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Name: Justin
Status: student
Grade: 9-12
Location: VA



Question:
How could Van der Waals forces be strong enough to hold paraffin molecules together to form a solid?


Replies:
Justin,

Remember that London Forces (or Van der Waals forces) depend on the center of negativity being offset from the center of positivity. This means that, if we consider electrons as particles, whenever the center of mass of all the electrons do not coincide with the nucleus (the center of mass of all the protons), then there is a momentary dipole. This now means that the more nuclei and electrons there are, the more possibility for these momentary dipoles to appear. Since the number of nuclei and electrons increase with molar mass, the collective strength of the London Forces should increase as molar mass increases.

Look up the trend of molar mass and boiling point of the following compounds: methane, ethane, propane, butane, pentane, etc.

Greg (Roberto Gregorius)


Paraffin waxes are long molecules, 20-30 carbons long, which means each molecule is quite large and heavy. Paraffin wax is a long "alkane" -- "alkane" is a chemical term that refers to molecules with the molecular formula C(n)H(2n+2), in other words just carbon and hydrogen, no other elements. You might know that the bigger the molecule, the more likely it is to be a solid at a given temperature. In the case of alkanes, the smallest ones, like methane CH4 or ethane C2H6, are gases at room temperature and pressure. Some larger alkanes, like octane or hexane, are liquids at room temperature and pressure. Very large ones, like the 25-carbon paraffin wax, are solids at room temperature and pressure.

Generally speaking, van der Waals forces are any non-covalent forces that attract molecules together. In terms of why a given substance is a gas or a liquid or a solid, you have figure out which state the substance is "happiest" in -- in this case "happy" means lowest energy. With very large molecules, there are simply more spots to be attracted, and because the molecule is larger, it is vibrating comparatively less than smaller molecules. The reason these molecules are attracted to each other is because it takes less energy to be next to another thing that is like them (you may have heard the saying "like attracts like") that being next to something different. So paraffin would rather be next to paraffin than it would next to air or water, or something else different. Pressure plays a big role here -- at higher pressures, the molecules are pushed closer together, and if they are attractive, this makes them more likes to condense instead of be a gas. Butane is a liquid inside a lighter (it is pressurized), but becomes a gas when the pressure is released. Add up all the effects together, and you can figure out if the molecules are "happiest" as a gas, liquid, or solid at a given set of conditions.

With paraffin, you have a substance that is "happier" being a solid than it is being a liquid or gas at room temperature. However, if you add a little energy -- make it jiggle a touch more -- you can get it to liquefy. Paraffin wax melts easy, at relatively low temperatures. What happens here is the thermal vibrations overcome the intermolecular attractions, and the solid melts.

Hope this helps,
Burr Zimmerman


Hi Justin,

One source of Van de Waal's forces is that momentary asymmetry in the electron distribution of even a symmetric molecule such as the long -CH2-CH2-CH2- chains that make up paraffin, act like temporary dipoles that induce similar dipoles in nearby molecules. Thus, the molecules attract each other and tend to form an orderly, almost crystalline arrangement. This is especially prevalent in long thin molecules like paraffin, and much less so in the shorter (but otherwise similar) molecules like Ethane, Butane, and so on. The relative strength of these forces in long chain molecules like paraffin, accounts for this material's being a solid at room temperature. Medium length molecules like Cetane are semi-oily liquids. Even shorter ones like Pentane or Octane are low viscosity liquids, and even shorter ones like Ethane or Butane are gases at room temperature.

Regards,
Bob Wilson.


Paraffin molecules are really big, so there are lots of van der Waals interactions holding them together. If a single pebble were to land on you, you could walk without difficulty. If a cubic meter of gravel were to fall on you, you probably would not be able to move. The van der Waals interactions that stick paraffin molecules to each other are like those pebbles. In large numbers, they add up.

Richard Barrans
Department of Physics and Astronomy
University of Wyoming


The origin of the van der Waals attraction is the dipole induced by the motions of electrons in a molecule (call it A) by all its neighbors, this dipole in A in turn induces dipoles in all the neighbors. There are two factors which make this apparently weak interaction significant. First, it is always attractive. That is the field of an induced dipole (+)------(-) always induces a dipole with the opposite direction (-)--------(+). That makes the interaction attractive (like the poles of a magnet, but one in which the interaction is turning "on" and "off" very rapidly. Second, the induced-dipole / induced-dipole interaction (an alternative name for the van der Waals interaction) occurs between all electrons, they don't have to have a particular molecular structure -- so the interaction is always "on" -- whether the molecule is ionic, dipolar, or whatever.

Hydrocarbons, in addition, have three other factors at work. First if the hydrocarbon has non-symmetrical structure, it has a permanent dipole, which can interact with similar dipolar hydrocarbons both by a dipole to dipole interaction. Second, by an interaction between the permanent dipole and by a transient dipole that it induces in its neighbors. Thirdly, if the hydrocarbon has more than 6 carbons in a row. the chains can get tangled producing a structure (in the extreme) of a bowl of cooked spaghetti. In high molecular weight waxy hydrocarbons this is a major contributor to holding the hydrocarbon together as a solid.

Vince Calder



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