On Sat, 13 Apr 2019 at 17:21, Sebastian Kozuch seb.kozuch[]= gmail.com <owner-chemistry]![ccl.net> wrote:

Sent to CCL by: Sebastian Kozuch [seb.kozuch(-)gmail.com] =20 =20 =20 =20
Dear Benoit,
Formally, you cannot know if a reaction is reversible or not with an Arrhenius plot, since you are plotting the "ln k vs. 1/T", an= d k is just the forward rate constant of an elementary step (i.e. it doesn't take into consideration the possibility of the reaction going back). However, if you graph the rate instead of the rate constant ("ln r" instead of "ln k", with r =3D kf -= kr,=C2=A0 kf=3Dforward rare constant, kr=3Dreverse rate constant), then the Arrhenius graph is completely different.
I tried it in excel, considering the reactant and product concentrations equal to one, and the forward and backward pre-exponential values (Af and Ar) equal. This is basically "r =3D exp(-Eaf/RT) - exp (Ear/RT)". The reverse activation energy must b= e larger than the forward one for an exothermic reaction. The result is that at low temperatures the classical Arrhenius plot (using ln kf) and this modified version (using ln r) are equal. But at high temperatures the two diverge. While using k at high T the graph converges to ln Af, using r the graph actually goes slowly to minus inf.
In summary, if you plot the logarithm of the rate of the elementary step instead of the logarithm of the forward rate constant, the Arrhenius plot will show you if the reaction is reversible if it diverges at high temperature. The lower the temperature it diverges, the more reversible the reaction.
I don't think anybody did this analysis before. Am I right? Do we have material for a new paper? ;)

Best,
Sebastian

On 13/4/19 12= :59 PM, Benoit Gearald ablux13:-:gmail.com wrote:
Sent to CCL b=
y: "Benoit  Gearald" [ablux13++gmail.com]
Dear CCLers,

I would like to enquire about the relationship between the Arrhenius plot=
=20
of rate constants of a given reaction with the=20
reversibility/irreversibility. In other words, is it possible to know=20
whether the reaction is reversible or irreversible from Arrhenius plot?

Regards,

BenoitE-mail to subscribers: CH=
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--0000000000005f02d905866b98e4-- From owner-chemistry@ccl.net Sat Apr 13 18:54:00 2019 From: "Eric V. Patterson eric.patterson|-|stonybrook.edu" To: CCL Subject: CCL: Arrhenius plot and reversibility/irreversibility Message-Id: <-53679-190413185153-7726-uEYk47gkxDFfRNRFQiuIJQ[*]server.ccl.net> X-Original-From: "Eric V. Patterson" Content-Type: multipart/alternative; boundary="Apple-Mail=_F2971F06-06E8-421C-8B19-DFF076D4F283" Date: Sat, 13 Apr 2019 18:51:44 -0400 Mime-Version: 1.0 (Mac OS X Mail 12.4 \(3445.104.8\)) Sent to CCL by: "Eric V. Patterson" [eric.patterson*_*stonybrook.edu] --Apple-Mail=_F2971F06-06E8-421C-8B19-DFF076D4F283 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=utf-8 To add my 2=C2=A2. One may infer something about the thermodynamics if one knows something = about the kinetics, and vice versa (see Hammond Postulate), but these = are correlations with plenty of exceptions. To truly know whether a = reaction is reversible, one must know the actual forward and reverse = barriers, and then decide what fraction of forward/reverse makes = something reversible or irreversible. Even that last step is not at all = straightforward as there are no hard cutoffs and it is subject to the = molecularity issue just mentioned by Peter. Best, Eric > On Apr 13, 2019, at 12:05 PM, Peter Kraus peter.kraus : = geeksonbikes.net wrote: >=20 > Dear Benoit, Dear Sebastian, >=20 > in some cases, an "Arrhenius plot" can show the "reaction" is = irreversible: if it's not linear (on a ln(k) vs 1/T scales), and has = kinks, it's very likely not an elementary step, and microscopic = reversibility won't hold. I doubt it's possible to prove that the = reaction you have is a reversible reaction by just looking at an = Arrhenius plot - proving anything happens the way we think in kinetics = is difficult! >=20 > As for the plotting of "ln r", the nomenclature I was taught was based = on the rate law: r_{f,r} =3D k_{f,r} [a]^z [b]^y ... [m]^n - rate is = always in 1/s (or mol/s), rate constants (k_f or k_r) depend on the = apparent molecularity of the reaction. You can only subtract them like = that, when each side has the same molecularity! As for the "temperature = and reversibility" behaviour, I believe you're describing an equilibrium = constant, which can be calculated from thermochemistry. >=20 > Best regards, > Peter >=20 >=20 > On Sat, 13 Apr 2019 at 17:21, Sebastian Kozuch seb.kozuch[]gmail.com =

