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.
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:
Outdoor components of a residential air-source heat pump A heat pump is a machine or device that transfers thermal energy from one location, called the “source,” which is at a lower temperature, to another location called the “sink” or “heat sink”, which is at a higher temperature. Thus, heat pumps moves thermal energy opposite to the direction that it normally ...
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 produced by the reaction. Vermiculite is also present and acts as an insulator for retaining the heat. If the vermiculite were not present, the heat would dissipate too quickly. This would have two negative consequences: the heat would be dissipated in a short amount of time causing a drastic elevation of temperatures which could cause burns to the skin, and the warmer would not last very long. Vermiculite is a non-toxic mineral that expands as it is heated. This expansion process is called exfoliation (wikipedia).
The water is present mainly to dissolve the salt into ions. These ions then act as catalysts in the oxidizing process.
To calculate the amount of iron present inside the pouch, a simple comparison of masses can be done. The difference between the mass of a fully reacted warmer and a freshly opened warmer will give us the mass of oxygen reacted with iron during the process. This assumes that no other reactions occur within the pouch and that the amount of carbon, vermiculite and salt remains constant. Knowing this, it is easy to calculate the number of moles reacted, and hence the total enthalpy. This will then allow us to calculate the heat content of the system.
By finding the heat capacity of carbon and air (21% oxygen and 79% nitrogen), the entropy of the system can be calculated using the following formula:
dS = (dq/T) = (dH/dT) = (Cp/T)dT (at constant pressure)
The (maximum) temperature of the surface of the pouch will be measured by placing the activated pouch inside of a standard ski mitt along with a thermometer to record temperature change and the hand of an experimenter. The temperature will be measured at various time intervals to investigate the variation of temperature with time. These results will be graphed. We will assume that heat lost through the mitt is negligible, and that enough air enters the mitt to initiate the reaction (an investigation of efficiency of the mitt could also be undertaken).
PLANNING Investigating the Kinetics of the reaction between Iodide ions and Peroxodisulphate (VI) ions By the use of an Iodine clock reaction I hope to obtain the length of time taken for Iodine ions (in potassium iodide) to react fully with Peroxodisulphate ions (in potassium Peroxodisulphate). I will do three sets of experiments changing first the concentration of iodide ions, then the ...
The rate of reaction can be controlled by limiting the amount of oxygen that the iron filings are exposed to. Another experiment could involve the application of specific amounts of oxygen to the iron and measuring the total times of reaction.