Intermolecular forces in liquids

FIGURE 10.12 Trends in the boiling points of hydrides of some main-group elements and the noble gases. 

Water makes up about 0.023% of the total mass of the earth. About 1.4 X 10 kg of it is distributed above, on, and below the earth s surface. The volume of this vast amount of water is about 1.4 billion km. Most of the earth s water (97.7%) is contained in the oceans, with about 1.9% in the form of ice or snow and most of the remainder (a small fraction of the total) available as freshwater in lakes, rivers, underground sources, and atmospheric water vapor. A small but important fraction is bound to cations in certain minerals, such as clays and hydrated crystalline salts. More than 80% of the surface of the earth is covered with water—as ice and snow near the poles, as relatively pure water in lakes and rivers, and as a salt solution in the oceans. 

If all possible hydrogen bonds form in a mole (Na molecules) of pure water, then every oxygen atom is surrounded by four H atoms in a tetrahedral arrangement its own two and two from neighboring molecules. This tetrahedral arrangement forms a three-dimensional network with a structure similar to that of diamond or SiOi. The result is an array of interlocking six-membered rings of water molecules (Fig. 10.14) that manifests itself macroscopically in the characteristic sixfold symmetry of snowflakes. 

FIGURE 10.14 The structure of ice is quite open. Each water moiecuie has oniy four nearest neighbors with which it interacts by means of hydrogen bonds (red dashed iines). 

FIGURE 10.15 The density of water rises to a maximum as it is cooied to 3.98°C, then starts to decrease siowiy. Undercooied water (water chiiied be-iow its freezing point but not yet converted to ice) continues the smooth decrease in density. When iiquid water freezes, the density drops abruptiy. 

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A common feature of all clathrates discussed so far is a host lattice, by itself thermodynamically unstable, which is stabilized by inclusion of the second component. The forces binding this component must be similar in nature to the intermolecular forces in liquids. It seems natural, therefore, to regard a clathrate compound as a solid solution of the second component in the (meta-stable) host lattice. 

At the same time that Dalton proposed his ideas on partial pressure, he developed the concept of vapor pressure. A vapor is the gaseous form of a substance that normally exists as a solid or liquid. A gas is a substance that exists in the gaseous states under normal conditions of temperature and pressure. The vapor pressure of a liquid is the partial pressure of the liquid s vapor at equilibrium. Liquids with strong inter-molecular forces exert lower vapor pressures than those with weak intermolecular forces. In liquids with strong intermolecular forces, it is more difficult for the molecules to leave the liquid state and enter the gaseous state. 

The properties of equations such as (3) and (4) which are not allowed by RMT are understood satisfactorily only in the relatively uninteresting linear case where, for example, rise and fall transients mirror each other as exponentials. When this frontier is crossed, the applied field strength is such that it is able to compete effectively with the intermolecular forces in liquids. This competition provides us with information about the nature of a molecular liquid which is otherwise unobtainable experimentally. This is probably also the case for internal fields, such as described by Onsager for liquids, for various kinds of intmial fields in int ated computer circuits, activated polymers, one-dimensional conductors, amorphous solids, and materials of interest to information tedmology. The chapters by Grosso and Pastori Parravidni in this volume describe with the CFP some important phenomena of the solid state of matter in a slightly different context. 

Sinanoglu, O. (1967) Intermolecular forces in liquids. Advances in Chemical Physics, Vol. 12, Wiley, New York, pp. 283-325. 

Intermolecular forces in liquids in general are described by the categories discussed above. In addition, hydrogen bondiing occurs in substances where H is covalently bonded to N, O, or F. 

Compare the strength of intermolecular forces in liquids with... 

Intermolecular forces in liquids are considerably stronger than intermolecular forces in gases. Particles are, on average. 

Kinetic energy needed to overcome intermolecular forces in liquid... 

The shape and combination of ions and molecules in solids determines a lot about their characteristics. The same is true of liquids. The size, strength, and shape of molecules along with the intermolecular forces in liquids have a big effect on the viscosity of different liquids. 

Gas molecules spend most of their time in transit between collisions and are only modestly affected by intermolecular forces. In liquids these forces are the dominant factor that determines the mobility of the molecules. They are notoriously difficult to quantify, and as a consequence, the prediction of liquid diffusivities has lagged behind theories describing the motion of gas molecules. 

During the last 80 years, hundreds of papers have appeared on the attempts to analyze intermolecular forces in liquids. Entailed in these analyses are arguments carrying a bewildering variety of connotations of the terms polar, nonpolar, normal, abnormal, associated, nonassociated, physical interaction, chemical interaction, ideal, and nonideal. I need only refer to the relevant Discussions of the Faraday Society held in 1937, 1953, 1965, and 1967. Certain workers pin the distinction of polar from nonpolar to the relative magnitude of the dielectric constant. Others connect these terms with the possession or otherwise of a dipole moment. I can find no systematic correlation between solubility, expressed as a mole ratio, and the dipole moment, or dielectric constant. 

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