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Introduction to Boiling Points

For general purposes to measure of the kinetic energy of all the atoms and molecules it is useful to consider temperature in a given system. As the temperature gets increased, there is corresponding increment in the vigor of rotation and translational motions of all molecules, with vibrations of the atoms and groups of atoms within molecules. Knowledge shows that various compounds exist normally in the liquid form and solid form, and at sufficiently low temperature and high pressure even low-density gases, like hydrogen and helium, can be liquified. A clear result drawn by this fact is that intermolecular attractive forces change considerably and that a compound's boiling point is a measure of the strength of these forces. So, it is necessary to increase the kinetic energy of their by raising the sample temperature to the characteristic boiling point of the compound to break the intermolecular attractions that hold the molecules of a compound in the condensed liquid state.

The table illustrates some of the facts that affect the intermolecular attractions' strength. Formula of each entry is followed by its boiling point in degrees Celsius and formula weight in parentheses. 1st there is molecular size. The Large molecules contain more nuclei and electrons that create van der Waals attractive forces, so compounds of large molecules generally have higher boiling points than smaller molecules that are made up of similar compounds. It is most important to apply this rule only to like compounds. The instances that are described in first two rows are the similar in that the shape of molecules or atoms are spherical and do not have permanent dipoles. Molecular shape is also important, because the second group of compounds illustrate. Upper row contains roughly spherical molecules, whereas in the lower row the isomers have linear or cylindrical shaped molecules. The attractive forces among latter groups are generally greater. At last, permanent molecular dipoles that are generated by polar covalent bonds result in even greater attractive forces among molecules provided they have mobility to line up in appropriate orientations. Last entries in the table compare non-polar hydrocarbons with same sized compounds having polar bonds to nitrogen and oxygen. By Halogens Polar bonds to carbon formed but they also raise molecular mass, making it not easy to distinguish among these factors.

Boiling Points (ºC) of Selected Elements and Compounds

Increasing Size

Atomic                Ar (40) -186         Kr (83) -153         Xe (131) -109

Molecular            CH4 (16) -161      (CH3)4C (72) 9.5  (CH3)4Si (88) 27  CCl4 (154) 77

Molecular Shape

Spherical: (CH3)4C (72) 9.5             (CH3)2CCl2 (113) 69           (CH3)3CC(CH3)3 (114) 106

Linear:     CH3(CH2)3CH3 (72) 36     Cl(CH2)3Cl (113) 121          CH3(CH2)6CH3 (114) 126

Molecular Polarity

Non-polar: H2C=CH2 (28) -104        F2 (38) -188                  CH3C≡CCH3 (54) -32           CF4 (88) -130

Polar:        H2C=O (30) -21             CH3CH=O (44) 20          (CH3)3N (59) 3.5                  (CH3)2C=O (58) 56                           HC≡N (27) 26                 CH3C≡N (41) 82             (CH2)3O (58) 50                   CH3NO2 (61) 101

The crystalline solids' Melting points cannot be categorized in as simple as boiling points. In a crystal lattice distance between the molecules is small and regular, with the intermolecular forces serving to constrain the motion of molecules more strictly than in the liquid state. Molecular size is significant but shape is also important, because the each molecules need to fit together cooperatively for the attractive lattice forces to be large. The molecules that are Spherical in the shape usually have comparatively high melting points, which sometimes approach to boiling point. This clarifies the fact that than the other shapes spheres can pack together more closely. This structure or shape sensitivity is one of the reasons that melting points are extensively used to identify particular compounds. Figures in table used to serve to illustrate these points.

Compound

Formula

Boiling Point

Melting Point

pentane

CH3(CH2)3CH3

36ºC

-130ºC

hexane

CH3(CH2)4CH3

69ºC

-95ºC

heptane

CH3(CH2)5CH3

98ºC

-91ºC

octane

CH3(CH2)6CH3

126ºC

-57ºC

nonane

CH3(CH2)7CH3

151ºC

-54ºC

decane

CH3(CH2)8CH3

174ºC

-30ºC

tetramethylbutane

(CH3)3C-C(CH3)3

106ºC

+100ºC

Note that the melting points of even-carbon chains increase more than those of the odd-carbon chains, but the unbranched alkanes' boiling points (pentane through decane) increase rather smoothly with molecular weight,. The Even-membered chains pack together in a uniform fashion more compactly than do odd-membered chains. Last compound, an isomer of octane is nearly spherical and has an exceptionally high melting point (only 6º below the boiling point).

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