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wallace.k
science forum beginner
Joined: 09 Apr 2006
Posts: 7
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 Posted: Sun Apr 09, 2006 5:17 am Post subject:
mech eng student with chem question
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Hi all,
I'm taking an introductory combustion class, and of course we are
studying the chemistry of the fuels involved.
This question is for my own information, not for any assignments; just
conceptual understanding.
I am looking at the aromatics, benzene etc. I understand that the
double bonds "rotate" or move around the ring.
These 'fuels' have energy potential- bond energy, enthalpy of formation
and combustion, and I understand that. What I am wondering is this-
where, if at all, is the kinetic energy of the 'motion' of the electrons
"around" the ring taken into account? The quotation marks are because I
fully understand that these molecules are not little connected Bohr
model atoms, with electrons circling like planets, but in potential
'clouds'. But if the bonds are moving, the electrons must be moving.
The masses are incredibly small, but the velocities must be incredibly
large, and in kinetic energy, 1/2mv^2, I just wonder if that is taken
into account anywhere?
thanks!
k wallace |
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Herman Family
science forum Guru Wannabe
Joined: 14 Jun 2005
Posts: 173
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| Posted: Sun Apr 09, 2006 5:59 am Post subject:
Re: mech eng student with chem question
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"wallace.k" <wallaceNOT.k@comNOSPAMcast.net> wrote in message
news:5cOdnS6Tl_hlBaXZRVn-vQ@comcast.com...
| Quote: | Hi all,
I'm taking an introductory combustion class, and of course we are studying
the chemistry of the fuels involved.
This question is for my own information, not for any assignments; just
conceptual understanding.
I am looking at the aromatics, benzene etc. I understand that the double
bonds "rotate" or move around the ring.
These 'fuels' have energy potential- bond energy, enthalpy of formation
and combustion, and I understand that. What I am wondering is this-
where, if at all, is the kinetic energy of the 'motion' of the electrons
"around" the ring taken into account? The quotation marks are because I
fully understand that these molecules are not little connected Bohr model
atoms, with electrons circling like planets, but in potential 'clouds'.
But if the bonds are moving, the electrons must be moving. The masses are
incredibly small, but the velocities must be incredibly large, and in
kinetic energy, 1/2mv^2, I just wonder if that is taken into account
anywhere?
thanks!
k wallace
|
Gee, for a mechanical engineer, you give a pretty good chemical question.
The answer is that it is taken into account, though not generally quite so
explicitly. The double bonds tend to be tighter and somewhat closer than a
single bond. This gives a different bond energy. When the bond is broken,
it releases that energy. The energy released for fuels has been quite well
studied empirically, and you can also get a pretty good idea of a fuel value
by looking at the number of each kind of bond.
Michael |
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wallace.k
science forum beginner
Joined: 09 Apr 2006
Posts: 7
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| Posted: Sun Apr 09, 2006 6:25 am Post subject:
Re: mech eng student with chem question
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| Quote: |
Gee, for a mechanical engineer, you give a pretty good chemical question.
The answer is that it is taken into account, though not generally quite so
explicitly. The double bonds tend to be tighter and somewhat closer than a
single bond. This gives a different bond energy. When the bond is broken,
it releases that energy. The energy released for fuels has been quite well
studied empirically, and you can also get a pretty good idea of a fuel value
by looking at the number of each kind of bond.
Michael
thanks, that's what I was wondering. Is the double bond then "harder" to |
break than the single one? Are there cases in which that bond would
break, rather than the single bond, releasing that higher bond energy?
It may have been a good question from a mechanical engineer, but there's
plenty I don't understand about the specifics of this- why one bond
would break rather than another, and exactly what makes them break in
the first place- intro to chem was a few years ago now!
thanks,
Karinne |
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rudnyi
science forum addict
Joined: 20 May 2005
Posts: 75
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| Posted: Sun Apr 09, 2006 7:52 am Post subject:
Re: mech eng student with chem question
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| Quote: | thanks, that's what I was wondering. Is the double bond then "harder" to
break than the single one? Are there cases in which that bond would
break, rather than the single bond, releasing that higher bond energy?
