Intermolecular forces Relatively weak

Intermolecular forces relatively weak interactions that occur between molecules. (16.1)... 

Intermediate a species that is neither a reactant nor a product but that is formed and consumed in the reaction sequence. (12.6) Intermolecular forces relatively weak interactions that occur between molecules. (10.1)... 

If intermolecular forces are weak, it is relatively easy to break up aggregates of molecules to form liquids (from solids) and gases (from liquids). 

An example of a factor influencing the diffusing molecule is whether the gases or vapour are difficult to condense or readily condensable. With gases such as oxygen, carbon dioxide and nitrogen, which are difficult to condense and where the intermolecular forces are weak, the solubility of a permanent gas in plastic is relatively low at normal temperature, i.e. usually less than 0.2%. Where condensable vapours are involved in which the intermolecular forces are stronger, then there are two possible modes of interaction. 

Molecular solids consist of atoms or molecules held together by dipole—dipole forces, dispersion forces, and/or hydrogen bonds. Because the.se intermolecular forces are weak, molecular solids are soft and have relatively low melting points (usually below 200 °C). Most substances that are gases or liquids at room temperature form molecular solids at low temperature. Examples include Ar, H2O, and CO2. 

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. 

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. 

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. 

In molecular covalent compounds, intermolecular forces are very weak in comparison with intramolecular forces. For this reason, most covalent substances with a low molecular mass are gaseous at room temperature. Others, with higher molecular masses may be liquids or solids, though with relatively low melting and boiling points. 

It is especially important to investigate the molecular structure of coordination compounds in the vapor phase because the relatively weak coordination interactions may be considerably influenced by intermolecular interactions in solutions and especially in crystals. It has been shown that the geometrical variations can be correlated with other properties of the molecular complexes ). In particular the structural changes in the F3B N(CH3)3 and CI3B N(CH3)3 molecules ) relative to the respective monomeric species unambiguously indicated boron trichloride to be a stronger acceptor than boron trifluoride. Data on the geometry and force field have also been correlated ). 

Organic solids have received much attention in the last 10 to 15 years especially because of possible technological applications. Typically important aspects of these solids are superconductivity (of quasi one-dimensional materials), photoconducting properties in relation to commercial photocopying processes and photochemical transformations in the solid state. In organic solids formed by nonpolar molecules, cohesion in the solid state is mainly due to van der Waals forces. Because of the relatively weak nature of the cohesive forces, organic crystals as a class are soft and low melting. Nonpolar aliphatic hydrocarbons tend to crystallize in approximately close-packed structures because of the nondirectional character of van der Waals forces. Methane above 22 K, for example, crystallizes in a cubic close-packed structure where the molecules exhibit considerable rotation. The intermolecular C—C distance is 4.1 A, similar to the van der Waals bonds present in krypton (3.82 A) and xenon (4.0 A). Such close-packed structures are not found in molecular crystals of polar molecules. 

PVC and CPVC are resistant to nonoxidizing adds, alkalis, and salts. They are also more resistant than hope or PS to weak oxidizing adds, such as 10% nitric add at room temperature. PVC has a solubility parameter of 9.5 H. Because of its moderate degree of crystallinity and relatively strong intermolecular forces it is difficult to dissolve. It is soluble in cydohexanone... 

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