Re: Instant Disposable Heat Packets . . . How do they Work?

There are two types of heat packs.  One based on the supercooling of
sodium acetate and these are reusable.  The use once and dispose kind,
that I think you are asking about, contains:  iron (Fe), water (H2O),
cellulose (saw dust), vermiculite (a mica mineral, a filler sometimes
used as a packing material), salt (NaCl), and activated carbon.  The
reactions that occur are complicated and my references indicate that
they are not completely understood.  So, I give the facts I can
determine as well as some educated guesses.

The reaction that is occurring is the rapid oxidation of iron.  In
other words the iron is rusting (corroding), very fast. This reaction
gives off the heat.  

Iron that is kept perfectly dry does not rust.  Rusting (oxidation of
iron) requires both water and oxygen (O2).  Salt in the water
accelerates the corrosion reaction.  The electrochemical processes
that are presumed are:
 
Fe  ---> Fe(+2)  +  2(e-)             (equation 1) 
2H3O(+)  +  2(e-)  ---->  2H  +  H2O      ( equation 2)
 
Dissolved oxygen reacts with the H atoms to form H2 gas:
2H  +  1/2 O2 (aq)  ---->   H2O   (equation 3)
Where (aq) means aqueous, the oxygen is dissolved in the water.

The oxygen and water also react with the Fe(+2)  ions from equation
(1) to produce hydrated iron(III) oxide [Fe2O3(H2O)x], rust:
 
4 Fe(+2)  +  O2  +  (12+x) H2O--------->
                          2Fe2O3(H2O)x  +  8H3O(+)   (equation 4)
The H3O(+)  (consider this to be an acid species) generated in
equation (4)   is consumed in equation (2). 

The salt accelerates the reaction.  I presume (guess) that the
activated carbon accelerates (facilitates) the reaction in equations
(2) & (3), the formation of water from the acid species and electrons
generated in the corrosion reaction.  I further guess that the
cellulose and vermiculite form a soupy or jelly like mass with the
water and help the heat to be more evenly distributed. 
The reaction is spontaneous since, under the conditions in the heat
pack, the oxidation of iron is energetically favored.  So much so that
a large amount of heat is released during the reaction. 
 
The reaction stops when the iron is used up.  

There are many references on corrosion in the library.  It is likely
that new information has been published since my reference came out
(1970).  Check with a Chemistry teacher/professor.  You might check if
the manufacturer of the heat pack might have literature references on
the chemical reactions or a patent reference.

Reference: F. Brescia, J. Arents, H. Meislich, A. Turk, Fundamentals
of Chemistry: A Modern Introduction, Academic Press, 1970, page 311.