Gold dust a fire hazard?
Jeroen Belleman
I received my Goodfellow catalogue today was leafing through it when
my eye fell on this oddity:
High purity gold dust with particle size of 250 micrometres or less is
marked as being highly flammable.
In fact, so is platinum.
I'm no professional chemist, but it strikes me as odd that that these
metals would burn.
Anyone knows for sure?
Best regards,
Jeroen Belleman
jer...@dxcern.cern.ch
A. McPherson
i am no expert in this area, but most dusts are flammable when finely divided.
in fact, many have measurable explosivity ranges, depending on the particle
size and test method. an interesting literature source is "Explosibility tests
for Industrial Dusts", Fire Research Technical Paper No. 21, which can be
ordered from the Application Services Division, Building Research Establishment,
Garston, Watford, WD2 7JR, UK. to get a feeling for burning metals, one need
not look any further than the effects of the exocet missles on some of the
vessel hulls in the Falklands war.
--
Tobacco, coffee, alcohol, hashish, prussic acid, strychnine are
weak dilutions: the surest poison is time.
Emerson. Society and Solitude: Old Age
JOHN ROBERT KITCHIN
there are many metallic dusts which are not only flammable when in a
fine dust, but are pyrophoric (self-igniting). This is definitely true
for metals such as aluminum and zinc, which are commonly used for
explosives and pyrotechnics, so I see no reason why it would be
different for other metals like gold and platinum.
Tim Rolfe
(JOHN ROBERT KITCHIN) writes:
I thought gold was so UNreactive that even concentrated nitric acid
by itself doesn't bother it --- you need aqua regia. Given this, I
do find it surprising to hear that gold can be oxidized, even finely
divided.
--
--- Tim Rolfe
ro...@dsuvax.dsu.edu
RO...@SDNET.BITNET
Jeroen Belleman
ro...@dsuvax.dsu.edu (Tim Rolfe) writes:
>I thought gold was so UNreactive that even concentrated nitric acid
>by itself doesn't bother it --- you need aqua regia. Given this, I
>do find it surprising to hear that gold can be oxidized, even finely
>divided.
Exactly my idea. Many people seem to think that any fine powder,
especially metal powder, is a fire hazard. But there has to be
something in the air it wants to react with. As far as I know,
the O2 in the air is not a sufficiently strong oxidizer for Au.
Even for a half noble metal as Hg, the reaction would rather go
the other way, (I.e. 2HgO -> 2Hg + O2) although admittedly at
room temperature, nothing much happens.
I rather suspect this was brought to us in the same spirit that
makes people write 'Can be harmful if swallowed' on a box of
sand for parakeets.
Best regards,
Jeroen Belleman
jer...@dcxcern.cern.ch
David Toland
>by itself doesn't bother it --- you need aqua regia. Given this, I
>do find it surprising to hear that gold can be oxidized, even finely
>divided.
I`ve been mildly curious about this also, so I finally looked in my CRC.
There is an oxide listed, Au2O3, but it *LOSES* oxygen as you heat it,
so oxygen combustion seems unlikely to me. I don't see a nitride, but
wouldn't really expect that would be very likely anyway. Possibly it
would inflame with halogens when finely divided, but I don't know if
that would give it a flammability hazard rating.
Probably the manufacturer is deciding to err on the side of caution,
as MOST finely divided metals can combust explosively.
A few years ago, BTW, I did some work with lithium dispersion. You can't
even expose it to an N2 atmosphere (it burns to lithium nitride). We
worked with in an Ar filled glovebox. And lithium is tame among the
alkali metals!
--
d...@phlan.sw.stratus.com | "Laddie, you'll be needin' something to wash
| that doon with."
Joe Collins
>
>i am no expert in this area, but most dusts are flammable when finely divided.
>in fact, many have measurable explosivity ranges, depending on the particle
>size and test method. an interesting literature source is "Explosibility tests
>for Industrial Dusts", Fire Research Technical Paper No. 21, which can be
>ordered from the Application Services Division, Building Research Establishment,
>Garston, Watford, WD2 7JR, UK. to get a feeling for burning metals, one need
>not look any further than the effects of the exocet missles on some of the
>vessel hulls in the Falklands war.
