Chemical Insights
Heat Packs
A skier is trapped by a sudden snowstorm. After
building a snow cave for protection, she realizes her
hands and feet are freezing; she is in danger of frostbite. Then she remembers the four small packs in
her pocket. She removes the plastic cover from each
one to reveal a small paper packet. She places one
packet in each boot and one in each mitten. Soon
her hands and feet are toasty warm.
These “magic” packets of energy contain a
mixture of powdered iron, activated carbon, sodium
chloride, cellulose (sawdust), and zeolite, all moistened by a little water. The paper cover is permeable
to air.
The exothermic reaction that produces the heat
is a very common one—the rusting of iron. The
overall reaction can be represented as
4Fe(s) 3O2(g) 88n 2Fe2O3(s)
H° 1652 kJ
although in reality it is somewhat more complicated.
When the plastic envelope is removed, O2 molecules
penetrate the paper, causing the reaction to begin.
The oxidation of iron by oxygen occurs naturally.
Any steel surface exposed to the atmosphere inevitably
rusts. But this oxidation process is quite slow—much
too slow to be useful in hot packs. However, if the
iron is ground into a fine powder, the resulting
increase in surface area causes the reaction with
oxygen to be fast enough to warm hands and feet.
The packet can produce heat for up to six hours.■