It may have been a good question from a mechanical engineer, but there's
plenty I don't understand about the specifics of this- why one bond
would break rather than another, and exactly what makes them break in
the first place- intro to chem was a few years ago now!
|
I should say that you are not alone. I am a chemist by background but I
am not sure if I understand this either.
Chemistry is still a descriptive or classification science by its
nature. Well, nowadays, you can run molecular simulation software to
predict the behavior of molecules. Try it, it is quite amazing what
modern software on modern hardware can do. You can find some slides on
molecular simulation at my site - look at teaching. However, the
relationship between simulation and understanding is not that
straightforward.
Best wishes,
Evgenii
--
http://Evgenii.Rudnyi.Ru/ |
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wallace.k
science forum beginner
Joined: 09 Apr 2006
Posts: 7
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| Posted: Sun Apr 09, 2006 8:22 am Post subject:
Re: mech eng student with chem question
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Evgenii Rudnyi wrote:
| Quote: | thanks, that's what I was wondering. Is the double bond then "harder" to
break than the single one? Are there cases in which that bond would
break, rather than the single bond, releasing that higher bond energy?
It may have been a good question from a mechanical engineer, but there's
plenty I don't understand about the specifics of this- why one bond
would break rather than another, and exactly what makes them break in
the first place- intro to chem was a few years ago now!
I should say that you are not alone. I am a chemist by background but I
am not sure if I understand this either.
Chemistry is still a descriptive or classification science by its
nature. Well, nowadays, you can run molecular simulation software to
predict the behavior of molecules. Try it, it is quite amazing what
modern software on modern hardware can do. You can find some slides on
molecular simulation at my site - look at teaching. However, the
relationship between simulation and understanding is not that
straightforward.
Best wishes,
Evgenii
|
thank you! There is a lot of information on your website- could you
tell me where in 'teaching' to find what you're talking about? I'm not
sure where to start.
I don't have to know this much detail for the course I'm taking on
combustion, but I find it fascinating.
thanks again,
Karinne |
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Herman Family
science forum Guru Wannabe
Joined: 14 Jun 2005
Posts: 173
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| Posted: Sun Apr 09, 2006 10:35 am Post subject:
Re: mech eng student with chem question
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"wallace.k" <wallaceNOT.k@comNOSPAMcast.net> wrote in message
news:9JmdnfSgOfN0NaXZRVn-qQ@comcast.com...
| Quote: |
Gee, for a mechanical engineer, you give a pretty good chemical question.
The answer is that it is taken into account, though not generally quite
so explicitly. The double bonds tend to be tighter and somewhat closer
than a single bond. This gives a different bond energy. When the bond
is broken, it releases that energy. The energy released for fuels has
been quite well studied empirically, and you can also get a pretty good
idea of a fuel value by looking at the number of each kind of bond.
Michael
thanks, that's what I was wondering. Is the double bond then "harder" to
break than the single one? Are there cases in which that bond would break,
rather than the single bond, releasing that higher bond energy? It may
have been a good question from a mechanical engineer, but there's plenty I
don't understand about the specifics of this- why one bond would break
rather than another, and exactly what makes them break in the first place-
intro to chem was a few years ago now!
thanks,
Karinne
|
The bonds in aromatics tend to be a little harder to break, because they are
resonating throughout the molecule and tend to protect themselves.
Typically, double bonds are a little easier to break than single bonds (I
think). We have a few problems with the double bonds in natural oils
reacting well before the rest of the molecule reacts. They are quite happy
to give up their high energy bond in favor of lower energy bonds.
As for why one bond would break rather than another, there can be several
reasons, most of which comes down to energy.
Michael |
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steve@nospam.com
science forum beginner
Joined: 16 Sep 2005
Posts: 9
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| Posted: Fri Apr 14, 2006 3:34 pm Post subject:
Re: mech eng student with chem question
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I don't remember a lot of my molecular chemistry, but your statement on the
double bond being easier to break than the single bond made me raise my
eyebrows (initially).
But then again - I do remember something about double bonds using a bit
"more" of the electron cloud (there's only so much "electron cloud" to go
around), and therefore exposing a bit more of the atom's nucleus. That
exposure meant the naked bits of the atom were positively charged, and
therefore a bit more attractive to other passing atoms/electrons that carry
negative charges.
