Eli5 How do heat patches work and how do they stay hot for so long?


Eli5 How do heat patches work and how do they stay hot for so long?

In: Chemistry

I’m assuming you mean chemical ones, and not electrical, given the tag is “Chemistry”.

It’s essentially analogous to a fire. A few chemicals combine in a way that reduces their total energy (sort of like how a ball will fall to earth to reduce its gravitational potential energy).

The potential energy the chemicals had before is turned into particle wiggles (sort of like how a ball falling to earth starts bouncing and making sound). Temperature is basically just a measure of how wiggly a substances particles are, so the energy converting into wiggling generates heat.

This is basically how a fire burns. It takes two chemicals that have high potential energy (oxygen and a hydrocarbon) and convert them to chemicals with lower energy, turning the excess energy into particle wiggling, aka heat.

Chemical heat patches keep their heat for long periods of time the same way a fire does. It just keeps converting chemicals to keep producing heat.

the most common form of heat patch happens to be a simple bag of iron powder. (and a little salt).

when you open up the outer bag, oxygen gets in, and the iron powder starts to oxidise. (eli5 the bag of iron dust starts to rust).

this process is exothermic, which means that it gives off heat.. that heat also means that the air trapped inside the bag expands, which stops any additional oxygen rich air getting into it quite so quickly.. which slows down the rate at which the reaction can happen at.

they can also control the size of the dust, as finer dust will have more surface area, and will get hotter, but not for as long, and coarser powder will have less surface area, so wont get as hot, but will last longer.

you can also granulate the powder (like the granules in instant coffee), so that when the outside of the granule has oxidised, it flakes off, revealing a fresh area to be oxydised.

most heat packs are also covered in a fairly thick layer of cotton with very carefully engineered holes in, which both slows down the reaction, AND makes sure that no part of the powder can get out.

(also, as a bonus, those Flameless ration heaters in MRE’s are also just small bags of iron dust.. just with a watersoluable coating on them, so that when they get wet they get REALLY hot for as long as possible)