In the flameless ration heater, the following chemical reaction
takes place:
Mg + 2 H2O ➔ Mg(OH) 2 + H2 + energy
Magnesium also reacts readily with oxygen in the air. As a result,
any magnesium metal that is exposed to air quickly develops a layer
of magnesium oxide (MgO). Before magnesium can react with water,
this coating of magnesium oxide on the outer layer of the metal must
be removed. This is where the salt comes in. The salt, in an aqueous
solution, dissolves this oxide layer, exposing the pure magnesium underneath, which then reacts with water. The reaction, while highly exothermic, is very slow. To increase the rate of the reaction, small amounts of
elemental iron are added, helping to speed it up.
In the above reaction, energy must be absorbed to break the bonds of
the reactant water molecules. But when magnesium hydroxide is formed,
energy is released. More energy is released than absorbed, so the reaction is exothermic. It is this excess energy that heats the food. Flameless
ration heaters are inexpensive and available for purchase at army surplus
stores, truck stops, and online.
To help visualize why the absorption of energy results in bond breaking, imagine ripping a piece of paper in half. To break the bonds of the
paper, you must add energy. Or consider holding an ice cube in your
hand. As energy from your hand goes into the ice cube, the hydrogen
bonds—which are intermolecular forces of attraction between water
molecules within the ice—are broken, and the ice melts. Anytime energy
is added to a system, bonds are broken.
It is a little harder to visualize why bond formation results in a release
of energy, but an analogy may help. Consider taking two powerful
magnets and allowing them to come together as their opposite poles
are placed near one another. You can hear an audible clack as the magnets snap together. What you hear is energy being released as sound.
(Separating the magnets would require an input of energy, and no sound
would be heard, because energy would be absorbed.)
If you examine the above reaction with the flameless ration heater, you
will notice that hydrogen gas is released. Problems could arise if this
device were used near an open flame, as hydrogen is highly flammable.
Also, it takes 10 to 12 minutes for the flameless ration heater to heat,
and ultimately cook, your food. So some safer (and faster) alternatives
were sought for the consumer market.
HeatGenie
One commonly used self-heating can is known as the HeatGenie.
The heating element is self-contained and is nestled into the bottom
of the can. It can heat up a 10-ounce (295-milliliter) product in about
2 minutes, reaching a temperature of 63 °C. The canister contains two
dry substances—powdered aluminum and silicon dioxide (SiO2). When
you push the button, a thermomechanical process generates heat, and
provides the energy needed to initiate the reaction. The reaction is exothermic and proceeds as follows:
4 Al + 3 SiO2 ➔ 2 Al2O3 + 3 Si + energy
HeatGenie uses a thermite reaction, which is a highly exothermic
oxidation-reduction (redox) reaction of a solid fuel with a solid oxidizer.
In the above reaction, aluminum is the fuel and silicon dioxide is the
oxidizer, which provides the oxygen for the reaction. The reaction occurs
quickly because the oxygen is supplied internally, as opposed to being
supplied from the outside air.
In a redox reaction, electrons are lost by one substance and gained by
another. The aluminum, being a free element, has a beginning charge of
zero. Aluminum ends up with a charge of + 3 in the product, aluminum
oxide (Al2O3). When each aluminum atom is oxidized, it loses three elec-
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A heating element at the bottom of a HeatGenie can contains aluminium
and silica, which are not mixed. By pressing a button on the bottom of
the can (see photo above), these two chemicals are mixed; they undergo
an exothermic reaction that gives off a large amount of heat. The heat is
absorbed by the food or beverage inside the can, which becomes hot within
a few minutes.
