Hard warmer conundrum…David? Marco?

The heat comes from a reaction of the iron in the packet and the oxygen in the air. So you start with iron, end with iron oxide. Mass isn’t being created, but you are bringing mass that was outside the packet (in the air) to inside the packet.

Some moisture from the air is also probably absorbed, but oxygen is going to be the primary addition.

Thank you, Jim. I wasn’t sure that the reaction was iron > iron oxide, but now it makes sense. So the iron in this case combined with 1.5 gm of oxygen. That seems like quite a bit, but it is what it is, I guess. I wonder if the same reaction would occur at altitude, but maybe slower due to the lower concentration of oxygen in the air. My house is at 5440′ elevation, so someone at sea level might see a different performance. And at 10,000′ it could be different as well.

I opened another one of the Hot Hands warmers just now, and I will let it sit overnight to see if it pulls in even more oxygen and be even heavier once the reaction is completed.

“Therefore the weight of the iron filings etc. was 19.7 gm, which one doesn’t have to carry out as trash (if one so chooses).”

Please, Please, Please – carry out this trash.  ALL trash.

We don’t need to add rusting iron to orange peels, egg shells, and coffee grounds.

Thanks

Gary, Jim is correct. Note that there are several other reactions going on, also. So, even with very precise measurements you would not work out even. Iron is a rather complex mineral, generally. Oxides can undergo phase differences Fe2O3 can be black(magnetite) or red(rust.) Some bacteria will use iron as a “food” source, though I forget which ones. Some is necessary to nutrition (hemoglobin, etc.) Iron is generally fairly well distributed over the planet and is the fourth or fifth most common element in soils. It is generally ok to scatter the remains from a packet.

I would think that iron would be different than egg shells, orange peels, and coffee grounds, especially when not in heavy traffic areas like the AT.

Hand warmers are ok for a night or two but quickly get heavier than gear that is warm enough without warmers.

Yes, iron to iron oxide adds the weight of the atmospheric oxygen consumed in the reaction.  But iron oxides also form hydrates, so there is some additional water weight, too.

Teaser alert: I have a new design for a aluminum Moulder Strip that is super cheap (between free and 28 cents), light (12 grams) and easy to make (scissors and a rolling pin).  I can’t be sure, not having tested it (we’re only getting down to 26F-ish now) but my sense is that it’s good for operation to at least 0F.

Back to the hand warmer: If you brought a sturdy(!) 2″x3″ ziplock, or maybe the thicker vinyl pouches to keep your wallet dry at the beach, and, you know, peed in it, you’d have an ounce or two of 98F water to place under your canister.  Reusable, the contents are disposable, and it’s only the baggie you need to pack in and out.  Also, the timing is better: the BTUs in the pee will be mostly transferred in tens of minutes, versus the iron-based hand warmer going on for hours afterwards.

If it was still April 1st, I would have explained that the warm air around the packet rises up in the cold surrounding air.  Since the air is being pushed up, something else must be pushed down by Newton’s Third Law of Motion and that something is the packet and scale pan causing the scale to indicate an apparently higher weight.

I am actually in the Mihalik camp myself. The only thing that I sometimes leave as a trace (knowingly) is a golf ball-size smooth/polished river rock that I come across a mile or so from the camp site I’m headed to. They are great for my rock bag to toss over the food poles in GNP and YNP and elsewhere. I leave them at the base of a tree near the food hang area for someone else to use. Probably the only way I could get busted for this is if some geologist comes along and thinks “This stone doesn’t belong here; it isn’t part of the local geology.”

So the iron in this case combined with 1.5 gm of oxygen. That seems like quite a bit…

Well, that is less than an 8% gain in weight.  Fe203 is 43% heavier than plain iron.  So that shows you how little of the iron is reacting to iron oxide.  Particularly when you consider that the 8% gain in weight is at least in part water.

After David brings in the bit about iron hydrates, you add even more to think about, Ben. Why do you suppose more of the iron doesn’t react with the oxygen? Is it possible that there are other chemicals added to prevent the hand warmer from getting TOO hot. Maybe they just don’t want it to get too heavy with lots and lots of iron oxide.