For something to be flammable implies it will burn. In order for something
to burn, this usually means it oxidizes. Paper burns, i.e. the various
components of paper (Carbon, Nitrogen, Hydrogen, etc) combine with oxygen
in a vigorous way.
The UK ship had an aluminum body and Aluminum is a fairly reactive metal.
Aluminum will readily burn under the right conditions and the only reason
it doesn't corrode is that Aluminum oxide (rust) quickly seals the metal.
With iron, the Iron Oxide doesn't seal the underlying metal and thus this
form of rust quickly corrodes through the metal - which is why iron must
be painted - to keep water and oxygen away from it so it can't rust.
When that exocet missle hit the hull, the ignition temperature of the
Aluminum was reached and the metal burned rather vigorously indeed. Anyone who
has seen Thermite burn (Aluminum+Iron Oxide => Iron + Aluminum Oxide + HEAT)
can appreciate how well Aluminum will burn.
Back to the question, less active metals like Gold, Silver would be
fairly unlikely to burn under most circumstances while more active
metals will burn with various degrees of difficulty. Magnesium is a rather
active metal and burns quite nice. Iron and Copper are less active and burn
with more difficulty. For a novelty, water can be made to burn, but it
requires an atmosphere of Fluorine. [Water is just hydrogen that has
burned, i.e. been oxidized into water].
I'm no expert either but thats my recollection from my university days when
I was a chem major.
David Knapp
I'll give you a reason. Redox potential.
While gold and platinum *do* form oxides, they are so weakly bound that
you can dissociate it just by shining a bright light on it! Aluminum,
on the other hand, forms such a strong oxygen bond that we call it a
'ceramic', when vitrified.
If you don't believe me, grab a handful of gold dust and throw it into
a flame. ;-)
--
David Knapp University of Colorado, Boulder
Perpetual Student kn...@spot.colorado.edu
Looks like hate *is* a family value after all and Colorado families are for it.
SMITH RANDALL SCOTT
It depends on what use you put gold to... NOTHING is unreactive in every
circumstance.
For instance, Hayashi and Ito have used gold complexed with chiral phos-
phinoferrocenyl ligands to catalyze asymmetric aldol reactions of
alpha-isocyano carboxylates (Ito, Sawamura, Hayashi, Tetrahedron Lett.,
(1987), _28_, 6125. Ito, Sawamura, Shirakawa, Hayashizaki, Hayashi,
Tetrahedron Lett., (1988), _29_, 235.) These studies obtained up to
96% enantiomeric excess! Moreover, the methodology is synthetically
useful, as they employed it in the synthesis of threo- and erythro-
sphingosines (Ito, Sawamura, Hayashi, Tetrahedron Lett., (1988), _29_ 239.)
Metallic gold, of course, is another matter entirely. However, as many
people have pointed out, almost all POWDERS , when suspended in air,
have flash points. For instance, wheat is not particularily explosive,
nor is flour, but grain and flour storage silos have a nasty habit of
exploding when emptied. The reason that these silos can explode is that
the dust within them is exceedingly (read explosively) combustible.
Moreover, metal POWDERS act very differently than the metals themselves,
in solid form. For instance, mix together an alkyl halide and a chunk of
magnesium and nothing will happen. However, mix together the same
alkyl halide and POWDERED magnesium and you'd better stand back, because
the formation of grignard reagent is rapid and rather exothermix. Why
the difference? Mainly, it has to do with the increased surface area of
the finely divided powder. More surface area is equivalent to more reactive
sites...and BOOM.
Anyway, my point is that just because one form of an element has a set of
properties, it doesn't mean that all forms of that element will have those
same properties. If this were true, we'd be in trouble seeing as how we're
made out of lots of carbon, and seeing as how diamond is so inert...
Randall Smith
smi...@ucsu.colorado.edu
the mad chemist
Rajat Kapoor,P303,2124
In my opinion, flammibility of fine powders is more of a heat transfer
effect than the conventional "hi surface area = hi energy" theory.
Oxidation of Al, highly exothermic, can lead on to formation of
hot-spots, which by definition is localized phenomena.
For finely divided micro-particles, the temperature rise is global,
owing to its comparatively low heat capacity (mass dependent). This
then further acclerates the oxidation rate, raising the temperature even
more, till ignition point is reached.
