how do those pocket hand-warmers (that you rub to generate heat) work?
In: Chemistry
The disposable sort are a packet filled with basically iron filings and a salt. The iron filings combined with the salt, moisture in the air, and oxygen from the air, rust (the salt helps this process). The process of iron rusting is an exothermic reaction (it gives off heat), which is what makes the hand-warmers warm.
If you seal the packet away from air (oxygen), it will stop rusting, and stop giving off heat. That’s why they sometimes seem to stop working if you put them in your boot or too airtight a glove or pocket. You could also arrest the reaction by sealing them in an airtight plastic bag until you wanted to use them again.
They have a limited lifespan, and stop giving off heat when all the iron filings inside have basically rusted completely. It’s pretty brilliant.
Disposable handwarmers basically purposely cause fine iron filings to rust when activated. This is an exothermic reaction and therefore releases heat.
Reusable packs are similar [(relevant Technology Connections)](https://youtu.be/Oj0plwm_NMs) but use a chemical solution instead. When cooled to the appropriate temperature then activated, they begin to crystallize and this process releases the heat. The video I linked does a good job of explaining this in more detail.
If you mean the ones I think, they are basically two or more chemicals sat in a pouch together, and they’ll react together, but they need something to start a reaction first. It’s like firewood sitting in air; there are all of the ingredients there for a fire but they need something to kick them off.
For those hand warmers, it’s generally a little metal piece inside, that you “click” to set it off. That provides the energy/prompt to start the reaction, and then it keeps going, giving off heat until it’s done.
I’m not entirely sure if the common model uses a burst of energy or an exposure of a catalyst, but the principle remains the same. Incidentally, this is basically how batteries work too; they’re just little cells full of chemicals, and when they’re prompted to work those chemicals react, pumping out energy (in that case, through electricity more than heat) until they run out.
Okay, so you know how oxygen interacts with iron to create rust? When that happens, a little bit of heat is released. Those hand warmers work on the same principle, they just have other chemicals to help the iron in them rust *really fast*.
When you shake them, or rub them, you’re exposing more of the filling to oxygen, keeping the reaction going.
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