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Simon Dexter
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 Posted: Fri Aug 01, 2003 5:57 am Post subject: Prediction of the outcomes of a reaction |
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If you are given 2 substances composed of 3 atoms each you may presume
initially all kinds of combinations of atoms that would form the products of
hypothetical reaction. Then however, somehow chemists know how to separate
possible outcomes from impossible.
Could anyone tell me, in general terms, how to do it? |
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Steve Turner
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| Posted: Fri Aug 01, 2003 8:27 am Post subject: Re: Prediction of the outcomes of a reaction |
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"Simon Dexter" <SimonDex@yahoo.com> wrote:
[quote]If you are given 2 substances composed of 3 atoms each you may presume
initially all kinds of combinations of atoms that would form the products of
hypothetical reaction. Then however, somehow chemists know how to separate
possible outcomes from impossible.
Could anyone tell me, in general terms, how to do it?
[/quote]
The only way I know is to spend several years studying chemistry.
This is not a question that can be adequately addressed in 30 words or
less.
Steve Turner
Real address contains worldnet instead of spamnet |
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David T. Croft
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| Posted: Fri Aug 01, 2003 1:50 pm Post subject: Re: Prediction of the outcomes of a reaction |
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Some combinations of atoms are more stable than others. There are tables of
(thermodynamic) properties that can be examined that will allow for
calculating the stability of various formations. There are also several
theories that allow one to make an educated guess about what bonds can or
will form.
"Simon Dexter" <SimonDex@yahoo.com> wrote in message
news:GViWa.113$mZ6.58@nwrdny02.gnilink.net...
[quote]If you are given 2 substances composed of 3 atoms each you may presume
initially all kinds of combinations of atoms that would form the products
of
hypothetical reaction. Then however, somehow chemists know how to separate
possible outcomes from impossible.
Could anyone tell me, in general terms, how to do it?
[/quote] |
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William David Thweatt
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| Posted: Fri Aug 01, 2003 6:04 pm Post subject: Re: Prediction of the outcomes of a reaction |
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Simon Dexter (SimonDex@yahoo.com) wrote:
: If you are given 2 substances composed of 3 atoms each you may presume
: initially all kinds of combinations of atoms that would form the products of
: hypothetical reaction. Then however, somehow chemists know how to separate
: possible outcomes from impossible.
: Could anyone tell me, in general terms, how to do it?
Chemical intuition.
--
--
William "Dave" Thweatt
Robert E. Welsh Postdoctoral Fellow
Chemistry Department
Rice University
Houston, TX
thweatt@ruf.rice.edu
dave.thweatt@us.army.mil |
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David T. Croft
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| Posted: Fri Aug 01, 2003 11:53 pm Post subject: Re: Prediction of the outcomes of a reaction |
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As was pointed out, the science of chemistry has evolved over the years in
an attempt to answer this question. An introductory course (or textbook)
can provide a great deal of insight. It may take several years to really
come to an understanding of the subject. The Lewis dot structures are
fairly simple ( that is, unless you want to know why they work*). Using
these gives an idea of what chemicals can form. Determining the relative
amounts of each possible reaction product requires thermodynamic data.
* Actually, I think the theory is based on fairly direct observation.
Quantum mechanical predictions of the structure of the hydrogen atom helped
tie things together.
"Simon Dexter" <SimonDex@yahoo.com> wrote in message
news:GViWa.113$mZ6.58@nwrdny02.gnilink.net...
[quote]If you are given 2 substances composed of 3 atoms each you may presume
initially all kinds of combinations of atoms that would form the products
of
hypothetical reaction. Then however, somehow chemists know how to separate
possible outcomes from impossible.
Could anyone tell me, in general terms, how to do it?
[/quote] |
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Michael Holliday
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| Posted: Fri Aug 01, 2003 11:59 pm Post subject: Re: Prediction of the outcomes of a reaction |
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In article <t0ojivg4mua5m7kj2drjsu20qsu5vanto5@4ax.com>,
Steve Turner <srturner1@spamnet.att.net> wrote:
[quote]"Simon Dexter" <SimonDex@yahoo.com> wrote:
If you are given 2 substances composed of 3 atoms each you may presume
initially all kinds of combinations of atoms that would form the products of
hypothetical reaction. Then however, somehow chemists know how to separate
possible outcomes from impossible.
Could anyone tell me, in general terms, how to do it?
The only way I know is to spend several years studying chemistry.
This is not a question that can be adequately addressed in 30 words or
less.
Steve Turner
Real address contains worldnet instead of spamnet
[/quote]
Back in 1968, John Polanyi>s group at U of Toronto made what I think was
the first a priori prediction of the results of a chemical reaction (the
simplest: D+ + H2 --> HD + H+) using a 3-dimensional classical
trajectory calculation of the dynamics of the reaction on a potential
energy surface obtained by ab initio calculation [I.G. Csizmadia, J.C.
