Quick chemistry question, for chemistry-knowing peeps...

[DanceswithUnix]

592kJ of heat is enough to heat a mole of reactants by around 10,000K, though in practice you'd lose a lot to the surroundings. It's easily enough heat to cause the magnesium to spontaneously combust.

My lunch was rated as 670kJ according to the label, a tin of carrot and corriander soup. I have yet to explode or combust, though the day is young

As Scary said, we are way under 1mol of reactants. But if you only have 2g of reactants then lets round that up to 6kJ of heat, enough to raise a litre of water by 1.4C in temperature. That could be quite exciting, I mean not take your head off exciting unless you actually swallow the stuff, but sounds like a potentially nasty burn.

(from 1J can raise the temperature of 1g of water by 0.24C).

[Butcher]

KMnO₄ has a molecular mass of 158g/mol, so your mass calculation's off there. You're looking at more like 200g of mixed reactants (rather than the 5 - 10g of mixed reactants in a worst case spill for the situation the OP's discussing).

158g of KMnO4 + 48.6g of Mg (2 moles) = 206.6g, however that's 3 moles of reactants. 1 mole is a third of that, or 68.9g. Hence approximately 70g. Not to mention my temperature calcs were per mole, if you scale it down you still get similar temperature rises (though likely greater loss to environment).

Only if the solids will spontaneously react. Here's a really good video:

Note how at the start they're prodding the powder around with an unlit match to mix it and nothing's happening? The reaction - as I thought I remembered - needs some activation energy ... and THEN it's pretty violent. But it takes a lit match to kick it off in the video - I don't think they're going to get enough energy just rattling around in a box if there's a small spill...

It's not so much the activation energy as getting Mg into contact with KMnO4. Mg shavings will have an oxide layer from exposure to air which is not reactive with KMnO4. Applying heat will reduce the oxide layer back to Mg (or it vaporises the MgO leaving exposed Mg) which allows the reaction to proceed. If you mixed them vigorously enough you could probably scrape the oxide layer off the Mg and start the reaction that way.

[Butcher]

My lunch was rated as 670kJ according to the label, a tin of carrot and corriander soup. I have yet to explode or combust, though the day is young

As Scary said, we are way under 1mol of reactants. But if you only have 2g of reactants then lets round that up to 6kJ of heat, enough to raise a litre of water by 1.4C in temperature. That could be quite exciting, I mean not take your head off exciting unless you actually swallow the stuff, but sounds like a potentially nasty burn.

(from 1J can raise the temperature of 1g of water by 0.24C).

See above for why 200g is 3 moles not one.

2g would give about 18kJ of heat, which can heat a mole of Mg by 750K, or a tenth of a mole (2.4g) by 7500K. So if you have a couple of grams of excess magnesium it's going to get very hot indeed.
Mg has an autoignition temperature of 475C so if you get it above this it will burst into flames at which point you have a magnesium fire which will likely set anything nearby on fire.

Also, for reference of ~600 kJ / mol not being much, burning hydrogen gives just 286 kJ / mol which is more than enough to heat things up and start fires. 600 kJ / mol is closer to the heat of burning methanol.

[Ttaskmaster]

Mg has an autoignition temperature of 475C so if you get it above this it will burst into flames at which point you have a magnesium fire which will likely set anything nearby on fire.

Sooo..... still not worth the risk?

[ik9000]

This is a hazard sheet for magnesium ribbon - which SFAIK is pure magnesium so akin to shavings. https://beta-static.fishersci.com/co...-m/S25397A.pdf

Choice quotes:

Reactivity: Reacts violently with water.
Chemical stability: Stable under normal conditions.
Possible hazardous reactions: Emits flammable gas when in contact with water.
Conditions to avoid: Air and moisture sensitive.Incompatible materials.
Incompatible materials: Strong oxidizing agents, acids, Acid chlorides, Halogens
Hazardous decomposition products: Magnesium oxide

Additional property: Hygroscopic (absorbs moisture from the air)

Conditions for safe storage, including any incompatibilities:
Store contents under inert gas.Air and moisture sensitive.Store in a cool location.Keep away from food and beverages.Protect from freezing and physical damage. Provide ventilation for containers. Keep container tightly sealed. Store away from incompatible materials

[Butcher]

Sooo..... still not worth the risk?

I'd say not.

[Ttaskmaster]

I'd say not.

I shall maintain the good fight, then!

Reactivity: Reacts violently with water.

Wow, didn't know that one...
Plenty of guys carry magnesium blocks for shaving slivers onto the tinder pile, and they get rained on all the time.

[Butcher]

Wow, didn't know that one...
Plenty of guys carry magnesium blocks for shaving slivers onto the tinder pile, and they get rained on all the time.

The reaction is very slow at room temperature (and not at all violent). If you chuck some magnesium in water it will slowly evolve hydrogen gas (and by slowly I mean after some minutes).

