Polar Molecules Attract One Another

You probably remember from Chapter 6 that a polar molecule is one in which the bonding electrons are unevenly distributed. One side of the molecule carries a slight negative charge, and the opposite side carries a slight positive charge. This separation of charge is a dipole. 

Ion-dipole attractions are much weaker than ionic bonds. However, a large number of ion-dipole attractions can act collectively to disrupt an ionic bond. This is what happens to sodium chloride in water. Attractions exerted by the water molecules break the ionic bonds and pull the ions away from one another. 

Electrical attractions are shown as a series of overlapping arcs. The blue arcs indicate negative charge, and the red arcs indicate positive charge. 

Sodium and chloride ions tightly bound in a crystal lattice are separated from one another by the collective attraction exerted by many water molecules to form an aqueous solution of sodium chloride. 

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In this section, you have pieced together the main components that determine the structure and polarity of molecules. Why is the polarity of a molecule important Polar molecules attract one another more than nonpolar molecules do. Because of this attraction, many physical properties of substances are affected hy the polarity of their molecules. In the next section, you will consider some of these physical properties for liquid and solid substances, and learn about other forces that have a significant effect on the interactions within and among molecules. 

FIGURE 10.4 (a) Polar molecules attract one another when they orient with unlike charges close together, but (b) repel one another when they orient with like charges together. 

M FIGURE 10.9 Polar and nonpolar molecules A mixture of polar and nonpolar molecules, like a mixture of magnetic and nonmagnetic particles, separates into distinct regions because the polar molecules attract one another, excluding the nonpolar ones. Question Can you think of some examples of this behavior ... 

As a result of these dipole-dipole forces of attraction, polar molecules will tend to attract one another more at room temperature than similarly sized non-polar molecules would. The energy required to separate polar molecules from one another is therefore greater than that needed to separate non-polar molecules of similar molar mass. This is indicated hy the extreme difference in melting and boiling points of these two types of molecular substances. (Recall that melting and boiling points are physical properties of substances.)... 

Both dipole-dipole and London Dispersion forces are subclasses of van der Waal interactions. When two polar molecules approach one another, a natural attraction known as dipole-dipole forces is created between oppositely charged ends. The relative intensity of dipole-dipole forces may be represented by Eq. 2 ... 

The nonpolar molecules in oil do not attract polar water molecules, so oil and water are immiscible. The polar water molecules attract one another strongly—they squeeze out the nonpolar molecules in the oU. Oil is less dense than water, so it floats on water. 

The importance of the surface polarity and the surface characteristics for polymer adhesion has been considerably discussed in scientific literature [87]. A useful generalized theory of adhesion, however, can be built upon the basis of electrical attractions. The electrical attractions, resulting from uneven surfaces, which are not normally considered to be electrical, participate easily in attractive interactions if adhesives can be found that will wet them. Thus the reason that polyethylene and poly(tetrafluoroethylene) are difficult to bond is simply that available adhesives are thermodynamically more stable if their molecules attract one another than if they interact with low energy surfaces. The solution to this problem would... 

It is well known that neutral molecules such as alkanes attract one another, mainly through van der Waals forces. Van der Waals forces arise from the rapidly fluctuating dipoles moment (1015 S-1) of a neutral atom, which leads to polarization and consequently to attraction. This is also called the London potential between two atoms in a vacuum, and is given as... 

London forces Deformable electron clouds in adjoining molecules distort one another, resulting in an instantaneous polarity with accompanying attraction between the molecules involved. The polarizability of a molecule (see Chapter 5, Section 5.2b) is a measure of its tendency to display this effect. 

Figure 6.28 illustrates how polar molecules electrically attract one another and as a result are relatively difficult to separate. In other words, polar molecules can be thought of as being sticky, which is why it takes more energy to separate them and let them enter the gaseous phase. For this reason, substances composed of polar molecules typically have higher boiling points than substances composed of nonpolar molecules, as Table 6.3 shows. Water, for example, boils... 

So what happens to polar molecules, such as water molecules, when they are near an ionic compound, such as sodium chloride The opposite charges electrically attract one another. The positive sodium ions attract the negative side of the water molecules, and the negative chloride ions attract the positive side of the water molecules. This is illustrated in Figure 7.1. Such an attraction between an ion and the dipole of a polar molecule is called an ion—dipole attraction. 

Ethers are not very soluble in water because, without the hydroxyl group, they are unable to form strong hydrogen bonds with water (Section 7.1). Furthermore, without the polar hydroxyl group, the molecular attractions among ether molecules are relatively weak. As a result, it does not take much energy to separate ether molecules from one another. This is why ethers have relatively low boiling points and evaporate so readily. 

If molecules are sufficiently polar, there is an additional electrical force pulling them toward each other. The negative partial charge on one side of a polar molecule attracts the positive partial charge on the other side of the next polar molecule. If you add polar molecules to other polar molecules, such as water, the attraction between the two is strong. An example is vitamin C dissolving in water. This is another part of the rule like dissolves like polar molecules dissolve other polar molecules. 

Consider the mixture resulting from vigorous shaking of salad oil (nonpolar) and vinegar (polar). Droplets of hydrophobic oil are temporarily suspended in the water. In a short time, however, the very polar water molecules, which attract one another strongly, squeeze out the nonpolar oil molecules. The oil then coalesces and floats to the top. If we add an emulsifying agent, such as egg yolk, and shake or beat the mixture, a stable emulsion (mayonnaise) results. 

The existence of attractive forces between non-polar molecules has been recognised since the classical work of van der Waals (1873), but their origin was not understood until London (1930) showed how they could be calculated by a quantum mechanical discussion of the interaction between fluctuating dipoles, arising from the motions of the outer electrons on the two molecules. According to the theoretical equations, these attractive forces increase in magnitude as the molecules approach one another, and at a separation of r between the nuclei of atoms (or the centre of mass of roughly spherical molecules) they are proportional to the inverse seventh power of the separation ... 

Hydrocarbons (compounds that contain only carbon and hydrogen) are nonpolar. The favorable ion-dipole and dipole-dipole interactions responsible for the solubility of ionic and polar compounds do not occur for nonpolar compounds, so these compounds tend not to dissolve in water. The interactions between nonpolar molecules and water molecules are weaker than dipolar interactions. The permanent dipole of the water molecule can induce a temporary dipole in the nonpolar molecule by distorting the spatial arrangements of the electrons in its bonds. Electrostatic attraction is possible between the induced dipole of the nonpolar molecule and the permanent dipole of the water molecule (a dipole-induced dipole interaction), but it is not as strong as that between permanent dipoles. Hence, its consequent lowering of energy is less than that produced by the attraction of the water molecules for one another. The association of nonpolar molecules with water is far less likely to occur than the association of water molecules with themselves. 

Neutral polar molecules attract each other when the positive end of one molecule is near the negative end of another, as in Figure 11.4(a) . These dipole-dipole forces are effective only when polar molecules are very close together, and fliey are generally weaker than ion-dipole forces. 

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