Intermolecular forces, weak

Intermolecular Forces = weak attractions between separate molecules (e.g., two H2O molecules)... 

Bfi and 022- However, in the second binary, intermolecular forces between unlike molecules are much stronger than those between like molecules chloroform and ethyl acetate can strongly hydrogen bond with each other but only very weakly with them-... 

In most covalent compounds, the strong covalent bonds link the atoms together into molecules, but the molecules themselves are held together by much weaker forces, hence the low melting points of molecular crystals and their inability to conduct electricity. These weak intermolecular forces are called van der WaaFs forces in general, they increase with increase in size of the molecule. Only... 

The attention of this article is focused on physical adsorption, which involves relatively weak intermolecular forces, because most commercial appHcations of adsorption rely on this phenomenon alone. Chemisorption is discussed only briefly in some sections on specific appHcations. 

Substitution of fluorine for hydrogen in an organic compound has a profound influence on the compound s chemical and physical properties. Several factors that are characteristic of fluorine and that underHe the observed effects are the large electronegativity of fluorine, its small size, the low degree of polarizabiHty of the carbon—fluorine bond and the weak intermolecular forces. These effects are illustrated by the comparisons of properties of fluorocarbons to chlorocarbons and hydrocarbons in Tables 1 and 2. 

The general properties of the resins are much as to be expected. They have very good heat resistance but are mechanically much weaker than the corresponding organic cross-linked materials. This weakness may be ascribed to the tendency of the polymers to form ring structures with consequent low cross-linking efficiency and also to the low intermolecular forces. 

In a thermoplastic material the very long chain-like molecules are held together by relatively weak Van der Waals forces. A useful image of the structure is a mass of randomly distributed long strands of sticky wool. When the material is heated the intermolecular forces are weakened so that it becomes soft and flexible and eventually, at high temperatures, it is a viscous melt. 

The intermolecular forces operative in nonpolar compounds are also electrostatic-in nature. These weak van der Waals forces involve attraction between nonbonded atoms and are effective over short ranges only. 

The degree of polarity has considerable influence on the physical properties of covalent compounds and it can also affect chemical reactivity. The melting point (mp) and boiling point (bp) are higher in ionic substances due to the strong nature of the interionic forces, whereas the covalent compounds have lower values due to the weak nature of intermolecular forces. 

Synthetic rubber (elastomers) are high molecular weight polymers with long flexible chains and weak intermolecular forces. They have low crystallinity (highly amorphous) in the unstressed state, segmental mobility, and high reversible elasticity. Elastomers are usually cross-linked to impart strength. 

The generally low melting and boiling points of molecular substances reflect the fact that the forces between molecules (intermolecular forces) are weak. To melt or boil a molecular... 

When iodine chloride is heated to 27°C, the weak intermolecular forces are unable to keep the molecules rigidly aligned, and the solid melts. Dipole forces are still important in the liquid state, because the polar molecules remain close to one another. Only in the gas, where the molecules are far apart, do the effects of dipole forces become negligible. Hence boiling points as well as melting points of polar compounds such as Id are somewhat higher than those of nonpolar substances of comparable molar mass. This effect is shown in Table 9.3. 

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. 

Thermal expansion — as elasticity — depends directly upon the strength of the intermolecular forces in the material. Strongly bonded materials usually expand little when heated, whereas the expansion of weak materials may be a hundred times as large. This general trend is confirmed by Table 5.1. The coefficient of thermal expansion a was found to be lower in the crosslinked polymers and higher in the less crosslinked or thermoplastic materials as observed by Nielsen [1], In addition, Table 5.1 presents the Young s moduli E of the polymers at ambient temperatures as well as the products a2E. The values of oc2E are all close to 13.1 Pa K 2 with a coefficient of variation of 1.6%. 

The presence of intermolecular forces also accounts for the variation in the compression factor. Thus, for gases under conditions of pressure and temperature such that Z > 1, the repulsions are more important than the attractions. Their molar volumes are greater than expected for an ideal gas because repulsions tend to drive the molecules apart. For example, a hydrogen molecule has so few electrons that the its molecules are only very weakly attracted to one another. For gases under conditions of pressure and temperature such that Z < 1, the attractions are more important than the repulsions, and the molar volume is smaller than for an ideal gas because attractions tend to draw molecules together. To improve our model of a gas, we need to add to it that the molecules of a real gas exert attractive and repulsive forces on one another. 

Fig. 15. The low-frequency vibrational mode spectrum for 8CB in its smectic phase. The broad and weak spectral feature at about 8 cm has been attributed to intermolecular forces extending across smectic layers...

Such weaknesses of the present implementation include the lack of an explicit inclusion of intermolecular forces other than excluded volume, resulting in a qualitatively inaccurate description of the equation of state. Another weakness is that the model shows lattice artefacts when dealing with problems of polymer crystallization or liquid-cristalline order only rather flexible poly-... 

Why is polyethylene a solid at room temperature when the only intermolecular forces it experiences are the weak dispersion forces ... 

Plasticization has been explained by a variety of theories in an attempt to explain how the plasticizer reduces the rigidity of the final part. All theories rely on the premise that the plasticizer reduces the strength of the intermolecular forces between the polymer chains. The theories fall into two broad categories interference mechanisms and expansion mechanisms. The interference mechanisms state that plasticizer molecules interact only weakly with the polymer chains after separating the chains from one another, thereby reducing the overall cohesion of the material. The expansion mechanisms state that the reduced rigidity arises from an increase in the free volume of the system as the system expands to incorporate bulky,... 

The term molecular crystal refers to crystals consisting of neutral atomic particles. Thus they include the rare gases He, Ne, Ar, Kr, Xe, and Rn. However, most of them consist of molecules with up to about 100 atoms bound internally by covalent bonds. The dipole interactions that bond them is discussed briefly in Chapter 3, and at length in books such as Parsegian (2006). This book also discusses the Lifshitz-Casimir effect which causes macroscopic solids to attract one another weakly as a result of fluctuating atomic dipoles. Since dipole-dipole forces are almost always positive (unlike monopole forces) they add up to create measurable attractions between macroscopic bodies. However, they decrease rapidly as any two molecules are separated. A detailed history of intermolecular forces is given by Rowlinson (2002). 

This structure contains a total of five bonds, which is an average of 2.5 bonds per NO unit. Therefore, there is no net increase in the number of bonds in the dimer compared to two separate molecules. The result is that there is not much energy advantage if dimers form. The melting point of NO is -164 °C and the boiling point is -152 °C. The low boiling point and small liquid range, about 12 °C, is indicative of only very weak intermolecular forces. The Lewis structure of the molecule can be shown as... 

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