Forces also boiling points, relation

The polarity of molecules also creates attractive forces between molecules that cause the molecules to stick together. These attractive forces are called Intermolecular Forces. The physical properties of melting point, boiling point, vapor pressure, evaporation, viscosity, surface tension, and solubility are related to the strength of attractive forces between molecules. 

When a molecule takes part in a reaction, it is properties at the molecular level which determine its chemical behaviour. Such intrinsic properties cannot be measured directly, however. What can be measured are macroscopic molecular properties which are likely to be manifestations of the intrinsic properties. It is therefore reasonable to assume that we can use macroscopic properties as probes on intrinsic properties. Through physical chemical models it is sometimes possible to relate macroscopic properties to intrinsic properties. For instance 13C NMR shifts can be used to estimate electron densities on different carbon atoms in a molecule. It is reasonable to expect that macroscopic observable properties which depend on the same intrinsic property will be more or less correlated to each other. It is also likely that observed properties which depend on different intrinsic properties will not be strongly correlated. A few examples illustrate this In a homologous series of compounds, the melting points and the boiling points are correlated. They depend on the strengths of intermolecular forces. To some extent such forces are due to van der Waals interactions, and hence, it is reasonable to assume a correlation also to the molar mass. Another example is furnished by the rather fuzzy concept nucleophilicity . What is usually meant by this term is the ability to donate electron density to an electron-deficient site. A number of measurable properties are related to this intrinsic property, e.g. refractive index, basicity as measured by pK, ionization potential, HOMO-LUMO energies, n — n ... 

We have seen (Section 12-15) how the presence of strong attractive forces between gas molecules can cause gas behavior to become nonideal when the molecules get close together. In liquids and solids the molecules are much closer together than in gases. As a result, properties of liquids, such as boiling point, vapor pressure, viscosity, and heat of vaporization, depend markedly on the strengths of the intermolecular attractive forces. These forces are also directly related to the properties of solids, such as melting point and heat of fusion. Let us preface our study of these condensed phases with a discussion of the types of attractive forces that can exist between molecules and ions. 

Consideration of these characteristics makes it clear that only very special liquid pairs could conceivably form ideal solutions. It would be necessary that the molecules of the constituents be very similar in every respect, for example in structure, size, and chemical nature. Thus, solutions of optical isomers, adjacent members of an homologous series, and similar mixtures would be expected to be nearly ideal, but actually all solutions can at best only approach ideality as a limit. Solutions which form immiscible liquid phases are of necessity extremely nonideal, and extraction operations depend upon this. The extent to which solutions depart from ideality is manifested by deviations of the properties of the solutions from the characteristics listed above, and a study of these deviations will permit us to some extent to predict their behavior in extraction operations. The most useful characteristics of the ideal solution for these purposes is that of vapor pressure, since considerable information has now been accumulated for many mixtures on this and related properties such as boiling points of solutions, azeotropism, and vapor-liquid equilibria. Classifications of compounds according to the effect of intermolecular forces on properties of mixtures also provide much useful material, but the second and third characteristics in the list above are of limited value owing to lack of experimental data to which we can refer. 

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