From: whitbeck@snowlake.math.uh.edu (Mike Whitbeck)
Newsgroups: sci.chem
Subject: Re: Gold dust a fire hazard?
Date: 18 Nov 92 03:27:19 GMT
Reply-To: whitbeck@snowlake.math.uh.edu (Mike Whitbeck)

In article <1992Nov16.221038.8023@ucsu.Colorado.EDU>,
knapp@spot.Colorado.EDU (David Knapp) writes:
|> In article <4843@equinox.unr.edu> whitbeck@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? 
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                                   knapp@spot.colorado.edu



whew... never try to inject science into a sci-dot newsgroup!

Mike