Methane London forces

Molecular solids have their lattices composed of molecules held in place by London forces, dipole-dipole forces, and hydrogen bonding. Solid methane and water are example of molecular solids. 

We now have three substances remaining methane, CH4, methyl fluoride, CH3F, and krypton difluoride, KrF2. We also have two types of intermolecular force remaining dipole-dipole forces and London forces. In order to match these substances and forces we must know which of the substances are polar and which are nonpolar. Polar substances utilize dipole-dipole forces, while nonpolar substances utilize London forces. To determine the polarity of each substance, we must draw a Lewis structure for the substance (Chapter 9) and use valence-shell electron pair repulsion (VSEPR) (Chapter 10). The Lewis structures for these substances are ... 

The molecular geometry of methane and of methyl fluoride is tetrahedral. In the case of methane, this symmetrical arrangement of polar covalent carbon-hydrogen bonds leads to a canceling of the bond polarities resulting in a nonpolar molecule. As a nonpolar molecule, the strongest intermolecular force in methane is a London force. In methyl fluoride, a fluorine atom replaces one of the hydrogen... 

It is interesting to note that methane, ethane, and ethylene are all gases hexane, octane, and nonane are all liquids (at room conditions) while low molecular weight PE is a waxy solid. This trend is primarily due to an increase in the mass per molecule and to an increase in the London forces per polymer chain. The London force interaction between methylene units is about 8 kcal/mol. Thus, for methane molecules the attractive forces are 8 kJ/mol for octane it is 64 kJ/mol and for PE with 1000 ethylene (or 2000 methylenes) it is 2000 methylene units X 8 kJ/mol per methylene unit = 16,000 kJ/mol, which is well sufficient to make PE a solid and to break backbone bonds before it boils. (Polymers do not boil because the energy necessary to make a chain volatile is greater than the primary backbone bond energy.)... 

It is of interest to note that under normal conditions methane, ethane, and ethene are all gases, and hexane, octane, and nonane are all liquids, whereas polyethylene is a waxy solid. This trend is primarily due to both an increase in mass per molecule and an increase in the London forces per molecule as the chain length increases. 

Salts are immiscible with non-polar solvents such as hexane and tetrachloro-methane. The forces between ions (such as Na" and Cl ) and molecules of non-polar solvents are London dispersion forces. Such forces are much weaker than the attractions between opposite ions which hold the sodium chloride lattice together. 

Intermolecular forces can be of three different tjq>es. The first t5q>e is called a London force or dispersion force. This very weak type of attraction generally occurs between nonpolar covalent molecules, such as nitrogen (Nj), hydrogen (H ), or methane (CE ). It results from the ebb and flow of the electron orbitals, giving a very wesik and very brief charge separation around the bond. 

Dispersion Forces If we consider a substance like methane where the particles are nonpolar molecules, we find that the melting point and boiling point are very low — 182.6°C and — 162°C, respectively. Instead of asking, Why does methane melt and boil at low temperatures a more appropriate question might be Why does methane, a nonionic, nonpolar substance, become a liquid or a solid at all The answer to this question can be given in terms of attractive intermolecular forces called dispersion forces or London forces. 

On the other hand, for aqueous systems, the geometric mean rule deteriorates and large correction parameters are needed, as shown here for the water-methane system. This is because of the hydrophobic effect, in general and, more specifically, the strong water-water interactions which are much more important than the water-methane interactions. Moreover, as indicated above, the geometric mean mle is strictly valid - derived for molecules having only dispersion (London) forces. 

While these two effects are undoubtedly relevant in solids in which molecules are polar, neither can play any part in the numerous structures, such as those of the inert gases, methane, hydrogen, benzene and many others, in which the atoms or molecules possess no permanent dipole moment. Moreover, even when a dipole moment does exist, the forces predicted are far smaller than those experimentally observed. Some other component of interatomic force must therefore exist, and it was London who first ascribed this to a dispersion effect associated with the dynamic polarization of an atom or molecule arising from its zero-point motion. Even if we consider a completely symmetrical system, with no permanent dipole moment, it will still be possible for such a system to possess an instantaneous moment, since at any given instant the electrons will not necessarily be distributed with the high symmetry 8... 

Weaker secondary bonds act between molecules. Thus, below — 182°C, methane is a solid, the covalent molecules being held in a solid lattice be weak secondary bonds. These weak forces are associated with interactions between dipoles. Three different types of interaction have been described by London, Debye and Keesom, known respectively as dispersion, induction and orientation forces see Table 1 and Dispersion forces and Polar forces. The three types of interaction are often referred to collectively as van der Waals forces, as indicated in Table 1. However, it is necessary to note that some authors use the term van der Waals to refer exclusively to dispersion forces, the other two types being referred to as polar forces . Table 2. (The term dispersive is sometimes used by francophone authors writing in English where dispersion would be correct.)... 

Molecular Atoms or molecules London dispersion, dipole-dipole forces, hydrogen bonds Fairly soft, low to moderately high melting point, poor thermal and electrical conduction Argon, Ar methane, CH4 sucrose, C12H22O11 DryIce ,C02... 

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