[curiouscat]

Came across an interesting blog post (link below) on the quite scary sounding  dioxygen difluoride:  FOOF. One comment there intrigued me:

"Sulfur compounds defeated him, because the thermodynamics were just too titanic. Hydrogen sulfide, for example, reacts with four molecules of FOOF to give sulfur hexafluoride, 2 molecules of HF and four oxygens. . .and 433 kcal, which is the kind of every-man-for-himself exotherm that you want to avoid at all cost."

Now, H2S + four gmoles FOOF together is ~314 gms (34 + 4 * 70). That'd mean an exotherm of ~6 MJ/kg (if I did the math right!)

So a kilogram of that H2S+FOOF cocktail  should just about be enough to evaporate 2.5 kg of water? Is that such a high exotherm? Am I doing this right? Hmmmm....

What gives?

PS. Note that there might be other things scary about this reaction, not denying that. (e.g. entropy, all those gas molecules, maybe speed, low initiation barriers, toxicity etc.) But in itself, is this exotherm of 433 kcal as scary as this guy makes it sound?

http://pipeline.corante.com/archives/2010/02/23/things_i_wont_work_with_dioxygen_difluoride.php

[discodermolide]

Sounds reasonably normal to me. Maybe he is scared about something else?

[Corribus]

Heat of combustion of acetylene is about -310 kcal/mol (http://www.nist.gov/data/PDFfiles/jpcrd6.pdf). The heats of combustion of higher order alkanes can be easily in the 1000s of kcals/mol. So, yeah it's a lot of heat, but I don't think heat-production would be my (only) metric for a scary reaction.

On the other hand, water takes crapload (technical term) of heat to evaporate (think how long it takes to fully evaporate a kettle of water on a gas stove at full power!), so I don't think that would be my metric for a scary reaction either.

(Maybe the copious amount of HF produced would qualify as a better metric for scary in this case.)

[curiouscat]

Heat of combustion of acetylene is about -310 kcal/mol (http://www.nist.gov/data/PDFfiles/jpcrd6.pdf). The heats of combustion of higher order alkanes can be easily in the 1000s of kcals/mol. So, yeah it's a lot of heat, but I don't think heat-production would be my (only) metric for a scary reaction.

Yes, but doing it on a per mole basis gives the higher alkanes an inherent advantage. Thinking in terms of mass seems more natural for a scariness evaluation. In any case, burning an alkane doesn't seem very scary.

To restate my question: On a pure exotherm/kg-reactants basis what would be a value (kJ/kg) that gets a seasoned chemist scared.

Anyone?

PS. ....or maybe kJ/kg is a metric irrelevant to scariness for an experienced chemist? Dunno.

[curiouscat]

On the other hand, water takes crapload (technical term) of heat to evaporate (think how long it takes to fully evaporate a kettle of water on a gas stove at full power!), so I don't think that would be my metric for a scary reaction either.

True but to me thinking in terms of water is intuitively relevant since in most lab or industrial situations that's a common, easy, cheap way to carry away your exotherm.

(Maybe the copious amount of HF produced would qualify as a better metric for scary in this case.)

Yep. Agree. I was only singling out his mention of that exotherm. To me it sounds like a bad argument in the limit of the data provided.

[curiouscat]

Sounds reasonably normal to me. Maybe he is scared about something else?

Thinking more on this, in addition to the kJ/kg value of a transformation, shouldn't "scariness" be fundamentally related to some form of kJ / kg-sec metric? i.e. the same 1000 J is more scary when evolved in a millisec. rather than 10 sec.

Is some sort of such hybrid unit of exotherminess in typical use? i.e. a hybrid of kinetics & thermochemistry. Sounds like a useful way to think about exotherm danger.

[Corribus]

Actually it occured to me while writing the above that the amount of heat released per mole of material probably isn't the best metric to use anyway, because there is a lot more mass in one mole of some materials than other. The amount of energy liberated per mass would be a better metric in my view. Consider that the combustion energy per mole of alkane climbs quite quickly as the number of carbons increases - this isn't because higher order alkanes combust more explosively or anything. It's just because a mole of heavier alkane has more combustible mass than a mole of lighter alkane. On a per mass basis, most of the alkanes have the same combustion energy (~11-12 kcal/g, because essentially the same amount and same type of bonds are breaking per unit mass).

Considering that the reaction under question gives 433 kcal/mol for the reaction of 4 moles of FOOF with H2S*, and a molecular weight of 70 g/mol for FOOF, this means that the reaction liberates 1.546 kcal/g of FOOF consumed. Compare that with acetylene, which liberates ~11.5 kcal/g of acetylene combusted, and it's really not all that impressive. As far as noncombustion reaction goes, it's pretty big - you are breaking a relatively weak O-O bond and forming some strong O=O bonds, not to mention H-F bonds) - but I wouldn't call it scary based on just quantity of heat alone.

*It's a little unclear whether this is the case from the way it's written in the article. Even if it's 433 kcal/mole of FOOF consumed, you're still left with a heat of reaction of ~6 kcal/g of FOOF reacted, which is much less heat produced than that for combustion of alkanes. I suppose we could figure it out by looking at bond energies involved, but I'm not so eager to spend the time.

@ curious,

Yes, I'd say kinetics would be an important factor in "scariness".

[curiouscat]

On a per kg reactants basis, the combustion of H2 in O2 is about 16 MJ/kg.

So that sounds a darn more impressive. 3 times as much eh?