Similarly, the double bonds involved pi bonding (I think) which is a bit of
a different physical arrangement to the sigma (?) bonding - I used to
conceptualise this as follows (don't know how right it any more): In a
double bond, there are too many electrons being shared between the two atoms
for them to fit on a straight line between the atoms - so they have to
spread out a bit, and tend be a bit more exposed as a result, which results
in regions that have more negative charge than your typical single bond.
In contrast to the double bond, the atoms involved in a single bond manage
to retain a greater proportion of their... err.... electron cloud "cover"
over the nucleus, making the net charge around the nucleus closer to neutral
(+ve charge from protons are cancelled out by the -ve charge of the
electrons). Therefore, the single bond molecules only have a small degree of
polarisation around the molecule and are more chemically stable as a result
(because there are more attractive reaction targets around).
Just my two cents, to see if it helps anyone else out.
- Steve. |
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wallace.k
science forum beginner
Joined: 09 Apr 2006
Posts: 7
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| Posted: Sat Apr 15, 2006 4:49 am Post subject:
Re: mech eng student with chem question
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| Quote: |
The bonds in aromatics tend to be a little harder to break, because they are
resonating throughout the molecule and tend to protect themselves.
Typically, double bonds are a little easier to break than single bonds (I
think). We have a few problems with the double bonds in natural oils
reacting well before the rest of the molecule reacts. They are quite happy
to give up their high energy bond in favor of lower energy bonds.
As for why one bond would break rather than another, there can be several
reasons, most of which comes down to energy.
Michael
|
Can you explain what you mean by resonating? I know my mechanical
knowledge is not helping here, as in solid dynamics, if an object
vibrates at its resonant frequency, it tends to go into catastrophic
failure- basically can 'shake itself apart'. I assume this resonance is
not the same sort of thing, but what does it mean on an atomic or
molecular level?
thanks,
Karinne |
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Herman Family
science forum Guru Wannabe
Joined: 14 Jun 2005
Posts: 173
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| Posted: Sat Apr 15, 2006 5:36 am Post subject:
Re: mech eng student with chem question
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"wallace.k" <wallaceNOT.k@comNOSPAMcast.net> wrote in message
news:tO6dnR_O2M7F5t3ZRVn-qg@comcast.com...
| Quote: |
The bonds in aromatics tend to be a little harder to break, because they
are resonating throughout the molecule and tend to protect themselves.
Typically, double bonds are a little easier to break than single bonds (I
think). We have a few problems with the double bonds in natural oils
reacting well before the rest of the molecule reacts. They are quite
happy to give up their high energy bond in favor of lower energy bonds.
As for why one bond would break rather than another, there can be several
reasons, most of which comes down to energy.
Michael
Can you explain what you mean by resonating? I know my mechanical
knowledge is not helping here, as in solid dynamics, if an object vibrates
at its resonant frequency, it tends to go into catastrophic failure-
basically can 'shake itself apart'. I assume this resonance is not the
same sort of thing, but what does it mean on an atomic or molecular level?
thanks,
Karinne
|
In a molecule, resonance means that electrons are being shared by several
atoms, each more or less in turn. In a benzene molecule, we sometimes think
of double bonds moving from one carbon pair to the next all the time. By
having many atoms share several electrons, it is possible to protect the
ring from breaking, as well as facilitate addition reactions. A good
chemical weenie could do a better job of explaining this than I can.
Michael |
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Unomor
science forum beginner
Joined: 01 Mar 2005
Posts: 2
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| Posted: Sat Apr 15, 2006 1:46 pm Post subject:
Re: mech eng student with chem question
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"Herman Family" <ecalptsudwaseht.in.reverse@frontiernet.net> wrote in
message news:nr%%f.5854$kg.3008@news02.roc.ny...
| Quote: |
"wallace.k" <wallaceNOT.k@comNOSPAMcast.net> wrote in message
news:tO6dnR_O2M7F5t3ZRVn-qg@comcast.com...
The bonds in aromatics tend to be a little harder to break, because they
are resonating throughout the molecule and tend to protect themselves.
Typically, double bonds are a little easier to break than single bonds
(I think). We have a few problems with the double bonds in natural oils
reacting well before the rest of the molecule reacts. They are quite
happy to give up their high energy bond in favor of lower energy bonds.
As for why one bond would break rather than another, there can be
several reasons, most of which comes down to energy.