Polanyi, A.C. Roach, and W.H. Wong, Can. J. Chem., 47: 4097 (1969)]. The
experimental verification came a little later [M.G. Holliday, J.T.
Muckerman, and L. Friedman, J. Chem. Phys., 54: 1058 (1971) and 54:3853
(1971)].
Not a reaction of 3-atom molecules, I know, but given the computing
power available to impecunious universites at the time, a pretty
impressive feat for the era nonetheless. I>m no longer in the field, but
I>m sure that with the computing power now readily available,
3-atom-molecule systems should be a piece of cake -- anyone care to
enlighten? |
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David Lloyd-Jones
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| Posted: Sat Aug 02, 2003 12:10 am Post subject: Re: Prediction of the outcomes of a reaction |
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William David Thweatt wrote:
[quote]Simon Dexter (SimonDex@yahoo.com) wrote:
: If you are given 2 substances composed of 3 atoms each you may presume
: initially all kinds of combinations of atoms that would form the products of
: hypothetical reaction. Then however, somehow chemists know how to separate
: possible outcomes from impossible.
: Could anyone tell me, in general terms, how to do it?
Chemical intuition.
[/quote]
Simon,
Dave is putting you on.
In chemistry, as in nearly everything else, the question of effort
determines what gets done. Dust bunnies collect under the bed, water
runs down hill, and chemical combinations take place when energy
dissipates, entropy increases, and everything spirals down a wee bit.
Think of chemistry as thermodynamics with a lot of wordy labels on it.
-dlj. |
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Joćo Antonio
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| Posted: Sat Aug 02, 2003 1:33 am Post subject: Re: Prediction of the outcomes of a reaction |
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In the XVI century (al)chemists already talked about "affinity", i.e.,
species
that tend to combine.
Some time later J. J. Berzelius came with a "dualistic theory' stating that
all
elements (or radicals) should have either a positive or negative charge, and
chemical reaction would be a leveling of those charges (and energy provided
or removed). Things evolved from thgem, with atoms, electrons, quantics,
etc...
But it always comes bacj to affinity, be it empirical or affinity between
orbital lobes
that overlap, doesn>t matter.
The chemist is an elemental psychoanalist.
"Simon Dexter" <SimonDex@yahoo.com> escreveu na mensagem
news:GViWa.113$mZ6.58@nwrdny02.gnilink.net...
[quote]If you are given 2 substances composed of 3 atoms each you may presume
initially all kinds of combinations of atoms that would form the products
of
hypothetical reaction. Then however, somehow chemists know how to separate
possible outcomes from impossible.
Could anyone tell me, in general terms, how to do it?
[/quote]
---
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Steve Turner
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| Posted: Sat Aug 02, 2003 6:30 am Post subject: Re: Prediction of the outcomes of a reaction |
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Michael Holliday <mgh@NO_SPAMmh-associates.com> wrote:
[quote]Back in 1968, John Polanyi>s group at U of Toronto made what I think was
the first a priori prediction of the results of a chemical reaction (the
simplest: D+ + H2 --> HD + H+) using a 3-dimensional classical
trajectory calculation of the dynamics of the reaction on a potential
energy surface obtained by ab initio calculation [I.G. Csizmadia, J.C.
Polanyi, A.C. Roach, and W.H. Wong, Can. J. Chem., 47: 4097 (1969)]. The
experimental verification came a little later [M.G. Holliday, J.T.
Muckerman, and L. Friedman, J. Chem. Phys., 54: 1058 (1971) and 54:3853
(1971)].
[/quote]
When I went through quantum mechanics in college, there was only one
atom -- hydrogen -- which had been rigorously solved. Everything
above that was too complex. Perhaps things have changed.
Still, it is possible to predict many reactions, at least in a
qualitative sense, without resort to ab initio methods. Acid +
hydroxide --> salt + water. Metal + oxygen ---> metal oxide. Etc.
But there are many pitfalls and exceptions, and learning about those
(and *why* they are exceptions) is all part of the science.
Steve Turner
Real address contains worldnet instead of spamnet |
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Eric Lucas
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| Posted: Sat Aug 02, 2003 10:04 am Post subject: Re: Prediction of the outcomes of a reaction |
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"David Lloyd-Jones" <dalj@rogers.com> wrote in message
news:3F2ABB2C.1020101@rogers.com...
[quote]
Think of chemistry as thermodynamics with a lot of wordy labels on it.
[/quote]
That>s funny, I>ve always thought of thermodynamics as chemistry with a lot
of ugly, Martian-looking equations on it.
