From: glhurst@onr.com (Gerald L. Hurst)
Newsgroups: sci.chem
Subject: Re: Does Heat Speed Up or Slow Down a Chem
Date: 24 Feb 1996 07:29:27 GMT

In article <4gldu2$cot@lll-winken.llnl.gov>, shaw@s119.es.llnl.gov (Henry
Shaw 510 423-4645) says:

>In article 6jn@reader2.ix.netcom.com, Alan \"Uncle Al\" Schwartz <uncleal0@ix.netcom.com> () writes:
>> Heat may speed up a reaction, slow it down, or have no effect upon it at 
>> all.  It depends upon the thermodynamics of the reaction.
>
>
> The equilibrium state may change as a function of temperature, but
>heat will *always* speed up the rate of a fundamental reaction.

What is a "fundamental" reaction? If an overall reaction slows
with increasing temperature does that automatically qualify it
as "non-fundamental"?


>> In general, hotter means faster, doubling reaction rates near room temp 
>> for every 10 celsius rise in temperature.
>
>You can't make a general statement like this. 

The heck you can't. Competent chemists have been using this
approximation as a first estimate for thought experiments
for a long time. The fact that the statement doesn't 
qualify as a fundamental law does not mean that it has 
no utility - especially for explaining the typical behaviour 
of simple homogeneous reactions and a lot of heterogeneous
phenomena.

Jerry (Ico)

From: rparson@spotNO.SPAMColorado.edu (Robert Parson)
Newsgroups: sci.chem
Subject: Re: LeChatelier
Date: 14 May 1999 19:50:12 GMT

In article <3738ABD6.DDEB0FF6@worldnet.att.net>,
Eric Lucas  <ealucas@worldnet.att.net> wrote:

>Only because the adiabatic compression will lead to a temperature
>increase, which will indeed change the equilibrium constant.  Under
>isothermal conditions, increasing the pressure with an inert gas will
>not shift the equilibrium.

 This is correct.

>It has no effect on the collision frequency
>of the two reagent gases, and therefore cannot affect the rate of their
>reaction, at least to a good first approximation.

 Be careful here, though. Many gas-phase reactions require three-body
 collisions in order to conserve energy and momentum. (For example,
 simple association reactions like O + O2 + M -> O3 + M; the third body
 partner "M" removes the excess kinetic energy.) The rate of the
 reaction then depends strongly upon the pressure of an inert
 gas. However, the equilibrium is still unaffected since the rate of
 the reverse reaction, collision-induced dissociation of O3 in this
 example, is increased by the same amount.

 ------
 Robert

From: rparson@spotNO.SPAMColorado.edu (Robert Parson)
Newsgroups: sci.chem
Subject: Re: LeChatelier
Date: 15 May 1999 18:03:22 GMT

In article <373CE8B8.212F1C4B@worldnet.att.net>,
Eric Lucas  <ealucas@worldnet.att.net> wrote:
>Robert Parson wrote:
>> >It has no effect on the collision frequency
>> >of the two reagent gases, and therefore cannot affect the rate of their
>> >reaction, at least to a good first approximation.
>>
>>  Be careful here, though. Many gas-phase reactions require three-body
>>  collisions in order to conserve energy and momentum. (For example,
>>  simple association reactions like O + O2 + M -> O3 + M; the third body
>>  partner "M" removes the excess kinetic energy.)
>
>True, but would it be fair to say that this generally only occurs in
>cases (like O3 formation) where the reaction is endothermic (and thus
>requires reactants with lots of kinetic energy), and the product is very
>tenuously stable

 No! (In fact O + O2 -> O3 is exothermic.) This is the _general_
 situation with gas-phase association reactions. A + B -> AB _cannot_
 go by two-body collisions - a third body is required to remove the
 kinetic energy, otherwise AB will fall apart again, as guaranteed by
 microscopic reversibility. (At _extremely_ low pressures, such as one
 finds in interstellar clouds, an alternative mechanism sometimes comes
 into play - the excess kinetic energy can be radiated away as a
 photon. However the transition probabilities for this are extremely low
 and only significant when every other mechanism is shut off.) Also,
 if the reactants are large molecules, it may take a long time for the
 AB reactant complex to dissociate again, by which time it's had a
 chance to hit a wall, dust particle, etc., but this is just another
 form of third-body collision.