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What does order of a reaction tell us?

What does order of a reaction tell us? - Chemistry Forum

What does order of a reaction tell us? - Chemistry Forum. Discuss chemical reactions, chemistry.


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  #1  
Old 10-16-2003, 01:06 PM
SNUMBER6
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Default What does order of a reaction tell us?



>From: [Only registered users see links. ] (Mark Tarka)


I have seen molecularity defined in many ways ...
One way is the number of molecules involved in the overall reaction ... and the
rate determining step does not enter into it ... In my previous oxygen tin
caustic reaction ... the molecularity would be 3 by that definition ... and the
order of the reaction is still 1/2 ...
Another way is the number of molecules involved in the rate determining step
....
Again in my previous reaction ... the molecularity would be one ... and the
order is still 1/2 ...
And the order of a reaction ... is in simple terms the sum of the exponents in
the equation ...
Rate = kA^2B^1 ... is third order ... and with no other information ... would
have a molecularity of 3 ...
but that rate constant k might be dependent on say the hydrogen ion
concentration (another reactant but kept constant in the first case) ...
so the rate is = k'[H+](1-1/[H+])A^2B^1 ... and the order is ... geesh ... how
the bleep can you add those exponents ...

Maybe in freshman chemistry ... but definitely not if one's training is in
kinetics and reaction mechanisms ...


Be seeing you
In the Village
Number 6

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  #2  
Old 10-16-2003, 09:43 PM
Mark Tarka
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Default What does order of a reaction tell us?

[Only registered users see links. ] (SNUMBER6) wrote in message news:<[Only registered users see links. ].com>...

Well, that was something like the point I
was trying to make. In olden times, the
gents doing the groundbreaking work weren't
mucking around in those areas where there
wasn't a definite definable for sure process
occurring, something along the lines of
forcing a system to one extreme or the other
in order to avoid the "gray" areas where
a simple theory didn't neatly apply -- sort
of like eating dessert first :-) Only later
did "bright" scholors invade those "forbidden"
zones, only to become dissolusioned with
the established theoritical platform.


Naw...or maybe naw...I probably picked up the
lingo from Richard Noyes at the U. of Oregon.
He's an old-timer. Even a search on the web
yields some suggestions that the two are related.
But, that is "but", as I said above, the view
is clouded by reports from people who've gone
into those areas where theory doesn't quite fit;
this, I suspect, leads to confusion in the open
literature/internet. Your half order reaction
is one of those areas?


Mark (All B.S., but _good_ B.S. :-)
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  #3  
Old 10-16-2003, 11:31 PM
SNUMBER6
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Default What does order of a reaction tell us?

>From: [Only registered users see links. ] (Mark Tarka)


I have the utmost respect for those olden times guys ... Armed with nothing
much more than baling wire and chewing gum ... the set up experiments ... and
took as perfect a measurement as possible ... and recorded all their
observations ... then ... they examined it all and formed an opinion ... Though
I am out of academia for 30 years or so (damn I'm getting old ... maybe time to
go there and research anti-aging drugs ...) ... I still keep up with things ...
but I've found ... that as bad as things were 30 years ago they've gotten many
times worse ... bad in the respect ... that experiments are chosen ... and
interpretations are made ... based on their preconceived theories ...
The one example from my youth ...occured in an interest group meeting ... where
one Ph.D. candidate got a second data point giving a positive slope ... and
proudly announced that the positive slope as predicted confirmed the <insert
mentor's name here> theory ... well without missing a beat ... our esteemed
mentor (who I did and do respect greatly) ... got up ... changed the value of
the initial data point ... saying that it was an error ... and the 2 points now
gave a negative slope ... to which he then explained triumphantly why the
negative slope confirmed his theory ...
And IMO ... things have gotten progressively worse over the years ...



Be seeing you
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Number 6

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  #4  
Old 10-16-2003, 11:41 PM
SNUMBER6
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Default What does order of a reaction tell us?

>But, that is "but", as I said above, the view

I'm posting twice so if you rightly ignore the rantings of the previous post
....you can see my response to this ...

I've never ... either in academia or school ... encountered a reaction that fit
the theory (molecularity=molecules in the transition state) ... there is always
some effect that screws things up ... mostly side equilibria like a protonated
specie reacting faster than the unprotonated one ...
Half order reactions are quite common ...


or a Ph.D. Piled High and Deep ...
Although I see in you someone even more flippant than I am ...

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  #5  
Old 10-17-2003, 09:13 AM
Mohammed Farooq
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Default What does order of a reaction tell us?

[Only registered users see links. ] (SNUMBER6) wrote in message news:<[Only registered users see links. ].com>...




