Core Concepts of Intermolecular
Forces: Vapor pressure-bioling point relationships and bond strengths lead to their selection
|With the engineering of a mechanical refrigerator in
hand, various refrigerants were tried. Of primary importance was their relationship of
vapor pressure to boiling point. In order to obtain an internal temperature in the cold
chamber of the refrigerator low enough to preserve, yet not freeze food, refrigerants
boiling in the range of -10 to -40oC./760 torr were needed. This
temperature can be obtained with a wide variety of liquified gases whose boiling points
are in that range.
But the number of candidates decreases considerably when
their vapor pressures, toxicity and flammability are taken into consideration.
and sulfur dioxide (SO2)
were initially chosen as refrigerants for commercial use. Both have desirably high latent heats of vaporization. However, they have the obvious
drawbacks (to the chemist, at least) of being toxic and corrosive, and with NH3,
flammable. The Frigidaire Division,
now a member of the Electrolux Group, but then a division of General
Motors, realized that their use would considerably limit markets for mechanical
refrigerants. And serious industrial accidents are documented when they were used.
EPA requires that Material Safety Data Sheets (MSDS) be prepared for
chemicals in commercial use. The DuPont Company, whose emphasis
on safety dates back to their origins as an explosives manufacturer,
maintains an MSDS Search
Engine that you can use to obtain MSDS information
|Compounds with high vapor pressures would
require heavy-duty equipment to contain them. To illustrate, carbon dioxide (CO2)
would seem to be ideal for many reasons. Carbon dioxide is low in cost, non-toxic,
non-flammable and relatively non-reactive.
But carbon dioxide's vapor pressure at -25oC,
the temperature at which we want to expand the gas and remove the heat from the
refrigerated substances, is about 15 atm. At the higher temperatures at which we
want to lose heat to the surroundings, above room temperature, the vapor pressure is above
60 atm. However, early on CO2 was used on naval vessels
where safety considerations are of utmost importance.
Therefore, Frigidaire recruited three scientists, Robert McNary, Thomas Midgley and Albert
Henne, to try to identify safe, effective refrigerants. Henne had been studying the
synthesis of fluorocarbons since the 1920's. They quickly concluded that small
molecules having C-F bonds (fluorocarbons) were the best bets. And CF2Cl2,
dichlorodifluoromethane had the best properties.
The very strong C-F bond imparts inertness, low toxicity and
non-flammability to the fluorocarbons. The low cohesive forces that fluorocarbons possess
lead to low boiling points, viscosities and surface tensions that are desirable in
refrigerants. However, this causes them to have latent heats of vaporization which are
lower than most other potential refrigerants. In addition to the desirable properties
already mentioned, fluorocarbons have been found to have good heat capacities and thermal conductivities.
Let's look at some of the physical properties of carbon dioxide, ammonia,
sulfur dioxide and CF2Cl2. We
can learn much about the relationship between intermolecular forces (cohesive forces) and physical
||Pressure at 0oC
||Pressure at 50oC
We order the gases by molecular weight.
- Compare the boiling points of ammonia and
CF2Cl2. These two compounds have about the same boiling
point, yet one has MW 17, the other 121. Ammonia molecules are held together by
hydrogen bonds. Ammonia behaves as if it had a higher molecular weight. Only
the weaker London dispersion forces hold the CF2Cl2 molecules
- Consider the boiling points of carbon dioxide and sulfur dioxide. What accounts for the
huge difference in boiling point. Molecular weight accounts for part of the
difference. But carbon dioxide is linear, sulfur dioxide is bent. The bent SO2
has a dipole and the dipole-dipole interaction increases the intermolecular forces and
raises its boiling point.
Initially, there was concern about the toxicity of
fluorocarbons based on earlier experiences of other researchers. This proved to be a false
concern related to toxic impurities present in early samples. Midgley dramatically
demonstrated the low
toxicity and non-flammability of purified CF2Cl2
(CFC*-12) at a 1930 American Chemical Society meeting by inhaling the gas and then
using the inhaled vapors to extinguish a lighted candle!
For these many reasons, CFC-12 quickly became the refrigerant of choice for most
* - Chlorofluorocarbon
Latent Heat of Vaporization: The higher the latent heat of
vaporization, the more heat can be removed in refrigeration per mole of refrigerant gas.