For macro-particles, the adiabatic heat rise on the surface could
be quickly conducted and absorbed by the larger mass, preventing
formation of hot-spots and quenching the reaction, allowing only a
protective oxide layer to develop.
One may, however, argue that higher surface area of smaller particles
would allow for more efficient heat transfer with surroundings. Well,
at t=0 the temp rise would be non-discriminating, though, later on
one may expect higher loss due to a higher temp differential between
the surface and surroundings, but again, heat generation would outpace
heat loss by convection.
In the present context of Au vs. Al, I would't be surprised if gold
oxide formation were endothermic.
While Al occurs as oxide in nature (bauxite), Au is native.
Rajat
rxn for Gold Oxide formation is
A. McPherson
gold might likely be the least reactive of all the metals in terms of
its combustibility, i would agree. it would be interesting to confirm
the prediction, which is based on fairly well known thermodynamic
principles, with a test, just in case we have missed some other
considerations. in any event, your logic is well based, and the
loose comparison i offered with respect to burning aluminum and
magnesium was not a very good one.
Mark Robert Thorson
form of ash? Surely not gold oxide.
Ken Shirriff
>>to get a feeling for burning metals, one need
>>not look any further than the effects of the exocet missles on some of the
>>vessel hulls in the Falklands war.
People in sci.military who should know say that the Sheffield's superstructure
was steel, not aluminum, and it did not burn. The flammable interior of
the ship burned as a result of the missile explosion, but the metal
superstructure did not burn. In 1975, the USS Belknap, which did have an
aluminum superstructure, suffered a severe fire; the superstructure melted,
but did not burn.
Ken Shirriff shir...@sprite.Berkeley.EDU
Stephen Jacobs
>In article <1992Nov13.0...@dsuvax.dsu.edu> ro...@dsuvax.dsu.edu (Tim Rolfe) writes:
>>In <1992Nov12.2...@ncsu.edu> jrki...@eos.ncsu.edu
>>(JOHN ROBERT KITCHIN) writes:
>>
>
>Metallic gold, of course, is another matter entirely. However, as many
>people have pointed out, almost all POWDERS , when suspended in air,
>have flash points. For instance, wheat is not particularily explosive,
>nor is flour, but grain and flour storage silos have a nasty habit of
>exploding when emptied. The reason that these silos can explode is that
>the dust within them is exceedingly (read explosively) combustible.
There's recently been some serious disagreement with this explanation of
explosions in grain-handling facilities. The main puzzle is why such
explosions aren't more common, since dusty storage areas and factories, and
ignition sources are rather common. The revisionist view (definitely minority,
but growing) is that these are methane explosions.
>
>Moreover, metal POWDERS act very differently than the metals themselves,
>in solid form. For instance, mix together an alkyl halide and a chunk of
>magnesium and nothing will happen. However, mix together the same
>alkyl halide and POWDERED magnesium and you'd better stand back, because
>the formation of grignard reagent is rapid and rather exothermix. Why
>the difference? Mainly, it has to do with the increased surface area of
>the finely divided powder. More surface area is equivalent to more reactive
>sites...and BOOM.
A most unfortunate example, because it isn't true. In the presence of a
good Grignard solvent (let's not go into that now), and with the usual wait
for things to get going, common alkyl halides will cheerfully munch up even
big pieces of magnesium. On the other hand, in the absence of a 'good' solvent
(and the above reads like one is to think of working neat), common alkyl
halides don't react much (or at all) with magnesium. For various reasons that
I consider incidental, Grignard reagent formation tends to be faster with the
usual turnings than with magnesium powder anyway. Now if you were to go to
alkali-metal-precipitated magnesium......
Steve s...@chinet.chi.il.us
Stephen Jacobs
statements that the metal of the destroyer Sheffield burned
after it was hit by an Exocet. This has been gone over endlessly in
sci.military, and consensus is very strongly that it didn't happen that way.
I am a great deal less sure of another point: while Sheffield was designed
with a very high aluminum content, I believe there was actually very much less
than intended when she entered combat.
Steve s...@chinet.chi.il.us
Jeroen Belleman
>
>Probably the manufacturer is deciding to err on the side of caution,
>as MOST finely divided metals can combust explosively.
>
Yeah, probably. Nevertheless, if supposedly knowledgable people start
warning for non-existent hazards, how much longer are we going to
heed them?
