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#Post#: 1013--------------------------------------------------
chemical kinetics
DIR By: chaitra
Date: October 29, 2012, 11:06 am
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how does rate constant k of a reaction depend on pressure ???
#Post#: 1014--------------------------------------------------
Re: chemical kinetics
DIR By: Cheminized
Date: October 29, 2012, 11:24 am
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as a shallow answer one might say that if the reaction mechanism
is pressure dependent this would happen because that's what
catalysis is all about , a catalyst should alter the reaction
mechanism so the resulting reaction path is more favorable
(energetic point of view ).
so if the pressure would alter the reaction path resulting in a
different activation energy then the rate constant is pressure
dependent and that's for the overall reaction:
assuming that a ----> d is our reaction , and in low pressures
it goes like this :
a---->c----->d and the rate constant for that recation is k
and in high pressures it goes like :
a---->b---->d and the rate constant is k'.
i hope that was helpful .
keep doing chemistry stuffs.
Cheminized ;D
#Post#: 1015--------------------------------------------------
Re: chemical kinetics
DIR By: chaitra
Date: October 29, 2012, 11:45 am
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thankyou sir :)
As i have understood from your reply the rate constant do vary
with pressure wrt CATALYSIS.
i got that doubt when i read from a book that The pressure
dependence of k is small and is usually ignored.
#Post#: 1016--------------------------------------------------
Re: chemical kinetics
DIR By: Cheminized
Date: October 29, 2012, 11:53 am
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i dont know wrt....
but i meant that like the catalyst would change the mechanism
,pressure might in some cases do the same thing but pressure is
not a catalyst , a catalyst is compound. ;D
#Post#: 1017--------------------------------------------------
Re: chemical kinetics
DIR By: chaitra
Date: October 29, 2012, 12:02 pm
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thanku sir and i know the use of a catalyst ;D and wrt is with
respect to (sorry for using the short form) :P
#Post#: 1018--------------------------------------------------
Re: chemical kinetics
DIR By: Chemist@
Date: October 29, 2012, 1:21 pm
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Pressure is directly proportional to kinetic energy. So, if you
increase the pressure in a container where gasses should react,
the kinetic energy of the gas' molecules (or atoms) will
increase, meaning that their speed will increase, meaning that
the collisions will be more frequent. Now, from Arrhenius'
equation you can conclude what will happen with k.
#Post#: 1019--------------------------------------------------
Re: chemical kinetics
DIR By: Cheminized
Date: October 29, 2012, 3:20 pm
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pressure is derictly proportional to the kenitic energy witch
means that if you give the gaseouse molecules more energy that
would result in an encrement in pressure and it is absolutly
right , and it explain why the rate of reaction will increase if
give the system extra energy, but if you where to apply an extra
pressure there would be more collisions thus an encrement in the
rate of reaction and that is the same effect as if you where to
increase the concentration of the reactant and in these cases
the dependency of rate constant on the pressure can be neglected
, but at some point you cant neglect the effect of pressure on
the rate constant as i stated above in cases where the mechanism
is pressure dependent , as an example : unimolecular
dissociation, in this kind of reactions the pressure would even
change the reaction order, youcan read about pressure dependent
mechanisms such like Lindman mechanism of unimolecular
dissociation.
Thank you for opening the discussion, im sure the topic is
clearer for any one who would read it.
#Post#: 1021--------------------------------------------------
Re: chemical kinetics
DIR By: chaitra
Date: October 30, 2012, 8:42 am
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thankyou so much sir :):) i learnt more than what i had thought
i would from this discussion :)
but when we refer REPLY #5, according to Arrhenius equation k
should increase with pressure(when we substitute RT by PV) ??
#Post#: 1023--------------------------------------------------
Re: chemical kinetics
DIR By: Cheminized
Date: October 30, 2012, 4:17 pm
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im so glad that you learned something out of the discussion...
PV=RT describes ideal gas but, even though we need to discuss
it:
that equation is for one mole of the gas so lets right it as
follows:
PV=nRT n: number of gaseouse moles in the system ====>
RT=PV/n
know lets ask a question how can we increase the pressure:
1) by decreasing the volume of the reactant .... now if we
decrease the volume to the tenth of the original volume we find
that the pressure of the system will be ten fold increased and
this would arise from same equation PV=nRT as long as the
temperature and n remain constants so decreasing the volume will
not change k.
2) the reactor volume is not changed but we pump more of the
reactant :
here the variable is n , and from the same equation you can find
that P/n is a constant as long as the temperature and the volume
are constants, so pumping more of the reactant in same reactor
will not affect k.
3) volume and temperature are constants but we pump an inactive
gas in the reactor:
the over all pressure is increased so as n and we have the same
situation as discussed above, but keep in mind that pumping an
inert gas in the system will decease the reaction rate.
our discussion is no sense because PV=nRT describes only IDEAL
gases, and ideal gases have no interactions between the
molecules , so you can't describe a gaseous reactant as an ideal
gas cause you cant have a reaction without having interactions
between molecules.
and im available for any further discussion.... ;D
#Post#: 1024--------------------------------------------------
Re: chemical kinetics
DIR By: chaitra
Date: October 30, 2012, 8:21 pm
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thankyou loads sir :) :) everything in clear. thankyou again
:)
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