Intermolecular forces in solids

The big difference in melting points suggests a difference in type of crystal binding. The intermolecular forces in solid CO2 must be very low to be overcome by a low-temperature sublimation. CO2 is actually a molecular lattice held together only by the weak van der Waals forces between discrete CO2 molecules. Si02 is a covalent lattice with a three-dimensional network of bonds each silicon atom is bonded tetrahedrally to four oxygen atoms and each oxygen is bonded to two silicon atoms. 

What physical properties should you consider in comparing the strength of intermolecular forces in solids and in liquids ... 

A common feature of all clathrates discussed so far is a host lattice, by itself thermodynamically unstable, which is stabilized by inclusion of the second component. The forces binding this component must be similar in nature to the intermolecular forces in liquids. It seems natural, therefore, to regard a clathrate compound as a solid solution of the second component in the (meta-stable) host lattice. 

Crystal engineering. Utilization of noncovalent intermolecular forces in the solid state to design new nanomaterials with desired functions. 

Every gas changes into a liquid if the pressure is high enough and the temperature is low enough. The atoms or molecules of a liquid or solid stick together in a finite volume rather than expanding, as a gas does, to fill all available space. This cohesiveness comes from electrical forces of attraction between the negative electron cloud of each atom and the positive nuclei of other atoms. We describe intermolecular forces in Chapter 11. 

Figure 5.2 (a) Electron density contour map of the CI2 molecule (see Chapter 6) showing that the chlorine atoms in a CI2 molecule are not portions of spheres rather, the atoms are slightly flattened at the ends of the molecule. So the molecule has two van der Waals radii a smaller van der Waals radius, r2 = 190 pm, in the direction of the bond axis and a larger radius, r =215 pm, in the perpendicular direction, (b) Portion of the crystal structure of solid chlorine showing the packing of CI2 molecules in the (100) plane. In the solid the two contact distances ry + ry and ry + r2 have the values 342 pm and 328 pm, so the two radii are r 1 = 171 pm and r2 = 157, pm which are appreciably smaller than the radii for the free CI2 molecule showing that the molecule is compressed by the intermolecular forces in the solid state. 

In ordinary solids such as crystalline or amorphous glassy materials, an externally applied force changes the distance between neighboring atoms, resulting in interatomic or intermolecular forces. In these materials, the distance between two atoms should only be altered by no more than a fraction of an angstrom if the deformation is to be recoverable. At higher deformations, the atoms slide past each other, and either flow takes place or the material fractures. The response of rubbers on the other hand is almost entirely intramolecular [4,5]. 

Plasticizers weaken the intermolecular forces in the PVC reducing crystallinity. A relatively stable suspension, called a plastisol, of finely divided PVC in a liquid plasticizer, can be poured into a mod and heated to about 175°C producing a solid flexible plastic as a result of fusion of the plasticizer in the PVC. 

This chapter continues to explore states of matter by focusing on the gas state. Table 8.1 summarized the main features of gases. In the gaseous state, molecules are much farther apart than in either solid or liquids. Because of this distance molecules in the gaseous state have virtually no influence on each other. The independent nature of gas molecules means intermolecular forces in this state are minimal. A gas expands to fill the volume of its container. Most of the volume occupied by the gas consists of empty space. This characteristic allows gases to be compressible, and gases have only about 1/1,000 the density of solids and liquids. 

At the same time that Dalton proposed his ideas on partial pressure, he developed the concept of vapor pressure. A vapor is the gaseous form of a substance that normally exists as a solid or liquid. A gas is a substance that exists in the gaseous states under normal conditions of temperature and pressure. The vapor pressure of a liquid is the partial pressure of the liquid s vapor at equilibrium. Liquids with strong inter-molecular forces exert lower vapor pressures than those with weak intermolecular forces. In liquids with strong intermolecular forces, it is more difficult for the molecules to leave the liquid state and enter the gaseous state. 

The increases in melting points are not as regular as the increases in boiling points because the intermolecular forces in a solid depend not only on the size of the molecules but on how well the molecules fit into the crystal lattice. 

The sudden expansion of liquid CO2 does work pushing back the atmosphere and overcoming intermolecular forces in the liquid. The energy to do that work comes from the molecules themselves, so the average energy of the molecules is lowered. The CO2 condenses to a solid because of this loss of energy. 

Since a spectrum is determined by the structure of the sample, which in turn is a con-.sequence of the forces (and particularly the intermolecular forces in the solid state), this technique is able to provide two-fold information. An introduction to the application of the discussed methods towards the investigation of intermolecular forces is provided in Sec. 5.2, including a short review of the quality which can be obtained. Another obvious application is the identification of samples. Table 4.5-1 demonstrates clearly that, if the formula is known, the spectra provide an indication of the phase of a sample. By using micro-techniques it is possible to identify samples or parts of samples in fields ranging as widely as geology, medicine (kidney stones), pharmacy, electronics, painting (mineral pigments), and materials research. 

It is generally possible to study the effects of changes of state on the intramolecular and intermolecular forces In inorganic and organic compounds by investigating the variations in the vibrational spectra with decreasing temperature as the gas phase changes to the liquid and then to the solid state. 

The intermolecular forces in a crystal lattice are often not homogeneous in all directions. If the solid consists of strong interactions among neighbors in specific layers, and weak interactions among molecules in neighboring layers e.g., graphite),... 

The properties of equations such as (3) and (4) which are not allowed by RMT are understood satisfactorily only in the relatively uninteresting linear case where, for example, rise and fall transients mirror each other as exponentials. When this frontier is crossed, the applied field strength is such that it is able to compete effectively with the intermolecular forces in liquids. This competition provides us with information about the nature of a molecular liquid which is otherwise unobtainable experimentally. This is probably also the case for internal fields, such as described by Onsager for liquids, for various kinds of intmial fields in int ated computer circuits, activated polymers, one-dimensional conductors, amorphous solids, and materials of interest to information tedmology. The chapters by Grosso and Pastori Parravidni in this volume describe with the CFP some important phenomena of the solid state of matter in a slightly different context. 

In the late 1800s and early 1900s, scientists were still struggling to understand intermolecular forces, so it is doubtful that Oscar Wilde had a clear picture of intermolecular forces in mind when he wrote of the subtle affinity between chemical atoms in The Picture of Dorian Gray. Nonetheless, his description of subtle affinity is quite apt. Intermolecular forces are complex, consisting of attractions as well as repulsions. Intermolecular attractions are those between water molecules that allow water to condense once it has been sufficiently cooled—and intermolecular repulsions are what make water feel like a solid mass when it is forcefully encountered. (Have you ever been knocked over by a wave ) If it were not for intermolecular attractions, our bodies would vaporize into gases, and if it were not for intermolecular repulsions, we would collapse into unimpressive puddles. 

Enzymes, such as the one we used in our demonstration, are governed by the principles of chemical kinetics—one of the many links between the basic principles of chemistry and the intricate chemistry of life. Our rapid and cursory survey of biochemistry here, combined with our previous discussions of biochemical systems, shows that in life all our chemical principles come into play acid-base reactions, redox reactions, chemical bonding, intermolecular forces, concentration, solids and solubility, kinetics, and even phase transitions and the gaseous phase. 

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