Michael
Can you explain what you mean by resonating? I know my mechanical
knowledge is not helping here, as in solid dynamics, if an object
vibrates at its resonant frequency, it tends to go into catastrophic
failure- basically can 'shake itself apart'. I assume this resonance is
not the same sort of thing, but what does it mean on an atomic or
molecular level?
thanks,
Karinne
In a molecule, resonance means that electrons are being shared by several
atoms, each more or less in turn. In a benzene molecule, we sometimes
think of double bonds moving from one carbon pair to the next all the
time. By having many atoms share several electrons, it is possible to
protect the ring from breaking, as well as facilitate addition reactions.
A good chemical weenie could do a better job of explaining this than I
can.
Michael
|
....Good chemical weenie...That's Unomor's cue to delurk.
First, some quickie clarification on the nature of "normal" (2-electron,
2-center, classical, G.N.Lewis-taught-us-about-it) bonding:
A double bond consists of two components, a sigma bond, in which the two
electrons being shared by the two (carbon) atoms being bonded are most
likely to be found *between* the two carbon nuclei, and a pi bond, in which
the two electrons being shared are most likely to be found in two regions on
either side of the sigma bond. The three regions thus described define a
plane.
Note that, although we can certainly have a sigma bond without a pi bond, we
never get the pi bond without the sigma bond. Therefore, we can only
*deduce* the contribution of the pi bond to the overall double bond
strength, we cannot measure it directly. If we analyze bond disruption
energy data for large numbers of organic compounds containing single and
double bonds, what we find is that the energies are as follows: double bond
| Quote: | single (sigma) bond > pi component of a double bond.
|
This simple model of two electrons shared by two atoms is a very powerful
insight, and it explains a great deal of chemical bonding (and not just in
organic compounds), but it fails in some equally important cases. One of
these is so-called "electron-deficient" compounds, like diborane, in which
it is necessary to invoke a model in which two electrons are shared by (at
least) three atoms. Another is the completely-misnamed "resonance"
phenomenon the OP asked about. What both situations feature is the
necessity to invoke electron "delocalization," the ability of one pair of
electrons to be shared by more than two atoms. In the (prototypical) case
of benzene, the physical and chemical properties of the substance cannot be
accounted for by three, localized, 2-center, 2-electron pi bonds in the
ring. It is necessary to write delocalized (in this case, over all 6 ring
carbon atoms) molecular orbitals. In the case of benzene, there are three
such orbitals with bonding character. At 2 electrons per orbital (that rule
is still good), these three molecular orbitals accommodate the six pi
electrons, with neither electrons nore "holes" left over. This
delocalization was originally called "resonance," for reasons I'm not sure I
understand (and I am a chemist, and I have been one since before the OP was
born). All I can say about the terminology is that the last thing a
mechanical engineering atudent should do is to take it literally. Think
delocalization explaining energy "anomalies" which can't be explained any
other way; you're much better off (and more accurate).
HTH,
Unomor
[Lurk Mode On] |
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rudnyi
science forum addict
Joined: 20 May 2005
Posts: 75
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| Posted: Sat Apr 15, 2006 4:07 pm Post subject:
Re: mech eng student with chem question
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| Quote: | thank you! There is a lot of information on your website- could you
tell me where in 'teaching' to find what you're talking about? I'm not
sure where to start.
|
I guess that what you need is quantum chemistry
http://evgenii.rudnyi.ru/doc/teaching/md/md6.pdf
where you can see an overview of methods to solve the Schroedinger
equation.
A few comment to the discussion.
1) Double bonds vs. single bonds.
The total engergy required to break a double bond (C=C) is, no doubt,
more that that for a single bond (C-C). Roughly speaking, a single bond
is made by two electrons, and a double bond is made by four electrons.
However, the energy required to break pi-bonding (that is, convert C=C
to C-C) is less than to break C-C.
2) Resonance in benzene.
I think that a good text to browse is
http://en.wikipedia.org/wiki/Benzene
So, one can write down a benzene molecule with three double and three
singe bonds but there are two alternative configurations. The energy to
break the six bonds determined experimentally was much more than the
sum for three single and three double bonds. Then, at some point,
chemists suggested that both structures dynamically switch between each
other - hence the word resonance.
Evgenii |
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