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Topic: Dissociation / Ionisation of OH radical  (Read 2725 times)

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Offline s.sally

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Dissociation / Ionisation of OH radical
« on: August 07, 2012, 10:32:00 AM »
Hi everyone!

For a retake exam of Molecular Physical Chemistry I have to solve the next exercise:

Calculate the degree of dissociation of the OH radical at 5000K. Is ionisation an issue at these energies?

For dissociation I have OH* -> O + H, and for ionisation OH* -> OH+ + e-.
To start with, is this correct?

I know how to solve this exercise, you basicly have to formulate an equilibrium constant K, with a translational, electronical, rotational and vibrational contribution.

For the dissociation I get a logical value for K and the degree of dissociation alpha I calculated from K, so I believe my way of working is correct, but the my problem is, for the ionisation reaction, I can't find any values in the CCCDCB database for the characteristic rotational and vibrational temperature for OH+ (I can only find values for OH), and no energy levels either to calculate Kel.

Does anyone have any idea of how to solve this problem?
Maybe OH+ and OH have the same θvib and θrot? But I'm not sure if this is correct?


Thanks in advance!

Offline dipesh747

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Re: Dissociation / Ionisation of OH radical
« Reply #1 on: August 07, 2012, 08:01:45 PM »
If you can't find data I wouldn't worry too much. They won't expect you to search through databases in your exam (at least I hope not!)

I think you have the right about what you need to do, but just so you know:

I would approach the second part of this question by calculating K at 5000k using the Eyring equation. Calculate K for both the dissociation and ionisation reaction, and then you can evaluate the individual rate constants and this should tell you whether or not ionisation plays a big part!

With regard to the last part of your question:

The electronic partition function is calculated using the formula (via transition state theory):

Σg e ^ (ΔE/RT) where g = degeneracy of transition state / degeneracy of reactants and ΔE is the potential barrier of going from OH* to ≠ (unknown transition state) - So I don't think you will find accurate Kel data, as there will be no accurate data for a transition state of that diatomic. (at what point in the electrons motion does it go from being part of an orbital to going to some half way house between being localised on a molecule and not)

With regard to the rotational and vibrational constants - think about what these figures actually encompass - what are the equations behind them, and then think, could they be the same if with OH+ and OH*?
Another way to think about it, I assume you would have dealt with electronic excitation of molecules. The potential energy curves change when they get excited, so if a molecule loses an electron its potential energy curve will change. If this changes, then surely a lot of other things will change too!


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