Intermolecular force bonding forces

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. 

We have to refine our atomic and molecular model of matter to see how bulk properties can be interpreted in terms of the properties of individual molecules, such as their size, shape, and polarity. We begin by exploring intermolecular forces, the forces between molecules, as distinct from the forces responsible for the formation of chemical bonds between atoms. Then we consider how intermolecular forces determine the physical properties of liquids and the structures and physical properties of solids. 

In the liquid state, the molecules are still free to move in three dimensions but stiU have to be confined in a container in the same manner as the gaseous state if we expect to be able to measure them. However, there are important differences. Since the molecules in the liquid state have had energy removed from them in order to get them to condense, the translational degrees of freedom are found to be restricted. This is due to the fact that the molecules are much closer together and can interact with one another. It is this interaction that gives the Uquid state its unique properties. Thus, the molecules of a liquid are not free to flow in any of the three directions, but are bound by intermolecular forces. These forces depend upon the electronic structure of the molecule. In the case of water, which has two electrons on the ojQ gen atom which do not participate in the bonding structure, the molecule has an electronic moment, i.e.- is a "dipole". 

In this chapter, you have learned about intermolecular forces, the forces between atoms, molecules, and/or ions. The types of intermolecular forces include ion-dipole forces, hydrogen bonding, ion-induced and dipole-induced forces, and London (dispersion) forces. 

The previous chapter dealt with chemical bonding and the forces present between the atoms in molecules. Forces between atoms within a molecule are termed intramolecular forces and are responsible for chemical bonding. The interaction of valence electrons between atoms creates intramolecular forces, and this interaction dictates the chemical behavior of substances. Forces also exist between the molecules themselves, and these are collectively referred to as intermolecular forces. Intermolecular forces are mainly responsible for the physical characteristics of substances. One of the most obvious physical characteristics related to intermolecular force is the phase or physical state of matter. Solid, liquid, and gas are the three common states of matter. In addition to these three, two other states of matter exist—plasma and Bose-Einstein condensate. 

These unusual properties can be explained by hydrogen bonding. This is a weak intermolecular force (bond) which occurs between water molecules because the bonds within the molecules are polar. 

Intermolecular force A force between two or more molecules tends to be weaker than the force of a chemical bond. 

The expansion of a material on heating is a phenomenon that depends on internal - mostly intermolecular - forces. Bond lengths between atoms are virtually independent of temperature. This also holds for bond lengths between segments of a polymer chain. Polymer systems, therefore, have lower expansivities than related low-molecular liquids. 

The difference is pronounced. In an alcohol solution a minimum of approximately six water molecules are required per soap to bring it into solution. A liquid carboxylic acid will dissolve the soap without water to a soap/acld molecular ratio of 1/2. It appears reasonable to evaluate these differences from terms of intermolecular forces. These forces, the strong hydrogen bonds and ligand bonds to the metal ion will be treated in the following section. 

Fig. 10. Intermolecular hydrogen-bond force constant (Fh. . o) as a function of R(0.. O) distance and of intramolecular O—H force constant (Fo-h)- Numbers are those of Table 5.

In contrast to intermolecular forces, intramolecular forces hold atoms together in a molecule. (Chemical bonding, discussed in Chapters 9 and 10, involves intramolec-... 

It is very important not to confuse the two terms intramolecular forces and intermolecular forces. Intramolecular forces are forces that act inside the molecules and thus constitute the bonds between the atoms. Intermolecular forces on the other hand are forces that act outside the molecules between molecules. The energies of chemical bonds (intramolecular forces) are much larger than the energies related to the intermolecular forces. Three different types of intermolecular forces can be distinguished ... 

Intermolecular forces Intramolecular forces Dipole-dipole attraction Hydrogen bonding London dispersion forces Normal boiling point Heating/cooling curve Normal freezing point Molar heat of fusion Molar heat of vaporization... 

We then examine four intermolecular forces disfjersion forces, dipole-dipole forces, hydrogen bonds, and ion-dipole forces. 

Intermolecular forces Attractive forces fhat act between molecules weaker than covalent bonds Hydrogen bonding Attraction between a hydrogen atom bonded to a highly electronegative atom (0. N, F) and the lone pair on an electronegative atom in another or the same molecule... 

Several critical temperatures and pressures cffe hsted in A Table 11.6. Notice that nonpolar, low-molecular-weight substances, which have weak intermolecular attractions, have lower critical temperatures and pressures them substcuices that are polar or of higher molecular weight. Notice also that water and ammonia have exceptionally high critical temperatures and pressures as a consequence of strong intermolecular hydrogen-bonding forces. 

In Chapter 7, we saw that the formation of chemical bonds is always governed by minimization of the energy of the atoms involved. As we turn our attention to the structures of solids, we ll see that the same principle still holds. The structure of any solid, whether crystalline or amorphous, is determined by the balance of attractive and repulsive forces between the atoms or molecules involved. In this section, we will consider the nature of intermolecular forces—the forces between molecules. 

Intermolecular Forces Intermolecular forces, which ate responsible for the nonideal behavior of gases, also account for the existence of the condensed states of matter— Uquids and solids. They exist between polar molecules, between ions and polar molecules, and between nonpolar molecules. A special type of intermolecular force, called the hydrogen bond, describes the interaction between the hydrogen atom in a polar bond and an electronegative atom such as O, N, or F. 

Molecular solids are solids whose composite units are molecules. Ice (solid H2O) and dry ice (solid CO2) are examples of molecular solids. Molecular solids are held together by the kinds of intermolecular forces—dispersion forces, dipole-dipole forces, and hydrogen bonding— that we just discussed in Section 12.6. For example, ice is held together by hydrogen bonds, and dry ice is held together by dispersion forces. Molecular solids as a whole tend to have low to moderately low melting points ice melts at 0 °C and dry ice sublimes at -78.5°C. 

The forces of attraction between molecules are known as intermolecular forces. Intermolecular forces vary in strength but are generally weaker than bonds that join atoms in molecules, ions in ionic compounds, or metal atoms in solid metals. Compare the boiling points of the metals and ionic compounds in Figure 5.8 (on the next page) with those of the molecular substances listed. Note that the values for ionic compounds and metals are much higher than those for molecular substances. 

Conventional failure considerations have neglected strain-related dimensioning for the obvious reason that failure caused by large deformations is rare for most components, especially machine elements made from metals. Plastics, however, exhibit low stiffness due to the structure of their macromolecules and intermolecular, physical bond forces. Their stiffness is often less than 1/100 of steel, while their strength is only 1/10 of steel. 

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