Eric Lucas |
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Josh Halpern
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| Posted: Sat Aug 02, 2003 7:30 pm Post subject: Re: Prediction of the outcomes of a reaction |
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Eric Lucas wrote:
[quote]"David Lloyd-Jones" <dalj@rogers.com> wrote in message
news:3F2ABB2C.1020101@rogers.com...
Think of chemistry as thermodynamics with a lot of wordy labels on it.
That>s funny, I>ve always thought of thermodynamics as chemistry with a lot
of ugly, Martian-looking equations on it.
No thermodynamics is from Venus, quantum mechanics is from[/quote]
Mars.
Quantum is the stuff about which it was said, not only is quantum
mechanics stranger than you think, it is stranger than you can think
josh halpern
[quote]
Eric Lucas
[/quote] |
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David T. Croft
Guest
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| Posted: Mon Aug 04, 2003 1:47 am Post subject: Re: Prediction of the outcomes of a reaction |
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The solution for the H atom alone has turned out to be very useful. I
suspect that solutions can be found today for any atom - and even for some
molecules - but they just don>t provide very much new information about
chemistry. In fact, the solution for the hydrogen atom itself, more so
confirmed information already known at the time, as opposed to serving to
provide new information, at least as far as information regarding chemical
reactivity is concerned.
"Steve Turner" <srturner1@spamnet.att.net> wrote in message
news:es4mivga4i0a48u55kq4jffatoaslq6lo2@4ax.com...
[quote]Michael Holliday <mgh@NO_SPAMmh-associates.com> wrote:
When I went through quantum mechanics in college, there was only one
atom -- hydrogen -- which had been rigorously solved. Everything
above that was too complex. Perhaps things have changed.
Still, it is possible to predict many reactions, at least in a
qualitative sense, without resort to ab initio methods. Acid +
hydroxide --> salt + water. Metal + oxygen ---> metal oxide. Etc.
But there are many pitfalls and exceptions, and learning about those
(and *why* they are exceptions) is all part of the science.
Steve Turner
Real address contains worldnet instead of spamnet[/quote] |
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Joćo Antonio
Guest
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| Posted: Mon Aug 04, 2003 7:53 am Post subject: Re: Prediction of the outcomes of a reaction |
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I>d say physical chemistry is from Mars and quantum mechanics is from Pluto
;-)
"Josh Halpern" <j.halpern@incoming.verizon.net> escreveu na mensagem
news:3F2BD3CF.5020901@incoming.verizon.net...
[quote]
Eric Lucas wrote:
"David Lloyd-Jones" <dalj@rogers.com> wrote in message
news:3F2ABB2C.1020101@rogers.com...
Think of chemistry as thermodynamics with a lot of wordy labels on it.
That>s funny, I>ve always thought of thermodynamics as chemistry with a
lot
of ugly, Martian-looking equations on it.
No thermodynamics is from Venus, quantum mechanics is from
Mars.
Quantum is the stuff about which it was said, not only is quantum
mechanics stranger than you think, it is stranger than you can think
josh halpern
Eric Lucas
[/quote]
---
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William David Thweatt
Guest
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| Posted: Wed Aug 06, 2003 8:07 pm Post subject: Re: Prediction of the outcomes of a reaction |
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Joćo Antonio (jas_bomfim@uol.com.br) wrote:
: > Someone mentioned chemical intuition. Some people develop a sense for
: > people. Some develop a sense for chemical reactions.
: Can we say that intuition is an inconscious knowledge one learns?
: This way, I totally agree.
I always have considered intuition to be the unconscious application of
previously compiled knowledge.
I hope that helps.
--
--
William "Dave" Thweatt
Robert E. Welsh Postdoctoral Fellow
Chemistry Department
Rice University
Houston, TX
thweatt@ruf.rice.edu
dave.thweatt@us.army.mil |
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William David Thweatt
Guest
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| Posted: Wed Aug 06, 2003 8:08 pm Post subject: Re: Prediction of the outcomes of a reaction |
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Joćo Antonio (jas_bomfim@uol.com.br) wrote:
: I am not a computational chemistry expert, but I think that even ab initio
: methods are aproximate. They don>t use any parameters, that>s right, but
: the maths are worked so you get things like 'self consistent fields' and
: 'minimise the gradient'.
Try getting ahold of a copy of Levine. The current edition is a must-read
for any chemist who wants to understand the power and limitations of
ab-initio quantum chemical calculations.
--
--
William "Dave" Thweatt
Robert E. Welsh Postdoctoral Fellow
Chemistry Department
Rice University
Houston, TX
thweatt@ruf.rice.edu
dave.thweatt@us.army.mil |
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