The vapor pressures of methanol and ethanol 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 methanol and ethanol are compared. Methanol and ethanol have similar structures and similar intermolecular forces, but differ in molecular size. Methanol has the higher vapor pressure because its molecules are smaller than those of ethanol, and so its intermolecular forces are less than ethanol's.
Methanol and ethanol have similar structures and similar intermolecular forces, but differ in molecular weight. Which has the higher vapor pressure?
When methanol is injected, the methanol rises to the top and vaporizes. The mercury column is depressed to 634 millimeters. When ethanol is injected, the ethanol rises to the top and vaporizes. Here, the mercury column is only depressed to 688 millimeters.
Methanol has the higher vapor pressure because its molecular weight is less than ethanol and so its intermolecular forces are less than ethanol's.
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