Just to add a remark in your discussion, I have a very old (1942
ed.) physical chemistry textbook by Samuel H. Glasstone, which says
"In the study of chemical kinetics reactions are divided into classes
either by molecularity, that is , by the number of atoms or molecules
taking part in each act leading to chemical reaction, or by the order
of the reaction, that is the number of atoms aor molecules whose
concentrations determine the velocity of the process. Until quite
recently these terms were ragrded as SYNONYMOUS, although molecularity
and order are often identical."

The point is that people had recongnized much before 1942 that order
and molecularity are different things. Laidler's "Chemical Kinetics"
says that the the term order was introduced by Ostwald in 1887 in his
general chemistry textbook (Lehrbuch der allgemeine Chemie).

Snumber6 , are you a chemical kineticist?
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  #6  
Old 10-17-2003, 11:53 AM
SNUMBER6
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Default What does order of a reaction tell us?

>From: [Only registered users see links. ] (Mohammed Farooq)


There is a lot of value in those old books ...


Olden days ... :-)


Yep ... by training 30 years ago ... and there have been many attempts to put
it in practice in industry ... Though now I am more concerned with finding the
mechanism to get bureaucrats to understand faster ...
For instance that 1/2 order reaction ... showing the O2 half order ...got me to
investigate catalysts to increase oxygen dot formation ... and we increased
production rates over 50% ...
One of my first research projects ...

Be seeing you
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Number 6

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  #7  
Old 10-17-2003, 04:05 PM
Mike Darrett
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Default What does order of a reaction tell us?

[Only registered users see links. ] (SNUMBER6) wrote in message news:<[Only registered users see links. ].com>...


Hmm... try fish oil supplements (or better yet, eat fish!), red wine
(port is my personal favorite) and lots of exercise. Cheaper than any
anti-aging drugs.


.... I still keep up with things ...


Um... don't you need THREE data points to get a standard deviation?

A classmate in chemical engineering took this to an extreme... he took
a sociology class (required general ed stuff), where the professor
went into detail about how some sociologist observed his daughter
doing something strange, and contributed some great theory to
sociology. My classmate then remarked, "You have an experiment with
N=1. HOW MANY data points do you need to get a standard deviation?
And you call sociology a Social SCIENCE???" The professor was not too
pleased with him after that...




Mike Darrett
[Only registered users see links. ]
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  #8  
Old 10-17-2003, 05:49 PM
SNUMBER6
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Default What does order of a reaction tell us?

>From: [Only registered users see links. ] (Mike Darrett)


I will experiment with different types of red wine ...


With two points one always gets a straight line ... getting a third takes too
many chances ...(unless your theory demands a parabola in which case 3 points
won't screw things up ...)


Now one data point is always best ... not only no error ... but one can draw
any line one wants ... :-)

In a serious vein ... I have seen one point lines drawn ... but only when the
slope or intercept were fixed by theory ... they do indeed look funny ...


Be seeing you
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Number 6

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  #9  
Old 10-17-2003, 06:24 PM
Mohammed Farooq
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Default What does order of a reaction tell us?

[Only registered users see links. ] (SNUMBER6) wrote in message news:<[Only registered users see links. ].com>...

Thats why I keep hunting for old textbooks from old bookstores. Old
texts are usually more comprehensive, emphasize experimental details.
Have you had the chance of reading Glasstone's P. Chem book?



Though my original question still remains unanswered, what is the
utility of the number "reaction order", except that it is an
experimental exponent in the rate law?
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  #10  
Old 10-17-2003, 07:41 PM
SNUMBER6
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Default What does order of a reaction tell us?

>From: [Only registered users see links. ] (Mohammed Farooq)


One can get ideas about the mechanism of the reaction from the reaction order
....
For instance, the reaction I'm referring to here ...

NaOH + Sn + O2 = Na2SnO3

Was zero order in sodium hydroxide ... and half order in O2 ...
The mechanistic implication is that the rate determining step ...
1) does not involve the OH concentration ...
2) does involve O2 in an equilibruim reaction ...

One can then write some equations ...

O2 <=> 2O
2O + Sn => SnO2 (fast) (or pseudo zero order)
SnO2 + 2NaOH => Na2SnO3 + H2O (fast)

Consistent with the "reaction order" ...
To do a more complete study ... One could look at the effect of temperature and
get enthalpy and entropy of reaction ... the addition of scavengers ...surface
modifiers ... etc. to further prove the mechanism ...
In industry ... as soon as we knew the rate determining step, we used that
knowledge to find ways to increase the reaction rate ... the true mechanism was
immaterial ...



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