The risk for the MSDS is that as temperature increases so does reaction rate. Magnesium reacts rapidly with steam producing a typical white flame. Hence if you try to douse a magnesium fire with water it will flash to steam and feed the fire, not to mention it will release hydrogen gas as part of the reaction which may then explode.

I find it weird that it's used for fire lighting given how awkward it can be to get going. I guess once it's caught you're sorted though as then any water will tend to feed it, so it can burn on damp tinder.

[Ttaskmaster]

I find it weird that it's used for fire lighting given how awkward it can be to get going. I guess once it's caught you're sorted though as then any water will tend to feed it, so it can burn on damp tinder.

I think any dampness in the tinder would be evaporated off by the flare, rather than being of any use.

Generally it's poured or shaved onto decent tinder and serves only to assist with a few seconds of blazing hot flash-flame, in order to get the kindling going. On it's own, it's pretty useless. Powdered magnesium especially tends to fall through the tinder, whereas what you really want is a pile of powder on something like a piece of birch bark, to which you then add a glowing ember from a fire drill... but again, only to maintain a flame long enough to get your kindling nest alight, and really that ember should be enough on its own.

Given the absolute shower of 3,000ºC sparks you can get off a decent ferrocerium rod, and how well things like waxed jute twine or crampball catch flame, magnesium is not really needed!
It's like having newspaper and kindling and hexamine and firecubes and petrol and white spirit, just to light your barbecue... which in today's world is probably piezoignition gas, anyway!!

[OilSheikh]

Test tubes are pretty durable. How are they going to be kept in the container ? In Test tube stands ?

[blueball]

https://www.youtube.com/watch?v=oZJnbSwjC0s

[Butcher]

Pfft don't mess about with magnesium and the like. Aniline with hydrogen peroxide (doubles as a disinfectant) and you're away!

[Peter Parker]

Sounds like you people are likely to enjoy (or have already read) Derek Lowe's "Things I Won't Work With" - http://blogs.sciencemag.org/pipeline...wont-work-with

People that cry with laughter when reading about chemistry are a small but excellent group.

[Butcher]

Sounds like you people are likely to enjoy (or have already read) Derek Lowe's "Things I Won't Work With" - http://blogs.sciencemag.org/pipeline...wont-work-with

People that cry with laughter when reading about chemistry are a small but excellent group.

Not read that. I can recommend John Clark's ignition though. Speaking of which, it has the ultimate fire starting chemical, chlorine trifluoride, which can ignite anything you like. Wet wood? No problem, it burns wood and water! Sand? No problem. Asbestos? No problem. No fire wood? No problem, it'll burn gravel or concrete.

[Ttaskmaster]

Test tubes are pretty durable. How are they going to be kept in the container ? In Test tube stands ?

As discussed in this post: https://forums.hexus.net/general-dis...ml#post4073633 , although I guess that got buried in the above discussion.

They're plastic pop-top vials (that's actually what they come in from the retailer), carried in a hinged metal tin in a rucksack pocket. All that bouncing around risks popping the vials open, which is where my concerns originate.
They do this for ease of one-handed use, in case you're already injured.

https://www.youtube.com/watch?v=oZJnbSwjC0s

Yeah, no, these guys are just cowboys replicating other peoples' videos. They were once mildly entertaining in passing, but seeing this "Potassium Po... Porm... Pomegranate" vid just put me right off them. I'd rather watch FPSRussia, I think!!

I'll have to dig around, but there's a good one where a British bushcraft guy does a similar demonstration, but in real time and in real British outdoors, to show you the reality of this. The PotPerm & Glycerine reaction takes a good few minutes to even spark off, and the colder it is the longer it takes. The minimum temperature for a favourable reaction is usually reckoned to be about 20ºC, whereas it's about half that here at the minute.
Bear in mind also that, in a survival situation, you're most likely lighting a fire because you're flippin' FREEZING COLD - Last thing you want to do is rely on a reaction that takes at least 10-15 minutes in cold weather and might not even work at all if it's in minus temperatures (not including any wind chill factors)!

Speaking of which, it has the ultimate fire starting chemical, chlorine trifluoride, which can ignite anything you like. Wet wood? No problem, it burns wood and water! Sand? No problem. Asbestos? No problem. No fire wood? No problem, it'll burn gravel or concrete.

And highly impractical/unsafe/stupid to carry in a survival kit/rucksack/pocket, I assume?

[scaryjim]

… They're plastic pop-top vials (that's actually what they come in from the retailer), carried in a hinged metal tin in a rucksack pocket. All that bouncing around risks popping the vials open, which is where my concerns originate. ...

Given this I can understand the concern. I think I'd personally be happy with the arrangement, but if it's easy enough to put them in separate containers then why not!

If there's a good reason for not carrying two tins - and I assume some people think there is, otherwise there'd be no point having this discussion - you could either pad the inside of the tin so the vials can't move around and pop open (although that might just be introducing additional flammable materials into the mix ) or put each vial in a small sealable baggy, so even if the top does pop open the contents won't be loose in the tin. Either choice would mitigate the vast majority of the risk you're concerned about.