>A few years ago, BTW, I did some work with lithium dispersion. You can't
>even expose it to an N2 atmosphere (it burns to lithium nitride). We
>worked with in an Ar filled glovebox. And lithium is tame among the
>alkali metals!
Powdered Li? That must be pretty reactive indeed. Here at CERN we had
a chap who played around with Li foil. He worked in a room which had
two huge dessicators taking turns at drying the air in the room. The
foil remained clean and shiny. If he took a piece outside
it would tarnish immediately.
Best regards,
Jeroen Belleman
jeroen@dxcern
David Knapp
>I am sure heat transfer is an important
>consideration however if the change in energy is
>not there then the reaction will not proceed. The
>main failing of most of the postings on this topic
>has been the neglect of surface area on the
>chemical potential.
>
>Once it is established that delta-G for the
>reaction (of size 's' particles) is favorable then
>kinetic, including heat transfer, considerations
>may be applied to see if the thermodynamically
>allowed reaction is also probable from a kinetic
>viewpoint.
>
>Personally I would be surprised (make that
>*shocked*) if there is any metal that will not
>spontateously combine with oxygen (in air) once
>reduced to sufficiently small size (like
>atomized!).
The metal would need to have a large enough affinity to break the O2 bond
which, while not particularly strong, is not trivial.
I believe that many 'spontaneous' combustion demonstrations of finely
divided metals are literally triggered by transient free oxygen from,
of all things, cosmic rays. I wish I could remember the article I read
this in to fend off the ensuing flames here, but I'm afraid I don't.
So sue me.
Anyway, the point of *that* is that not all metals have oxidation potentials
above the O2 bond energy. Hence, unless you have free oxygen around to
start things off with a bit of exothermicity, no matter how finely divided
the metal is, there are some that won't 'spontaneously' oxidize.
>
>just my 2 cents... Mike W.
In the end, it is the facts that count, not the amount of money...
Michael Whitbeck
>
>...interesting to confirm
>the prediction, which is based on fairly well known thermodynamic
>principles, with a test, just in case we have missed some other
I would like to point out, amidst all the *wild*
speculation, that the chemical potential of a
*finely* divided solid differs from that of bulk
material:
$\mu = \mu_0 + f(S) $
where $\mu_0$ is the bulk chemical potential in
some standard state and f(S) is a function of
surface area (and hence particle size).
This of course changes the outcome of any classic
thermo calculation for $\Delta G$
As for the reactivity of Au metal; sure conc nitric
is rather ineffective at attacking it-- but try a
solution of NaCN!
Mike W.
___________________________________________________________
|Mike Whitbeck | whit...@equinox.unr.edu |
|Desert Research Inst. | whit...@wrc.unr.edu |
|POB 60220 | |
|RENO, NV 89506 | 702-673-7348 |
|__________________________|______________________________|
All academics have the potential for being insatiable...
but the chemists are the most expensive and insatiable among the
expensive and insatiable. - J. Martin in "To Rise Above Principle"
Michael Whitbeck
>From article <1992Nov12.2...@ncsu.edu>, by jrki...@eos.ncsu.edu (JOHN ROBERT KITCHIN):
>>
>> there are many metallic dusts which are not only flammable when in a
>> fine dust, but are pyrophoric (self-igniting). This is definitely true
>> for metals such as aluminum and zinc, which are commonly used for
>> explosives and pyrotechnics, so I see no reason why it would be
>> different for other metals like gold and platinum
>
>In my opinion, flammibility of fine powders is more of a heat transfer
>effect than the conventional "hi surface area = hi energy" theory.
consideration however if the change in energy is
not there then the reaction will not proceed. The
main failing of most of the postings on this topic
has been the neglect of surface area on the
chemical potential.
Once it is established that delta-G for the
reaction (of size 's' particles) is favorable then
kinetic, including heat transfer, considerations
may be applied to see if the thermodynamically
allowed reaction is also probable from a kinetic
viewpoint.
Personally I would be surprised (make that
*shocked*) if there is any metal that will not
spontateously combine with oxygen (in air) once
reduced to sufficiently small size (like
atomized!).
just my 2 cents... Mike W.
PENELOPE E. SMITH
avid Knapp) writes:
...
