NetNews Usenet Archive 1992 #27/NN_1992_27.iso/spool/sci/chem/4694

Text File | 1992-11-18 | 6.7 KB | 193 lines

Newsgroups: sci.chem
Path: sparky!uunet!boulder!ucsu!spot.Colorado.EDU!knapp
From: [e-mail] (David Knapp)
Subject: Re: Gold dust a fire hazard?
Message-ID: <[e-mail]>
Sender: [e-mail] (USENET News System)
Nntp-Posting-Host: spot.colorado.edu
Organization: University of Colorado, Boulder
References: <[e-mail]> <[e-mail]> <[e-mail]>
Date: Wed, 18 Nov 1992 23:46:11 GMT
Lines: 180

In article <[e-mail]> [e-mail] (Mike Whitbeck) writes:
>In article <[e-mail]>,
>[e-mail] (David Knapp) writes:
>|> In article <[e-mail]> [e-mail] (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.




--
David Knapp                                      University of Colorado, Boulder
Perpetual Student                                   [e-mail]