This is getting more and more complicated by the hour. I knew that all (or most) oxidation reactions are exothermic, but I didn’t think about iron hydrates (which I assume is not an exothermic reaction, is that correct?).

One thing I learned today – if that impish David Thomas would have posted that bit on April 1st, there would have been a LOT of you learned engineers bash him hard. I love the amount of knowledge, and the willingness to share, here on BPL. I just wish that my pre-med course schedule would have allowed me to study more physical chemistry (as well as forestry and wildlife management), but I was too bogged down with calculus, physics, and things like embryology, cell biology, and the other chemistry classes to do much more than a relaxing minor in anthropology. The Viet Nam war just gave me 4 years of undergrad work, and I now wish I’d had a couple more to learn the other good stuff. Oh well…

Thread drift, since I’m logged in now – my snow melting trials showed that I could get 2 liters of snow melt in roughly 18.5 minutes, using 15-16 gm of canister fuel with my JB MiniMo pot, BRS-3000T stove, and the pot riser disk. The MSR Reactor took the same amount of time, but it consumed 20.0 gm of fuel. It’s somewhat of a fuel hog, which I’ve suspected. And it’s  heavy (but pretty wind-proof).

hy·drate
ˈhīˌdrāt/Submit
nounCHEMISTRY
plural noun: hydrates
1.
a compound, typically a crystalline one, in which water molecules are chemically bound to another compound or an element.

“wish that my pre-med course schedule would have allowed me to study more physical chemistry”

One of the other fathers on our Father-Daughter-Math-Pioneering backpacking trips first got his ChemEng degree.  That wasn’t challenging enough, so he went back to med school.  No surprisingly, his (and his MD wife’s) daughters are stand outs on the math team I coach.

“Why do you suppose more of the iron doesn’t react with the oxygen? Is it possible that there are other chemicals added to prevent the hand warmer from getting TOO hot.”

Well, Iron filings are not all that reactive except when a packet is opened and exposed to oxygen. The outside layers (don’t forget, we are talking a large particle here, not anywhere near molecule sized) will oxidize first. This will act as an insulator, to diffusion, of more oxygen into the particle. So, the reaction slows down. Since rust will “flake” off, being larger than iron, of iron particle, it will open up areas to continue the reaction but not the entire surface area. In the mean time the reduced oxygen means other compounds will form: Fe3, Fe4, Fe5 etc will bond with appropriate O molecules in a “lean” environment. OH will also bond with it. With the hydrogen bonding with another oxygen making it highly reactive (H30) and (another OH and 0). Like I say, it gets complicated. All rections generally push towards stability. More stable molecules are generally smaller, but, iron is an exception, hence the “flaking”. For the same reason, your car does not rust from the inside out. Only at the surface and only slowly.

This means that no inhibitors are needed. It just slows down naturally. Increasing the surface area, by finely dividing the iron, helps speed the reactions. A drop of water will speed the reactions, of course, making the pad a bit warmer, I think. Never tried it.

about-to-expire hand warmer

This could have had something to do with it.  The contents may have already partially reacted or been otherwise compromised.

Is it possible that there are other chemicals added to prevent the hand warmer from getting TOO hot.   -GD

Not to slow it down, but to speed it up.  They pack these things with salt, water, and a water absorbent to keep the water in there.  Anyone who has been to the snow belt knows what water and salt does to the iron oxidation reaction!  James has a good explanation on why you want to speed up the reaction and you won’t react everything.

Man, how do you guys know all of this stuff anyway? I’ve been using hand warmers for what, 30+ years? Over this time I’ve occasionally asked others how they worked, and nobody I’ve ever talked to could tell me. This thread should be archived somehow, as it contains all the information anybody would ever need to know about hand warmers.

I think I will follow Stephen’s suggestion and open a “not-at-all-close-to-expiring” hand warmer and see how it performs compared with the one that would have expired in May. Then I will drive into Boulder and pick up my on-line order at REI. For some giggles, I will wander around the camping section and chat with my buddies that work the floor. I will steer the conversation over to hand warmers and, you know, see what happens. They will think I am a genius, thanks to y’all.