinetic energy of a collection of gas molecules decreases as the temperature is lowered. As a sample of gas is cooled and compressed, the rapid, random motion of gaseous molecules decreases. The molecules approach one another, and the intermolecular attractions increase. Eventually, these increasing intermolecular attractions overcome the reduced kinetic energies. At this point the gas changes to a liquid this process is called condensation (liquefaction). The temperatures and pressures required for condensation vary from gas to gas, because different kinds of molecules have different attractive forces. 

Maginn [87] and Margulis [24, 88] presented their studies on the transport properties of ionic liquids and explained why these media have to be regarded as a different class of solvents. Several other groups associated spectroscopic techniques and molecular simulation calculations to assess the interfacial behaviour of the solutions containing ionic liquids. The use of AA force fields allows an accurate balance between the specific intermolecular interactions and the structural/conformational effects responsible for the properties determined experimentally, whether they are thermodynamic or spectroscopic. 

Elasticity is a macroscopic property of matter defined as the ratio of an applied static stress (force per unit area) to the strain or deformation produced in the material the dynamic response of a material to stress is determined by its viscosity. In this section we give a simplified formulation of the theory of torsional elasticity and how it applies to liquid crystals. The elastic properties of liquid crystals are perhaps their most characteristic feature, since the response to torsional stress is directly related to the orientational anisotropy of the material. An important aspect of elastic properties is that they depend on intermolecular interactions, and for liquid crystals the elastic constants depend on the two fundamental structural features of these mesophases anisotropy and orientational order. The dependence of torsional elastic constants on intermolecular interactions is explained, and some models which enable elastic constants to be related to molecular properties are described. The important area of field-induced elastic deformations is introduced, since these are the basis for most electro-optic liquid crystal display devices. 

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