POLE-DIPOLE FORCES

Polar molecules, like nonpolar molecules, are attracted to one another by dispersion forces. In addition, they experience dipole forces as illustrated in Figure 9.9, which shows the orientation of polar molecules, such as Id, in a crystal. Adjacent molecules line up so that the negative pole of one molecule (small Q atom) is as dose as possible to the positive pole (large I atom) of its neighbor. Under these conditions, there is an electrical attractive force, referred to as a dipole force, between adjacent polar molecules. 

Polar molecule A molecule in which there is a separation of charge and hence positive and negative poles, 183-185 dipole force, 237 orientation, 183 Polarimeter, 600 Polarity, 184-185 Pollutants, 6 Polyamide, 615-616 Polyatomic ion, 36,39 Polyatomic molecules, 654 Polyester A large molecule made up of ester units, 614-615 Polyethylene, 611-612... 

Dipole-Dipole forces The attractions of the positive and negative poles of molecules are called dipole-dipole forces. 

As the temperature of a gas system is lowered, the speed of the molecules decreases. When these lower-speed molecules collide with one another, attractive forces between the molecules become more significant, and a temperature will be reached where condensation occurs - the vapor state converts to liquid. Di-pole-dipole attractive forces are most important in causing condensation, and molecules with substantial partial charges, resulting from polar covalent bonds, t3q3ically have high condensation temperatures. (Condensation temperature will be the same as the boiling point of a liquid, approached from the opposite direction. )... 

The weakest dipole-dipole force is between two instantaneous dipoles. These di-pole-dipole bonds are called London Dispersion Forces. Although London Dispersion Forces are very weak, they are responsible for the phase changes of nonpolar molecules. 

The aim hae is to take a microscopic view of an ion inside a solvent. The central consideration is that ions orient dipoles. The spherically symmetrical electric field of the ion may tear water dipoles out of the water lattice and make them point (like compass needles oriented toward a magnetic pole) with the appropriate charged end toward the central ion. Hence, viewing the ion as a point charge and the solvent molecules as electric dipoles, one obtains apicture of ion-dipole forces as the principal source of ion-solvent interactions. 

When an ion and a nearby polar moleeule (dipole) attraet eaeh other, an ion-dipole force results. The most important example takes plaee when an ionic compound dissolves in water. The ions become separated because the attractions between the ions and the oppositely charged poles of the H2O molecules overcome the attractions between the ions themselves. Ion-dipole forces in solutions and their associated energy are discussed fully in Chapter 13. 

In Figure 10.12, you saw how an external electric field can orient gaseous polar molecules. When polar molecules lie near one another, as in liquids and solids, their partial charges act as tiny electric fields that orient them and give rise to dipole-dipole forces the positive pole of one molecule attracts the negative pole of another (Figure 12.11). 

Figure 12.11 Polar molecules and dipole-dipole forces. In a solid or a liquid, the polar molecules are close enough for the partially positive pole of one molecule to attract the partially negative pole of a nearby molecule. The orientation is more orderly in the solid left) than in the liquid (right) because, at the lowertemperatures required for freezing, the average kinetic energy of the particles is lower. (Interparticle spaces are increased for clarity.)...

The van der Waals radius determines the shortest distance over which intermolecu-iar forces operate it is aiways larger than the covalent radius. Intermolecular forces are much weaker than bonding (intramolecular) forces. Ion-dipole forces occur between ions and poiar molecules. Dipole-dipole forces occur between oppositely charged poles on polar molecules. Hydrogen bonding, a special type of dipole-dipole force, occurs when H bonded to N, O, or F is attracted to the lone pair of N, O, or F in another molecule. Electron clouds can be distorted (polarized) in an electric field. Dispersion (London) forces are instantaneous dipole-induced dipole forces that occur among all particles and increase with number of electrons (molar mass). Molecular shape determines the extent of contact between molecules and can be a factor in the strength of dispersion forces. 

Dipole forces or multi-pole forces, if the lattice components have dipoles or multipoles. Dipoles can be either intrinsic or induced. Depending on the orientation, the dipole-dipole interaction energy for linear-orientation (Figure 2.34a) is... 

Although incorporated in the poles and the dipole, the forces are not connected in this graph because they do not participate in the dynamics of the system. If friction (due to a viscous medium) were present for relating forces to velocities (see case study AlO Motion with Friction in Chapter 4), it would have been necessary to connect the pole forces to the dipole force. The dipole would enter the category of mixed dipoles, being of the inductive-conductive type instead of the fundamental type. 

The other poles-dipole connection involves the pressures, and a simple difference of energy-per-entity (effort) is adequate for modeling a conduction process. It corresponds to the classical expression of a driving force or pressure drop. 

The dipole force is defined by the general relationship between dipole effort and pole efforts which uses a separability factor Sq... 

Dipole-dipole forces occur between oppositely charged poles on polar molecules. 

Dipole-induced dipole forces, also based on polarizability, arise when a polar molecule distorts the electron cloud of a nonpolar molecule. They are weaker than ion-induced dipole forces because the charge of each pole is less than an ion s (Coulomb s law). The solubility in water of atmospheric O2, N2, and noble gases, while limited, is due in part to these forces. Paint thinners and grease solvents also use them. 

Dipole forces. A polar molecule is sometimes described as a dipole. The attraction between dipoles is between the positive pole of one molecule and the negative pole of another. Figure 15.8 shows the alignment of dipoles, one of several ways polar molecules attract each other. 

A polar molecule can induce a dipole in a nonpolar molecule by temporarily attracting its electrons. The result is a short-range intermo-lecular force that is somewhat weaker than the dipole-dipole force. The force of an induced dipole accounts for the solubility of nonpolar water. The positive pole of a water molecule attracts the outer electrons... 

Note the r dependence of these tenns the charge-indiiced-dipole interaction varies as r, the dipole-indiiced-dipole as and the quadnipole-mduced-dipole as In general, the interaction between a pennanent 2 -pole moment and an induced I -pole moment varies as + L + l) gQ enough r, only the leading tenn is important, with higher tenns increasing in importance as r decreases. The induction forces are clearly nonadditive because a third molecule will induce another set of miiltipole moments in tlie first two, and these will then interact. Induction forces are almost never dominant since dispersion is usually more important. 

A magnetic field exerts a force on each of the two poles of a dipole (particle), forcing it to ahgn itself with the hnes of magnetic force. 

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