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Topic: Rate constant units  (Read 5326 times)

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Offline Johnny010

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Rate constant units
« on: August 17, 2009, 04:50:28 PM »
Hey. Quick question:

What are the units for k in a 3rd order rxn (in mol cm-3)?

I get:

rate=[A][X][Z]
mol1 s-1 = k * mol1cm-3 * mol1cm-3 *mol1cm-3
mol1s-1 = k * mol3cm-9
k = mol1s-1 / mol3cm-9

cancel terms:

k = s-1/mol2cm-9

Then bring up the denominator:
k = mol-2cm9s-1

Is that correct?
Thank you in advance.
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Offline Dan

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Re: Rate constant units
« Reply #1 on: August 17, 2009, 05:19:47 PM »
I think the rate is normally measured in concentration per unit time - in this case mol cm-3 s-1 - your method is fine but I think you need to make that change and see what you get.
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Offline Johnny010

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Re: Rate constant units
« Reply #2 on: August 17, 2009, 05:41:49 PM »
The question asked for it in those units...no idea why but it is normally mol dm-3 s-1.
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Offline Dan

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Re: Rate constant units
« Reply #3 on: August 18, 2009, 03:36:49 AM »
yes, mol cm-3 is an unusual unit for concentration but my point is that I think the rate should be mol cm-3 s-1 and not mol s-1

ie. mol cm-3 s-1 = k.mol cm-3.mol cm-3.mol cm-3
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