>I believe that many 'spontaneous' combustion demonstrations of finely
>divided metals are literally triggered by transient free oxygen from,
>of all things, cosmic rays. I wish I could remember the article I read
>this in to fend off the ensuing flames here, but I'm afraid I don't.
>So sue me.
Please appear in court 9:00 AM 22 September 1993 in the matter of.... :-)
I think that Electrostatic discharge may have a lot more to do with this,
since it is quite common below a few hundred volts and imperceptible without
instrumentation. Of course, a cosmic ray might create a conductive path to
carry the discharge or recieve its radiation. There is a lot more energy
available in a very short time!
Eric Christenson
--
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Penelope E. Smith PS...@NS.CC.LEHIGH.EDU
Department of Mathematics PS...@LEHIGH.BITNET
Lehigh University
Bethlehem, PA
Dave Blackburn
the definitive experiment, attempting to ignite some gold dust in air.
Anyone who wishes co-author credit may assist by mailing me a few kilos of the
needed reagents.
Oh, yeah, :-)
--
* Dave Blackburn dbla...@alleg.edu flames to se...@home.dev.null *
* Chemistry, Macs, Hypercard, cats, <>< religion, & the Dead *
* (Anyone else who held these opinions would be as crazy as I am.) *
* "Sometimes the songs that we hear are just songs of our own" *
Mike Whitbeck
Whitbeck) writes:
|>
|> >I am sure heat transfer is an important
|> >consideration however if the change in energy is
...... stuff deleted about delta G and particle size ......
|>
|> The metal would need to have a large enough affinity to break the O2 bond
huh?
For the reaction to be thermo allowed one considers the NET change from
products to
reactants at the specified reaction conditions.
***************************** <- shouting for the
hard of hearing
|> which, while not particularly strong, is not trivial.
Yep, I agree O2 is moderately stable. Still...
Fe + O2 = FeO + O (atom) just pick a high enough T
If you want a sample of FeO to play with you might find some inside your
TV screen.
How about 2Fe + 3O2 = Fe2O3 ? steel wool + heat (a 9V battery will do nicely)
Or how about Mg ribbon + O2 + heat ---> more heat and more light
|>
|> I believe that many 'spontaneous' combustion demonstrations of finely
are you speaking thermodynamically spontaneous or 'gee look at that
sucker go !'
spontaneous? Under the 'gee look at ...' category don't expect find a lot on
metal (of any particle size) going poof at 25C in air (with some obvious
exceptions).
But under the thermo definition of spontaneous you can expect that
there will be
a T (for P= 1atm air) for oxidation for every metal.
The oxidation may be kinetically slow in the absence of an initiator or
catalyst.
However small particles are more reactive (re chemical potential) than
large particles-
lowering the particle size alters the thermodynamics.
****************************************************
|> divided metals are literally triggered by transient free oxygen from,
|> of all things, cosmic rays. I wish I could remember the article I read
|> this in to fend off the ensuing flames here, but I'm afraid I don't.
|> So sue me.
|>
Well I must admit this is new to me but I'll hold back on the legal
action for now.
|>
|> Anyway, the point of *that* is that not all metals have oxidation potentials
|> above the O2 bond energy. Hence, unless you have free oxygen around to
|> start things off with a bit of exothermicity, no matter how finely divided
we seem to be butting heads over the distinction between thermo allowed
(i.e. possible)
versus kinetically *fast*
|> the metal is, there are some that won't 'spontaneously' oxidize.
again is this the lay persons' spontaneous or the chemists' ? The quotes
lead me to
think that you mean the lay definition. In which case so what! IF the
reaction is thermo
spontaneous (the chemists usage) THEN conditions might be achievable to
make it
'spontaneous' (the lay usage of the word). [however if you did mean
thermo spontaneous
then you had best take chemical thermodynamics over again :-)]
The delta G for the reaction should be the appropriate change
in energy for the CONDITIONS-- getting hot enought and having particles
small enought.
Pyrophoric metal powder is a well recognized phenomenon- There are
shipping regulations
(CFR 49) even! That is (I believe) what started this circus!!
|>
|>
|> >
|> >just my 2 cents... Mike W.
|>
|> In the end, it is the facts that count, not the amount of money...
Sadly facts are few and far between on the internet news.
