Thermodynamics of exothermic reactions in hand warmers

[sfensphan]

I was reading online about the reactions used in hand warmers. One website that their version always heats up to 54 C (http://www.heatinaclick.com/Default.asp?sType=0&PageId=81390). This one works by a supersaturated fluid of sodium acetate crystallizing. Other hand warmers work by oxidation of iron(2) to iron(3). This reaction is usually listed as an amount of energy released per kg or mole. Both are a reaction of some sort but there seems to be a contradiction here. How could a handwarmer guarantee a certain temeprature? If the chemicals in the handwarmer were really cold, couldn't the energy that is released be insufficient to raise it to a specific temperature. On the other hand, I remember in chemistry lab that a defining characteristic of a material is it's melting temperature, when it shifts from solid to liquid. Is there a contradiction here or is it because one is a chemical reaction and the other is a change of state that allows them to specify what temperature it occurs at?If it was really cold then the reaction would have to generate more energy than if it was really warm to achieve the 54 degrees. That seems to violate a principal.

My apologies if this is confusing. I am as well. Many thanks for your comments. I greatly appreciate your time and thoughts.

 

[Aaron Bex]

That's a great question! It's definitely confusing to think about how a hand warmer could guarantee a certain temperature while also having to produce more energy when it's colder out. One possibility is that, when the hand warmer is exposed to a colder environment, the reaction rate of the chemicals inside the hand warmer would increase, leading to a larger amount of energy being released and thus a higher temperature. The other possibility is that the hand warmer is designed to produce a constant amount of energy, regardless of the external temperature, meaning that when it's colder out, the energy is more concentrated in a smaller area and thus has a larger effect on the temperature. It's hard to say without knowing more information about the specific hand warmer, but those are two possibilities.