> wrote: > Sent to CCL by: Sebastian Kozuch [seb.kozuch(-)gmail.com = ] > Dear Benoit, > Formally, you cannot know if a reaction is reversible or not with an = Arrhenius plot, since you are plotting the "ln k vs. 1/T", and k is just = the forward rate constant of an elementary step (i.e. it doesn't take = into consideration the possibility of the reaction going back). However, = if you graph the rate instead of the rate constant ("ln r" instead of = "ln k", with r =3D kf - kr, kf=3Dforward rare constant, kr=3Dreverse = rate constant), then the Arrhenius graph is completely different. > I tried it in excel, considering the reactant and product = concentrations equal to one, and the forward and backward = pre-exponential values (Af and Ar) equal. This is basically "r =3D = exp(-Eaf/RT) - exp (Ear/RT)". The reverse activation energy must be = larger than the forward one for an exothermic reaction. The result is = that at low temperatures the classical Arrhenius plot (using ln kf) and = this modified version (using ln r) are equal. But at high temperatures = the two diverge. While using k at high T the graph converges to ln Af, = using r the graph actually goes slowly to minus inf. > In summary, if you plot the logarithm of the rate of the elementary = step instead of the logarithm of the forward rate constant, the = Arrhenius plot will show you if the reaction is reversible if it = diverges at high temperature. The lower the temperature it diverges, the = more reversible the reaction. > I don't think anybody did this analysis before. Am I right? Do we have = material for a new paper? ;) >=20 > Best, > Sebastian >=20 > On 13/4/19 12:59 PM, Benoit Gearald ablux13:-:gmail.com = wrote: >> Sent to CCL by: "Benoit Gearald" [ablux13++gmail.com = ] >> Dear CCLers, >>=20 >> I would like to enquire about the relationship between the Arrhenius = plot=20 >> of rate constants of a given reaction with the=20 >> reversibility/irreversibility. In other words, is it possible to know=20= >> whether the reaction is reversible or irreversible from Arrhenius = plot? >>=20 >> Regards, >>=20 >> BenoitE-mail to subscribers: CHEMISTRY:+:ccl.net = or use:= >>=20 >> E-mail to administrators: CHEMISTRY-REQUEST:+:ccl.net = or use >> =http://www.ccl.net/chemistr= y/sub_unsub.shtml >>=20= >>=20=20 >> Conferences: = http://server.ccl.net/chemistry/announcements/conferences/ = >>=20 >> Search Messages: = http://www.ccl.net/chemistry/searchccl/index.shtmlhttp://www.ccl.net/spamm= ers.txt >>=20= >>=20 >>=20 >=20 > --=20 > xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx > ..........Sebastian Kozuch........... > xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx > .Ben Gurion University of the Negev . > ..........Beer Sheva, Israel......... > .......... kozuch:+:bgu.ac.il ......... > ....http://www.bgu.ac.il/~kozuch/.. .. > xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx > -=3D This is automatically added to each message by the mailing script = =3D-E-mail to subscribers: CHEMISTRY-,-ccl.net = or use: == E-mail to = administrators: CHEMISTRY-REQUEST-,-ccl.net = or use == Before posting, check = wait time at: http://www.ccl.net Job: = http://www.ccl.net/jobs Conferences: = http://server.ccl.net/chemistry/announcements/conferences/ = Search = Messages: http://www.ccl.net/chemistry/searchccl/index.shtml = http://www.ccl.net/spa= mmers.txt RTFI: = http://www.ccl.net/chemistry/aboutccl/instructions/ = >=20 > --=20 > Sent from your iPhone. No, really. --Apple-Mail=_F2971F06-06E8-421C-8B19-DFF076D4F283 Content-Transfer-Encoding: quoted-printable Content-Type: text/html; charset=utf-8 To = add my 2=C2=A2.