I maintain that from the discussions on the net thus
1) no data or arguments have been given to support or refute the KINETICS of
pyrohoric metals reacting in air. This requires a lot more in the way of
data and parameters than have even been mentioned so far here, and
2) classical chemical thermodynamics, properly applied, can and should
show that the
phenomenon is *possible* BUT that the reaction conditions must be applied-
NOT the mish-mash about bond strengths of ONE reactant at standard state.
[that's my point-] ....(I'll shout again for emphasis)
IT IS THE DELTA G FOR THE REACTION UNDER REACTION CONDITIONS THAT COUNTS
******** *******************
really loud shouting-> ********** *********************
******** *******************
|>
|>
|>
|>
|> --
|> David Knapp University of
Colorado, Boulder
|> Perpetual Student kn...@spot.colorado.edu
whew... never try to inject science into a sci-dot newsgroup!
Mike
Rajat Kapoor
> For the reaction to be thermo allowed one considers the NET change from
> products to
> reactants at the specified reaction conditions.
> ***************************** <- shouting for the
> hard of hearing
Your point was made, but seriously, all this while there was a tacit
understanding that the conditions were 1 atm, 25 deg C...
>
> The oxidation may be kinetically slow in the absence of an initiator or
> catalyst.
> However small particles are more reactive (re chemical potential) than
> large particles-
> lowering the particle size alters the thermodynamics.
> ****************************************************
>
>
> The delta G for the reaction should be the appropriate change
> in energy for the CONDITIONS-- getting hot enought and having particles
> small enought.
>
> IT IS THE DELTA G FOR THE REACTION UNDER REACTION CONDITIONS THAT COUNTS
Lets hammer this out (flames welcome)
dG = dH - TdS Now, dH is the heat released, and TdS is the unavailable
energy, i.e., the irrecoverable loss of energy due to, say,
reorganization of bonds, etc.
dG, then, gives a measure of tendency, not heat released.
This brings me back to my original argument (summarized)
For a given dH (metal specific,cond.specific), the heat released
would raise the particle temp globally if heat capacity ( not sp. ht.
cap. ) is low. This rise in T increases the rate, which in turn
increases the global T faster. The rise in T helps to overcome
diffusional resistance of metal in the interior. All in all, after a
certain temp is reached, the particle burns uniformly.
Remember, we used heat capacity in the argument. Now heat capacity
is mass dependent, and therefore on particle size, given density.
To conclude, IMHO, all metals have a characteristic critical specific surface
( depending on dH of rxn, and assuming Cp for most metals is near) at which
it will burn, and beyond which it wouldn't matter.
> ******** *******************
> really loud shouting-> ********** *********************
> ******** *******************
Kapoor
A. McPherson
mail a few kilos to test the explosivity of gold dust - why, what a
noble suggestion. according to thermodynamic principles we should
at least recover 100% of the gold as Au.
"flamed by an inert metal"
David Knapp
>In article <1992Nov16....@ucsu.Colorado.EDU>,
>kn...@spot.Colorado.EDU (David Knapp) writes:
>|> In article <48...@equinox.unr.edu> whit...@equinox.unr.edu (Michael
>Whitbeck) writes:
>|>
>|> >I am sure heat transfer is an important
>|> >consideration however if the change in energy is
>
>...... stuff deleted about delta G and particle size ......
>
>|>
>|> The metal would need to have a large enough affinity to break the O2 bond
>
>huh?
Odd you mention hard of hearing below... ;-)
>For the reaction to be thermo allowed one considers the NET change from
>products to
>reactants at the specified reaction conditions.
> ***************************** <- shouting for the
>hard of hearing
>
Catalysis in a common auto catalytic converter uses a three step process.
If it isn't hot enough to start process number one, namely, dissociating
O2, nothing will happen. If you have free oxygen sitting around, you don't
need to have a hot surface and can catalyze the process at lower temps.
This is the project I'm working on right now. CO oxidation catalysis in a
plasma.
>|> which, while not particularly strong, is not trivial.
>
>Yep, I agree O2 is moderately stable. Still...
>
>Fe + O2 = FeO + O (atom) just pick a high enough T
>If you want a sample of FeO to play with you might find some inside your
>TV screen.
