TertAmyl
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How can I calculate the ~ microwave energy required to decarboxylate a carboxylic acid?
I want to know how I can come to an equation that I can use to approximate the energy required to go from R-COOH --> R + CO2 in a microwave. All I
need is a ball park estimate of the seconds required in the microwave at a given Wattage.
I'm familiar with Hess's Law Hrxn = H1 + H2..
Using:
C-C = 88Kcal/mol
C=O = 127Kcal/mol
C-O = 84Kcal/mol
O-H = 111Kcal/mol
If I add up the bond disassociation energies I get
Hrxn = 199Kcal/mol + -265Kcal/mol OR
Hrxn = -66Kcal/mol OR Hrxn = -276Kj/mol
My initial thought was to calculate the moles per gram of the carboxylic acid (0.00168 moles) then divide by the 276Kj = 463J/g. I get the impression
that this is incorrect though.
If a 1000W microwave is ~50% efficient and I use the equation E(J)=P(W) * S
I come up with 463J / 500W = 0.926 seconds
How far off base am I in how I'm determining what I want? What would be the required steps for me to determine something like this? Could I
approximate the specific heat of the material by taking temperature measurements after specific power/time intervals in the microwave, and some how
use that to calculate what I need?
I appreciate any help pointing me in the right direction.
Edit: To clarify... The reaction is obviously spontaneous and occurs naturally at STP. I want to speed up the process and know how much energy
approximately is needed to theoretically decarboxylate 100% of the carboxylic acid present using a microwave.
[Edited on 24-10-2016 by TertAmyl]
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DraconicAcid
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The enthalpy of reaction won't help you in this case. You'd need to find out the activation energy of the reaction.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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blogfast25
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Quote: Originally posted by TertAmyl  | I want to know how I can come to an equation that I can use to approximate the energy required to go from R-COOH --> R + CO2 in a microwave. All I
need is a ball park estimate of the seconds required in the microwave at a given Wattage.
|
Breaking C-C bonds is hard. in the presence of air you're likely to end up with... CO2 and H2O only!
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TertAmyl
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The reaction happens spontaneously are STP - it is just far too slow to be useful.
I found and attached a publication that contains the energy of activation required for decarboxylation of the molecule. If I understand the paper
correctly the Ea = 85 kJ/mol
Would I use the equation ln k = lnk0 - (E/RT) ?
The paper says E & K0 are determined to be 84.8kj mol-1 & 3.7 x 10^8s-1 respectively
How can I use that information to determine % decarboxylated relative to time & power in a microwave?
Attachment: Journal of Molecular Structure 987 (2011) 67-73.pdf (1.1MB)
This file has been downloaded 575 times
[Edited on 26-10-2016 by TertAmyl]
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wg48
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| Quote: Originally posted by TertAmyl | The reaction happens spontaneously are STP - it is just far too slow to be useful.
I found and attached a publication that contains the energy of activation required for decarboxylation of the molecule. If I understand the paper
correctly the Ea = 85 kJ/mol
Would I use the equation ln k = lnk0 - (E/RT) ?
The paper says E & K0 are determined to be 84.8kj mol-1 & 3.7 x 10^8s-1 respectively
How can I use that information to determine % decarboxylated relative to time & power in a microwave?
[Edited on 26-10-2016 by TertAmyl] |
The short answer is you cannot.
The best you could do is determine it empirically from a series of carefully controlled experiments and hope its reproducible
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blogfast25
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I think you're conflating negative Gibbs free energy changes with kinetics.
Just because:
$$\Delta G^T<0$$
(at some temperature T) doesn't mean the reaction proceeds 'spontaneously'. That's a misconception that continues to do the rounds in amateur
chemist circles. Briefly put:
| Quote: | | Thermodynamics says nothing about kinetics. |
[Edited on 27-10-2016 by blogfast25]
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