Intermolecular attraction forces, weak

In linear polymers, cohesion results from weak (compared with covalent bonds) intermolecular attractive forces (Van der Waals) of various types London, Debye, Keesom, and hydrogen bonding. In a first approach, they can be considered undistinguishable, and one can define cohesive energy as the whole energy of intermolecular interactions. For small molecules, cohesive energy is easy to determine from calorimetric measurements since... 

Because unbranched alkanes are neutral, nonpolar molecules, it is difficult to explain the existing intermolecular force between such alkanes that increases as the alkane molecules become larger. We will see that this attractive force is weak and tenuous. These molecules do not become overly friendly with each other. In theory, as atoms within one alkane molecule approach the atoms of another alkane molecule, the electrons around these atoms, for an instant, arrange themselves asymmetrically around the atoms so that instant dipoles are formed—the positive side of one atom attracts the negative side of another atom. This weak intermolecular attractive force is called a London Force. When there is a weak intermolecular attractive force between polar molecules, the force is called a dipole-dipole force. Together, London forces and dipole-dipole forces are called Van der Waals forces. 

Unvulcanized rubber consists of a large number of flexible long molecules with a structure that permits free rotation about single bonds in the primary chain. On deformation the molecules are straightened, with a decrease in entropy. This results in a retractive force on the ends of the polymer molecules. The molecular structure of the flexible rubber molecules makes it relatively easy for them to take up statistieally random conformations under thermal motion. This property is a result of the weak intermolecular attractive forces in elastomers and distinguishes them chemically from other polymers which are more suitable for use as plastics or fibers. 

Elastomers. Elastomers are polymeric materials with irregular structure and weak intermolecular attractive forces. Elastomers are capable of high extension (up to 1000%) under ambient conditions. That is, they have the particular kind of elasticity characteristic of rubber. The elasticity is attributed to the presence of chemical and/or physical crosslinks in these materials. In their normal state, elastomers are amorphous, and as the material is stretched, the random chains are forced to occupy more ordered positions. Releasing the applied force allows the elongated chains to return to a more random state. Thus, the restoring force after elongation is largely because of entropy. (Fig. 14.3)... 

Apart from its dependence on the surrounding atmospheric pressure, the boiling point depends on the nature of the attractive forces between the liquid molecules. Polar liquids, such as water, with large intermolecular attractive forces have higher boiling points than nonpolar liquids, such as gasoline, which exhibit weak attractive forces. 

Not all plastics can be converted into practical fibers, however, because the intermolecular forces or crystallization tendency may be too weak to attain high strength by axial orientation. Synthetic fibers are therefore made generally from polymers whose chemical composition and geometry enhance intermolecular attractive forces and crystallization. Such polymers can be converted from plastics to fibers by suitable treatment to cause axial alignment, as explained above. 

Gas-Liquid Solutions Gases that are nonpolar, such as N2, or are nearly so, such as NO, have low boiling points because their intermolecular attractions are weak. Likewise, they are not very soluble in water because solute-solvent forces are weak. In fact, as Table 13.3 shows, for nonpolar gases, boiling point generally correlates with solubility in water. 

Disulfide bridges may be strong, but there are usually only a few of them. Van der Waals forces are weak, but they outnumber all the other intermolecular attractive forces. 

Dispersion forces Weak intermolecular attractive forces that arise between molecules are close together, because the fluctuating electron distributions of the molecules become synchronized so as to produce a shght electrostatic attraction. These forces play a role in the internal packing of many biomolecules. 

Dispersion forces (0.08—8 y/mol, 0.02-2 kcal/mol) are the weakest intermolecular attractive forces. It is the existence of dispersion forces that accounts for the fact that low-molecular-weight, nonpolar substances such as methane can be liquefied. When we convert methane from a liquid to a gas at —164 °C, for example, the process of separating its molecules requires only enough energy to overcome the very weak dispersion forces. 

The physical properties of arenes resemble those of other hydrocarbons. Although weak, intermolecular attractive forces are somewhat stronger than those of other hydrocarbons of similar size. 

Induced-dipole/induced-dipole attractions are ver-y weak forces individually, but a typical organic substance can par ticipate in so many of them that they are collectively the most impor tant of all the contributor s to intermolecular- attraction in the liquid state. They are the only forces of attraction possible between nonpolar- molecules such as alkanes. 

We have now discussed three types of intermolecular forces dispersion forces, dipole forces, and hydrogen bonds. You should bear in mind that all these forces are relatively weak compared with ordinary covalent bonds. Consider, for example, the situation in HzO. The total intermolecular attractive energy in ice is about 50 kj/mol. In contrast, to dissociate one mole of water vapor into atoms requires the absorption of928 kj of energy, that is, 2(OH bond energy). This explains why it is a lot easier to boil water than to decompose it into the elements. Even at a temperature of 1000°C and 1 atm, only about one H20 molecule in a billion decomposes to hydrogen and oxygen atoms. 

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