Hand Warmers

INTRODUCTION:

      Hand warmers were first invented in 1924 and were used extensively by Japanese soldiers during the winters of World War Two.   Contemporary hand warmers come in three basic types, all of which function through exothermic reactions.   Temperatures can range from 50°C to 65°C and can last anywhere from 30 minutes to 24 hours.   The first type generates heat through crystallization of a super-saturated solution (usually sodium acetate).   Bending a small metal strip inside the pouch creates a nucleation site.   Boiling this type of warmer and letting it cool returns the pouch to its initial state, making it completely reusable (2).
The second type of hand warmer generates heat through combustion of lighter fluid in the presence of a platinum catalyst.   These warmers are encased in a metal container and generate radiant heat without a flame (1).   These are also reusable, but must be recharged by adding lighter fluid.
The third type, which we propose to investigate further, involves the reaction of powdered iron with oxygen.   This reaction is non-reversible, and is similar to the rusting process of iron.   The active ingredients are iron filings, activated carbon, vermiculite, salt and water.

INVESTIGATION:

When the pouch is opened and exposed to air, the iron filings begin to oxidize.   The main chemical reaction that generates heat is as follows:

2Fe(s) + O2   (   2FeO + energy

      To calculate the Gibbs Free Energy for the total reaction we need to find the enthalpies of formation:

Fe ( 2e- + Fe2+ G° = Hf= 0
½ O2 +2e- ( ½ O22- G° = Hf= 0
½ O22- + Fe2+ ( FeO G° = Hf= -278 kJ/mol
Fe + ½ O2 ( FeO G° = Hf= -278 kJ/mol

The Gibbs Free Energy ( ΔG°)of this reaction is _______.
The molar Enthalpy ( ∆H) of this reaction is -278 kJ/mol.   This is an exothermic reaction.
      In addition to iron filings, carbon is also present in the hand warmer.   The main function of the carbon in the pouch is to disperse the heat...