Re: Fw: ORGLIST:Activation energy

Date view Thread view Subject view Author view Attachment view

From: Jacob Zabicky (zabicky$##$bgumail.bgu.ac.il)
Date: Thu Sep 18 2003 - 14:00:44 EDT


Thanks Thomas for the rules of thumb.

  It is nice to have a gut feeling for things and that is really the
point of having those rules. But one shouldn't use them in actual
experimental design, unless one is desperately out of information about
the system.

For instance, if you know the normal boiling point of a liquid you may
say it's about 20 degrees less at a pressure (Pvp) of one half
atmosphere. One may even try to extrapolate e couple of steps more. No
big deal if one fails in the prediction by 10 or 20 degrees.
Examination of actual vapor presure-temperature values may point to a
good fit or large departures in any sampling of organic compounds. I
have on my desk an old edition of the Chemical Rubber Handbook,
carrying a table of Pvp vs T up to 1 atm. The tabulated pressures are
760 torr and 400 torr, which is fine because the drop is not even 1/2
atmosphere, and many of the compounds show a T difference of more than
20 degrees, some exactly 20 (still a little bit over the rule) and a
few somewhat less than 20. The same goes for a table listing 1 and 2
atm for the more volatile organic compounds.

Another rule of thumb dealing with boiling points that may save the
day, is that the normal boiling point of a pure compound is about
two-thirds of the critical temperature on the absolute scale. That may
give the limiting temperature at which one may still have a liquid
state, provided the compound is stable when heated that much. This rule
is a real beauty and utterly out of the blue, because there is nothing
theoretically special with the pressure of 1 atm that defines normal
boiling points.

As for reaction rates, the rule doubling the rate of reaction for every
10 degrees raise may give a relatively good appreciation for simple
processes in the "chemical regime", proceeding in the lab at a moderate
rate. For reactions that are too sluggish or too fast at ordinary
temperatures, complex mechanisms, heterogeneous reactions, or if the
process is in a diffusion regime the rule may be totally misleading.

All the best,

Jacob

On Thursday, Sep 18, 2003, at 19:42 Asia/Jerusalem, Jacob Klug wrote:

>
> ----- Original Message -----
> From: "Hoyer" <hoyer$##$webspeed.dk>
> To: "'Organic List'" <everybody$##$orglist.net>
> Sent: Wednesday, September 17, 2003 9:50 AM
> Subject: Re: ORGLIST:Activation energy
>
>
>> Thanks for your reply, Jacob.
>>
>> There is a rule of thumb stating that the boiling ponit of a liquid
>> drops
> 20
>> degrees when you halve the pressure. Of course it depends on the
>> liquid,
> but
>> as a rule of thumb, it works. A similar, even more crude, rule of
>> thumb
>> might exist for reaction rates and temp.
>>
>> I appreciate the examples you give of higher temperatures in a small
> domain
>> of a mixture. I am more suspicious about the explanation of the
>> microwave-effect (the mere existence of which is under debate), given
>> in a
>> book on microwaves in organic synthesis. The author invokes the
>> concept of
>> an instantaneous temperature much higher than the bulk temperature
>> originating in the local event of absorption of a quantum of microwave
>> energy. Working through a realistic example, the author uses an
>> activation
>> energy of 50 kcal/mol - but is that within bounds?
>>
>> yours
>> Thomas
>>
>> _______________________________________________
>> ORGLIST - Organic Chemistry Mailing List
>> Website / Archive / FAQ: http://www.orglist.net
>> To post a message (TO EVERYBODY) send to everybody$##$orglist.net
>> To unsubscribe, send to everybody-request$##$orglist.net the message:
> unsubscribe your_orglist_password your_address
>> List coordinator: Joao Aires de Sousa (coordinator$##$orglist.net)
>>
>
>
***************************************************
Prof. Jacob Zabicky
Institutes for Applied Research
Ben-Gurion University of the Negev
P. O. Box 653, Beer-Sheva, 84105, Israel

Phone: +972-8-6461271; Fax: +972-8-6472969
zabicky$##$bgumail.bgu.ac.il
http://www.bgu.ac.il/IAR/Zabicky.htm
-------------------------------------------------
Private:
P. O. Box 12366, Beer-Sheva, 84863, Israel
Phone: +972-8-6496792

_______________________________________________
ORGLIST - Organic Chemistry Mailing List
Website / Archive / FAQ: http://www.orglist.net
To post a message (TO EVERYBODY) send to everybody$##$orglist.net
To unsubscribe, send to everybody-request$##$orglist.net the message: unsubscribe your_orglist_password your_address
List coordinator: Joao Aires de Sousa (coordinator$##$orglist.net)


Date view Thread view Subject view Author view Attachment view

This archive was generated by hypermail 2.1.4 : Wed Oct 01 2003 - 10:37:01 EDT