>How about 2Fe + 3O2 = Fe2O3 ? steel wool + heat (a 9V battery will do nicely)
>Or how about Mg ribbon + O2 + heat ---> more heat and more light
>
>|>
>|> I believe that many 'spontaneous' combustion demonstrations of finely
>
>are you speaking thermodynamically spontaneous or 'gee look at that
>sucker go !'
>spontaneous? Under the 'gee look at ...' category don't expect find a lot on
>metal (of any particle size) going poof at 25C in air (with some obvious
>exceptions).
Spontaneous, as I referred to it, involved having the reaction start in
its cascade without external influence.
Mg in air is a wonderful example of what happens when you get the reaction
going by putting a flame on it. Without someone starting it, you miss the
major action.
>But under the thermo definition of spontaneous you can expect that
>there will be
>a T (for P= 1atm air) for oxidation for every metal.
>The oxidation may be kinetically slow in the absence of an initiator or
>catalyst.
>However small particles are more reactive (re chemical potential) than
>large particles-
>lowering the particle size alters the thermodynamics.
I guess I don't see your point. Can you summarize?
>****************************************************
>
>|> divided metals are literally triggered by transient free oxygen from,
>|> of all things, cosmic rays. I wish I could remember the article I read
>|> this in to fend off the ensuing flames here, but I'm afraid I don't.
>|> So sue me.
>|>
>
>Well I must admit this is new to me but I'll hold back on the legal
>action for now.
?
>
>|>
>|> Anyway, the point of *that* is that not all metals have oxidation potentials
>|> above the O2 bond energy. Hence, unless you have free oxygen around to
>|> start things off with a bit of exothermicity, no matter how finely divided
>
>we seem to be butting heads over the distinction between thermo allowed
>(i.e. possible)
>versus kinetically *fast*
No, I'll buy our point regarding thermodymically allowed and the fast
reaction. You are right, aluminum powder in air will oxidize anyway
(because it can) and "ignites" in air due to what I described above.
There is a difference (which I guess I was unclear about) between igniting
because there is enough potential to first dissociate O2, and igniting
because you *provide* free oxygen through other means. That's all I was
trying to say. This whole class of reactions we're discussing are all
thermodynamically allowed.
>
>|> the metal is, there are some that won't 'spontaneously' oxidize.
>
>again is this the lay persons' spontaneous or the chemists' ?
Laypersons.
> The quotes
>lead me to
>think that you mean the lay definition. In which case so what!
Maybe nothing. I'm not trying to win a Nobel Prize with comments on the
net.
>IF the
>reaction is thermo
>spontaneous (the chemists usage) THEN conditions might be achievable to
>make it
>'spontaneous' (the lay usage of the word). [however if you did mean
>thermo spontaneous
>then you had best take chemical thermodynamics over again :-)]
>
>The delta G for the reaction should be the appropriate change
>in energy for the CONDITIONS-- getting hot enought and having particles
>small enought.
>Pyrophoric metal powder is a well recognized phenomenon- There are
>shipping regulations
>(CFR 49) even! That is (I believe) what started this circus!!
>
>|>
>|>
>|> >
>|> >just my 2 cents... Mike W.
>|>
>|> In the end, it is the facts that count, not the amount of money...
>
>Sadly facts are few and far between on the internet news.
>I maintain that from the discussions on the net thus
>
>1) no data or arguments have been given to support or refute the KINETICS of
> pyrohoric metals reacting in air. This requires a lot more in the way of
> data and parameters than have even been mentioned so far here, and
And still haven't been...
>2) classical chemical thermodynamics, properly applied, can and should
>show that the
> phenomenon is *possible* BUT that the reaction conditions must be applied-
> NOT the mish-mash about bond strengths of ONE reactant at standard state.
> [that's my point-] ....(I'll shout again for emphasis)
>
> IT IS THE DELTA G FOR THE REACTION UNDER REACTION CONDITIONS THAT COUNTS
> ******** *******************
>really loud shouting-> ********** *********************
> ******** *******************
>
>
>whew... never try to inject science into a sci-dot newsgroup!
Right, we're still waiting.
Care to add any of the numbers or kinetics you've mentioned? Not often do
people sit there with their Merck Index ready to beat someone with numbers
unless it is really called for. This was just a descriptive discussion. If
you have numbers, post them.