One of the most revealing of all physical properties for a chemical substance is its boiling point. Boiling point reflects the strength of the intermolecular attractive forces that hold the molecules of a substance together in a condensed phase, and as such, it is useful to compare the boiling points for related compounds to see how structural differences account for the differences in intermolecular attractions. After briefly reviewing the nature of intermolecular attractive forces, this page will examine trends in boiling points for various groups of compounds to help the reader understand how size, shape, and functional group polarity affect boiling point.
|dipole moument||electronegativity||hydrogen bond|
|inductive effect||lone pair electros||normal alkane|
|Non-uniform motionof electrons
in atoms and molecules
|2. Dipole-dipole interactions||Electrostatic attraction between (+) and (-) ends of molecules||all molecules having polar covalent bonds|
|3. Hydrogen bonding||An especially strong sort of dipole-dipole interaction||...molecules having H atom(s) covalently bound to N,O, or F|
|Compound||Formula (mol. wt.)||B.P.|
|propane||C3 H8 (44)||-42°C|
|n-butane||C4 H10 (58)||0°C|
|n-pentane||C5 H12 (72)||36°C|
Molecular nitrogen (N2) and carbon monoxide (CO) have identical molecular weights: 28g/mol. Which has the higher boiling point? Before looking at the answer, how do you know you are right?
Self-test question #3
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Version 1.1.7, 3/17/97