Chemistry Question #1803

B.Krystol, a 19 year old male from Jasper asks on January 13, 2004,

Why is Isobutane ( C4H8) a gas at 25 C and Dodecahedrane ( C20H20) a solid at 25 C?

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The answer

Reg Mitchell answered on January 20, 2004

Molecules like the hydrocarbons have only weak attractive forces between them, called Van der Waals forces, and approximately these attractions between molecules increase with the area of contact between molecules. So molecules of the same molecular weight that are sausage shaped like isobutane generally have higher melting points and boiling points than those that are sphere shaped like dodecahedrane. Molecules with more atoms usually are larger than those with less atoms, so have more attractions. Larger molecules thus have more attractive forces to break on going from solid to liquid to gas than smaller molecules. Hence C4H8 is a gas, C20H20 is a liquid, or maybe a solid. The maybe comes in because melting point also has to do with how well the compound packs in to a crystal lattice. The better the packing, usually the higher the melting point.

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