The vapor pressures of pentane, hexane and heptane are compared using barometers to show the effect of molecular size on vapor pressure.
The measurement of pressure exerted by a vapor is demonstrated using barometers. Vapor pressure varies with the strength of the intermolecular forces in the liquid.
The vapor pressures of pentane, hexane and heptane are compared. Pentane, hexane and heptane differ only in the length of their carbon chain, and have the same type of intermolecular forces, namely London dispersion forces. London forces increase with molecular size (number of electrons in a molecule). The London forces between the smaller pentane molecules are much weaker than the forces between the larger heptane molecules. Therefore pentane has the largest vapor pressure and heptane has the smallest vapor pressure.
Pentane, hexane and heptane differ only in the length of their carbon chain, and have the same type of intermolecular forces, namely dispersion forces. Dispersion forces increase with molecular weight.
Pentane, the smallest of the three, is injected (into the open end of the barometer, it rises to the top) and vaporizes. Here, the mercury is depressed from 730 to 285 millimeters. When hexane is injected, the hexane rises to the top and vaporizes. Here, the mercury is depressed to 596 millimeters. When heptane, the biggest of the three, is injected, the heptane rises to the top and vaporizes. Here, the mercury is depressed to only 686 millimeters.
The dispersion forces between the smaller pentane molecules are much weaker than the forces between the larger heptane molecules. Therefore pentane has the largest vapor pressure and heptane has the smallest vapor pressure.
The magnitude of London dispersion forces depends on the polarizability of a molecule, that is, the ease with which the molecule's electron cloud can be distorted. The bigger the molecule is and the more electrons it has, the bigger the London forces are. Bigger molecules usually have larger molecular weights; hence the correlation of vapor pressure with molecular weight.
- Design and Demonstration
- Kristin Johnson University of Wisconsin - Madison, Madison, WI 53706
- Text
- Kristin Johnson University of Wisconsin - Madison, Madison, WI 53706
- John W. Moore University of Wisconsin - Madison, Madison, WI 53706
- Video Production
- Jerrold J. Jacobsen University of Wisconsin - Madison, Madison, WI 53706
- Graphics
- Kristin Johnson University of Wisconsin - Madison, Madison, WI 53706