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Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: emu on September 07, 2017, 05:33:39 AM

Title: I can't find enhalpies of combustion for isomers of Decanoic & Eicosanoic acids!
Post by: emu on September 07, 2017, 05:33:39 AM

Hi! I'm new to the forum, and apologise if I'm writing in the wrong forum..

I need to prove in a statement, that the ACID isomers of the LINEAR ACIDs c10h20o2 and c20h40o2 (decanoic and eicosanoic) are different when it comes to heat/enthalpy of combustion (preferably in liquid).

I have searched and searched but just can't seem to find these numbers anywhere (meaning for the isomers, I do have them for the linear molecules) 

Could someone who knows a site where to find them, link them for me, or if you have the numbers (kJ/mol) please write them here and tell the source.

Million thanks to whoever has the answers!!! I'm in a rush with this, so the sooner the better! Thank you.

Title: Re: I can't find enhalpies of combustion for isomers of Decanoic & Eicosanoic acids!
Post by: Babcock_Hall on September 07, 2017, 09:29:03 AM

Is this a homework assignment?  Can you tell us where you have searched?

Title: Re: I can't find enhalpies of combustion for isomers of Decanoic & Eicosanoic acids!
Post by: Corribus on September 07, 2017, 09:37:52 AM

Well for one thing you won't find heats of combustion for liquid phase because combustion happens in the gas phase.

The data you want also may simply not exist - as molecules get larger it's often difficult to find thermodynamic data for different isomers because of the difficulty (or cost) of purifying them.

If you checked NIST database, and the CRC, and didn't find the molecules you are looking for, you may be out of luck.

Title: Re: I can't find enhalpies of combustion for isomers of Decanoic & Eicosanoic acids!
Post by: Arkcon on September 07, 2017, 03:43:22 PM

Quote from: emu on September 07, 2017, 05:33:39 AM

I need to prove in a statement, that the ACID isomers of the LINEAR ACIDs c10h20o2 and c20h40o2 (decanoic and eicosanoic) are different when it comes to heat/enthalpy of combustion (preferably in liquid).

Also, why are you highlighting the word 'ACID', are you trying to compare the enthalpy of acid to base form?

Title: Re: I can't find enhalpies of combustion for isomers of Decanoic & Eicosanoic acids!
Post by: sjb on September 07, 2017, 05:10:59 PM

Quote from: Arkcon on September 07, 2017, 03:43:22 PM

Also, why are you highlighting the word 'ACID', are you trying to compare the enthalpy of acid to base form?

As opposed to e.g. esters, or keto-alcohols I guess?

Title: Re: I can't find enhalpies of combustion for isomers of Decanoic & Eicosanoic acids!
Post by: Enthalpy on October 10, 2017, 07:14:58 AM

Heats of formation or combustion are scarce data. They must be measured individually but there are so many compounds.

If the isomerism, like branching, is at least 2 carbons away from the acid, then you can apply to the acids the difference observed on the alkanes. Some data exists for the alkanes; the "CRC Handbook of Chemistry and Physics" is a convenient compilation. Doing this needs some caution. Whether this will be a "proof"? To my eyes, only measures in a lab would prove it.

The difference is small, especially for a combustion.