From owner-chemistry@ccl.net Wed Nov  5 12:19:00 2008
From: "David Gallagher gallagher.da-*-gmail.com" <owner-chemistry-.-server.ccl.net>
To: CCL
Subject: CCL: Question: Arrhenius parameters with MOPAC
Message-Id: <-38036-081105121334-25470-8JEhQrscR1FaR/YPzFvZDQ-.-server.ccl.net>
X-Original-From: David Gallagher <gallagher.da(a)gmail.com>
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Date: Wed, 05 Nov 2008 09:13:28 -0800
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Sent to CCL by: David Gallagher [gallagher.da() gmail.com]
Hi Carlos,

In order to calculate the activation energy for a reaction to use in 
the Arrhenius equation, you would first need to consider all the 
possible mechanisms in the appropriate environment, then characterize 
and verify the transition state(s) for each reaction path.  The 
activation energy (Ea) will be the free energy of the transition 
state (of the lowest energy path) minus the free energy of the reactants.

The heats of formation of reagents and products will provide the heat 
of reaction (thermodynamics) but not the activation energy 
(kinetics).  There is more explanation and some notes about reaction 
modeling at: http://www.cacheresearch.com/presentations.html posted 
on 22 April 2008 (CCL:G: problems with a transition state)

The reaction you mention is not balanced as there is an extra water 
molecule on the right.  However, in the gas phase, this reaction 
would presumably be a proton transfer from phosphoric acid to ammonia 
which, in MOPAC2007 PM6 occurs spontaneously when optimizing the 
neutral molecules in close proximity, suggesting a very low or zero 
activation energy (i.e. very fast).

In aqueous solution, assuming the ammonium and phosphate ions already 
exist, the main reaction would be solvated proton plus hydroxyl 
forming covalent water.  The fusion of two such ions is also likely 
to have little or zero activation energy (i.e. very fast), and is 
known to be exothermic.

I hope this helps.

David Gallagher
CACheResearch.com


At 01:19 PM 11/4/2008, Carlos Abraham Diaz cancerbero_85%%hotmail.com wrote:

>Sent to CCL by: "Carlos Abraham Diaz" [cancerbero_85|hotmail.com]
>Hello. I need some help in the determination of the activation 
>energy of this reaction:
>
>   NH3  +      H3PO4         ------>   NH4H2PO4              +    H2O
>ammonia   phosphoric acid           ammonium  phosphate        water
>
>It's a neutralization and exothermic reaction. And however a fast 
>raction rate (i can't measure with standard equipment) I assume that 
>is a elementary reaction.
>
>My questions are:
>
>IT'S POSSIBLE CALCULATE THE Ea WITH MOPAC IN A NORMAL PC?
>THERE IS NOT EXPERIMENTAL DATA ABOUT THIS REACTION?
>
>I have the heats of formation of reagents and products and i tried the
>bell-evans-polanyi correlation:
>
>Ea = E* + BHf
>
>And i found for a exothermic reaction that
>
>Ea = 40 + 0.25Hf
>
>But the result is a negative Ea.
>
>I don't have so much knowledge in computational chemistry and i only 
>need the numeric valor of the Ea and/or the reference.
>Thank you.
>
>Carlos Abraham Diaz
>cancerbero_85*|*hotmail.com
>Ciudad Obregon, Sonora, Mexico.