CCL: Pressure



Hi

I suppose, Bonoit is talking about reaction rate, nor chemical equilibrium.

Usually, the effect of the pressure is the alteration of energy (mainly stabilization) of a reaction intermediate (complex or product) by non reactive collisions with a third partner.
As this third partner is not explicit included in the rate constants, it is accounted as an effect of the total pressure.

João Brandão
Universidade do Algarve, Portugal


Em 22-07-2013 03:03, Van Dam, Hubertus J HubertusJJ.vanDam~!~pnnl.gov escreveu:
 Sent to CCL by: "Van Dam, Hubertus J" [HubertusJJ.vanDam(-)pnnl.gov]
 Hi Bonoit,
 I assume you are talking about gas-phase reactions. In gas-phase reactions the
 pressure at which a reaction is performed shifts the equilibrium composition. If
 the pressure is increased the equilibrium shifts towards the side with fewer
 molecules and for pressure lowering the opposite happens. Differences in
 temperature have similar effects, with higher temperatures shifting the
 equilibrium to the endothermic side and vice versa. This is a straightforward
 application of Chatelier's principle (in the Netherlands it is referred to as
 the van 't Hoff principle but I haven't been able to find an obvious reference
 to his contribution).
 Best wishes,
 Huub van Dam
 Pacific Northwest National Laboratory
 Tel:  509-372-6441
 -----Original Message-----
   
From: owner-chemistry+hubertus.vandam==pnnl.gov^^^ccl.net [mailto:owner-chemistry+hubertus.vandam==pnnl.gov^^^ccl.net]
 On Behalf Of bonoit bonoit bonoit_10+*+yahoo.fr
     
Sent: Sunday, July 21, 2013 11:11 AM
 To: Van Dam, Hubertus J
 Subject: CCL: Pressure
 Sent to CCL by: "bonoit  bonoit" [bonoit_10[-]yahoo.fr] Dear CCLers,
 I would like to enquire about the meaning of high-pressure limit and
 low-pressure limit and what are their influence on the rate of a given reaction.
 I mean why some products form in the high-pressure limit and other low-pressure
 limit?
 What is the relation of these two pressure limits with temperature?
 Regards
 Bonoithttp://www.ccl.net/cgi-bin/ccl/send_ccl_messagehttp-:-//www.ccl.net/chemistry/sub_unsub.shtmlhttp-:-//www.ccl.net/spammers.txtE-mail
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