Is the reaction rate constant, k(T), of a molecular chemical reaction affected by the kind of a third inert species (CO2 or N2 for example) under the constant temperature condition ?
The reaction considered could be for example : H2+ O = H + OH
"if it is inert how can that affect? " Read textbooks on gas-phase chemical kinetics'
Reaction of Oxygen Atoms with Hydrogen
Thrush, BA Clyne, M A A Thrush, B A
Nature. , 1961, Vol.189(475), p.135-136
Sometimes the gas-phase reactions have pressure dependence. It can be important if an elementary reaction is complex-forming: It means that there are deep potential wells on the potential energy surface (PES) of the reactions. See the attached schematic PES of the S + HO2 reaction. During such kind of reactions the systems may spend a few time period in the deep potential well when the two body collision happens. Such kind of "complex" is not stable, it will dissociate after a while.
But...when the pressure is high, there are third body collision (they will be more frequent). A third body (He, Ar, N2, CO2) may collide with this unstable (transient) complex. In such kind of third body collisions it is possible that the collision with the third body stabilizes the complex through energy transfer or change the result of the original two body collision (S+HO2). That is why one can observe pressure dependence of elementary reactions.
Dear Mirosław Grzesik,
Your answer is misleading and wrong ! These molecules are not reagents, there is no reaction. In this case there are only inelastic collisions that are responsible for the energy transfer.
Why do you give answers if you are not an expert in this field?
I'm not sure what the question is really asking but reaction rates are usually proportional to the concentration of reactants (to some power often 1) and diluting with a nonreactive component will reduce the concentration/partial pressure and reduce the reaction rate. If it is a catalytic reaction the non reactive species could also occupy active sites on the catalyst. Is that really the question you wanted an answer to?
Dear Grzegorz Boczkaj,
This isn't so straightforward to give a Yes or No. It strongly depends on the nature of elementary reactions and the pressure of the buffer gas.
Oh my God...here are a lot of charlatans (not you Yurii Geletii)!
I really can't imagine...why does somebody give pointless advices if he/she isn't an expert of this field?
Such kind of reckless, arrogant behaviour is extremely destructive and dangerous.
Before you try to give any advice think it through again and again (and again) what you want to write....or open a basic textbook, please.
Rick Manner, Grzegorz Boczkaj and Mirosław Grzesik...please, open a basic level reaction kinetics book and refresh your knowlede. (Check the chapter on the pressure dependence of gas phase reactions
Best,
Peter
Dear All,
In combustion kinetic models, the H2+ O = H + OH
reaction is considered to be pressure independent.
(supported by both experiments and theory)
See a review of the rate parameterisation of this reaction in
Article Uncertainty of the rate parameters of several important elem...
in chapter 7.3.3.
Best,
Tibor
Péter Szabó Wow you have not made a very good first impression. I don't expect Kemal Büyükakın will bother to clarify his question but I am pretty sure that your response did not answer his query about "the kind of a third inert species". Are you really trying to suggest that a transient third body complex is an inert species?
Also I suspect that I know quite a bit more about chemical reaction kinetics than you will ever know. That’s not your fault. You simply have not had the opportunity to explore the wide range of commercial operations that I have so your experience is extremely limited.
In gas phase chemical reactions are commonly affected by the third inert species ( in other words by the total gas pressure). This is classic and is described in text-books. The answer for the main question is YES. There is nothing to discuss.
Concerning the reaction H2 + O -> H + OH. I'm sure that this reaction is very well studied. I don't know whether it depends on total pressure. Kemal, if your interested in this reaction check NIST data base on reaction rate constants.
Thanks for the answers. I am studying on gas phase combustion phenomena. As a mechanical engineer, i don't know much about chemical kinetics in details. I just wanted to ask you, for an enlightment, if the reaction rate constant of the two reactants could be changed by changing the kind of the third inert species (CO2 or N2), under the same temperature, pressure and reactants concentrations @ Rick Manner
This is strictly true for pressure dependent reactions, whose rate is sensitive to collisional energy transfer, as energization (unimolecular reactions) or stabilization (recombination reactions and chemically activated reactions) of a reactive intermediate plays an role in them.
please see:
https://www3.nd.edu/~powers/ame.60636/chemkin2000.pdf
chapter 2.4.3
Efficiency of collisional energy transfer with a third body is characterized by the third body efficiency, which is larger for molecules with more and softer vibrational modes with whom collisions are more inelastic.
see for example:
http://combustion.berkeley.edu/gri-mech/version30/files30/grimech30.dat
O+H+MOH+M 5.000E+17 -1.000 .00 H2/2.00/ H2O/6.00/ CH4/2.00/ CO/1.50/ CO2/2.00/ C2H6/3.00/ AR/ .70/
Reaction O+H2H+OH is the most influential reaction in combustion kinetics, and it is not pressure dependent. (Our review and recommendation (see in my previous post) takes into account all data in the NIST database - recommended by Yurii V Geletii ).
see also:
“Third-Body” collision efficiencies for combustion modeling: Hydrocarbons in atomic and diatomic baths
https://www.sciencedirect.com/science/article/pii/S1540748914001084
So, if we just replace N2 with CO2, which are both inert species in the considered reaction, by keeping the pressure, temperature and reactants' concentrations, can we change the reaction rate constant, k(T) ? This is the best i can do to clarify my question @Rick Manner. Thanks
It depends on the reaction.
For O+HOH yes, but not for O+H2H+OH.
Considering reaction
O+H+MOH+M 5.000E+17 -1.000 .00
H2/2.00/ H2O/6.00/ CH4/2.00/ CO/1.50/ CO2/2.00/ C2H6/3.00/ AR/ .70/
efficiency of 1 should be used for N2.
At low pressures in CO2 inert gas, the reaction will proceed twice as fast as in N2. This true only in the low pressure limit. At high pressures there will be no difference.
Please try to read this to fully understand:
https://www3.nd.edu/~powers/ame.60636/chemkin2000.pdf
chapter 2.4.3
This is common knowledge in combustion kinetics.
Elsewhere at this forum, I have considered the effect of hydrogen dilution in helium, for the case of high-temperature thermal dissociation of hydrogen (H2) to atomic hydrogen ― supposing that equilibrium is attained: https://www.researchgate.net/post/how_can_we_calculate_gamma_specific_heat_ratio_for_real_gas_chemical_non-equili
"if it is inert how can that affect? " Read textbooks on gas-phase chemical kinetics'
Dear V. V. Naumov,
First, we have to agree how we define "the reaction rate constant." You are right: "it is not so simple as it seems."
Two years ago I posted a question:
https://www.researchgate.net/post/What_is_the_reaction_rate_constant_and_how_to_measure_it
Strictly speaking, the answer will definitely be NO. Since the probability that THREE molecules meet simultaneously is almost 0. However, when two molecules meet, fairly living complexes can form. And if the lifetime of this complex is enough to meet with the third molecule, then we can talk about the influence of the third body.
Concerning the reaction H2 + O -> H + OH the answer is no at pressures of one atmosphere.
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