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Physical Properties
Some of the first things you notice about a chemical are its physical properties - its phase (solid, liquid, gas), volatility, viscosity, melting point, boiling point, solubility in water and other solvents. Even having a smell is partly a physical property because the molecules have to evaporate to reach your nose. (Odor can also be considered a chemical property because the molecules have to "react" with your nose.) These physical properties depend on the forces of attraction between molecules. Because alkanes are nonpolar molecules they have very weak van der Waals bonds between the molecules. Large molecules have more points of contact between molecules, so some people say they have stronger van der Waals bonding. My view is that there are more bonds, rather than stronger bonds. Take your choice. Either way the consequence is that the larger molecules hold together better, have higher melting and boiling points, more viscosity and less volatility than the smaller molecules. Alkanes having small molecules (1-4 carbon atoms) are gases at room temperature. Those having larger molecules (5-17 carbon atoms, depending on arrangement) are liquids. Those with still larger molecules (18+ carbon atoms, depending on arrangement) are solids. The fact that larger molecules are heavier and require more energy to move around is also a factor. Because alkanes are nonpolar, they are insoluble in water. They are also less dense than water and thus float on top of it. The reason that alkanes are nonpolar is a combination of two things. First, there is very little difference in electronegativity between carbon and hydrogen. Thus, neither element pulls electrons away from the other very much. Second, the hydrogen atoms so nearly surround the carbon atoms that what little bond polarity does exist is effectively canceled out. That's true not only for the small molecules but for the larger ones as well.
E-mail instructor: Eden Francis Clackamas Community College |