One = may infer something about the thermodynamics if = one knows something about the kinetics, and vice versa (see Hammond = Postulate), but these are correlations with plenty of exceptions. = To truly know whether a reaction is reversible, one must know the = actual forward and reverse barriers, and then decide what fraction of = forward/reverse makes something reversible or irreversible. Even = that last step is not at all straightforward as there are no hard = cutoffs and it is subject to the molecularity issue just mentioned by = Peter.

Best,

Eric

On Apr 13, 2019, at 12:05 PM, Peter Kraus peter.kraus : geeksonbikes.net <owner-chemistry##ccl.net> wrote:
Dear Benoit, Dear Sebastian,

in some cases, an = "Arrhenius plot" can show the "reaction" is irreversible: if it's not = linear (on a ln(k) vs 1/T scales), and has kinks, it's very likely not = an elementary step, and microscopic reversibility won't hold. I doubt = it's possible to prove that the reaction you have is a reversible = reaction by just looking at an Arrhenius plot - proving anything happens = the way we think in kinetics is difficult!

As for the plotting of = "ln r", the nomenclature I was taught was based on the rate law: r_{f,r} = =3D k_{f,r} [a]^z [b]^y ... [m]^n - rate is always in 1/s (or mol/s), = rate constants (k_f or k_r) depend on the apparent molecularity of the = reaction. You can only subtract them like that, when each side has the = same molecularity! As for the "temperature and reversibility" behaviour, = I believe you're describing an equilibrium constant, which can be = calculated from thermochemistry.

Best regards,
Peter
On Sat, 13 Apr 2019 at 17:21, Sebastian = Kozuch seb.kozuch[]gmail.com = <owner-chemistry-,-ccl.net> wrote:
Sent to CCL by: Sebastian Kozuch [seb.kozuch(-)gmail.com] =20 =20 =20 =20
Dear Benoit,
Formally, you cannot know if a reaction is reversible or not with an Arrhenius plot, since you are plotting the "ln k vs. 1/T", and k is just the forward rate constant of an elementary step (i.e. it doesn't take into consideration the possibility of the reaction going back). However, if you graph the rate instead of the rate constant ("ln r" instead of "ln k", with r =3D kf - kr, = kf=3Dforward rare constant, kr=3Dreverse rate constant), then the Arrhenius graph is completely different.
I tried it in excel, considering the reactant and product concentrations equal to one, and the forward and backward pre-exponential values (Af and Ar) equal. This is basically "r =3D exp(-Eaf/RT) - exp (Ear/RT)". The reverse activation energy must be larger than the forward one for an exothermic reaction. The result is that at low temperatures the classical Arrhenius plot (using ln kf) and this modified version (using ln r) are equal. But at high temperatures the two diverge. While using k at high T the graph converges to ln Af, using r the graph actually goes slowly to minus inf.
In summary, if you plot the logarithm of the rate of the elementary step instead of the logarithm of the forward rate constant, the Arrhenius plot will show you if the reaction is reversible if it diverges at high temperature. The lower the temperature it diverges, the more reversible the reaction.
I don't think anybody did this analysis before. Am I right? Do we have material for a new paper? ;)

Best,
Sebastian

On 13/4/19 = 12:59 PM, Benoit Gearald ablux13:-:gmail.com wrote:
Sent to =
CCL by: "Benoit  Gearald" [ablux13++gmail.com]
Dear CCLers,

I would like to enquire about the relationship between the Arrhenius =
plot=20
of rate constants of a given reaction with the=20
reversibility/irreversibility. In other words, is it possible to know=20
whether the reaction is reversible or irreversible from Arrhenius plot?

Regards,

BenoitE-mail to subscribers: CHEMISTRY:+:ccl.net or use:
      http://www.ccl.net/cgi-bin/ccl/send_ccl_message

E-mail to administrators: CHEMISTRY-REQUEST:+:ccl.net or use
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=20
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
..........Sebastian Kozuch...........
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
.Ben Gurion University of the Negev .
..........Beer Sheva, Israel.........
.......... kozuch:+:bgu.ac.il .........
....http://www.bgu.ac.il/~kozuch/....
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
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