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trons. (Remember the acronym “OIL RIG”: Oxidation Is Losing, Reduction Is Gaining.) The oxidation reaction is as follows:
Al ➔ Al3+ + 3 e–
Silicon begins with a + 4 charge in silicon dioxide and ends up with
a zero charge in its elemental form. It has been reduced, since it has
gained electrons. The reduction reaction is as follows:
Si4++ 4 e– ➔ Si
Self-cooling can
Self-cooling cans are more recent than self-heating cans.
PepsiCo, Inc.,was set to launch a self-chilling can in 1998 but
scrapped the idea, due to environmental concerns over the coolant that was being used. Today, self-chilled cans are a reality, with
beverages such as West Coast Chill Pure Energy, an energy drink
that sells for about $4 per can.
While the self-heating can is the result of an exothermic
reaction, the self-cooling can is the result of an endothermic
reaction. To understand how an endothermic process works,
it is important to distinguish between the system and its surroundings. In an exothermic process, energy is released from
the system to the surroundings, heating up the surroundings.
In an endothermic process, energy moves from the surroundings to the system. So if energy is removed from the surroundings, they will experience a decrease in temperature.
In the base of the self-cooling can is a cylindrical
chamber of high-pressure carbon dioxide gas, which
ends in a valve that extends through the base of the can
and is capped by a button. When the user pushes the
button, the valve opens and the carbon dioxide rushes
out of the bottom of the can into the air.
As a stream of carbon dioxide gas is released,
a temperature drop of 30 °F ( 14 °C) occurs, taking
about 3 minutes. Heat always travels from hot to cold,
so through the process of conduction, the heat from
the can enters the cooler cartridge, and the can and its contents
become cooler. Anytime gas is released through a small opening, a
drastic temperature change occurs. The same thing occurs when a gas
grill is used—the propane tank gets very cold.
To explain these phenomena, it is important to realize that because
the carbon dioxide gas in the canister is under high pressure, when it
is allowed to escape, it undergoes a rapid decrease in pressure and, as
it expands, it absorbs energy from the surroundings. In this case, the
liquid beverage in the can represents the surroundings.
In order for the carbon dioxide to expand, intermolecular forces of
attraction between the molecules of carbon dioxide need to be overcome. Separating the molecules requires work, and the energy for that
work needs to come from the surroundings. This energy is supplied
by the liquid beverage and is used to move the carbon dioxide molecules apart from one another.
A second effect further cools the gas. As a gas expands and pushes
its way through a small opening, more intermolecular forces of attrac-
tion are overcome. Imagine trying to push a large
object through a small opening—you would likely
break it apart. To break apart anything requires the
absorption of energy.
Likewise, when gas molecules that form intermo-
lecular forces among one another try to exit through
a small opening, these intermolecular forces need
to be broken. In this case, other gas molecules sup-
ply that energy, breaking apart the molecules next to
the hole and cooling the surrounding molecules that
provided that energy. This cooling effect, which results
from a gas passing through a small opening, is known
as the Joule-Thomson effect.
Self-heating and self-cooling cans are not the only con-
sumer products that use thermodynamic processes. Hand
warmers and cold packs have been on the market for quite
some time. Seat cushions that keep you warm and pillows
that keep you cool are also available on the market today. You can even
buy self-cooling and self-heating clothes. Each of these products make
use of some really cool (or hot) chemistry. What will scientists, engi-
neers, or inventors think of next?
SELECTED REFERENCES
Scott, D.; Meadows, R. Hot Meals. ChemMatters, Feb 1992, pp 12–13.
Tinnesand, M. Why Cold Doesn’t Exist. ChemMatters, Oct/Nov 2013, pp 10–11.
Birkett, D. All Wrapped Up. Chemistry World, Oct 1, 2003: http://www.rsc.org/
chemistryworld/Issues/2003/October/ wrappedup.asp [accessed Sept 2016].
Horovitz, B. Single-Serving Coffee Can Heats Itself. USA Today, Dec 19, 2004:
http://usatoday30.usatoday.com/money/industries/food/2004-12-19-self-
heat_x.htm [accessed Sept 2016].
HeatGenie: http://www.heatgenie.com/ [accessed Sept 2016].
Chill-Can®: http://www.chillcan.com/ [accessed Sept 2016].
Brian Rohrig is a science writer who lives in Columbus, Ohio. His most recent
ChemMatters article, “Guilty or Innocent? Fingerprints Tell the Story,” appeared
in the October/November 2016 issue.
18 ChemMatters | DECEMBER 2016/JANUAR Y 2017 www.acs.org/chemmatters
Fire Extinguishers and
Self-Cooling Cans
Old fire extinguishers used carbon dioxide gas. When sprayed, the carbon dioxide that was
released was so cold that you could actually see
little flecks of dry ice—solid carbon dioxide. One
early inventor of a self-cooling can using carbon
dioxide got his idea from a cook he knew during
the Vietnam War. The cook would cool down cans
of beverages for the officers by spraying them
with a carbon dioxide fire extinguisher.
—Brian Rohrig
