May 07, 2021, 08:55:12 PM
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Topic: Desperately need enthalpy of combustion values for alcohols (and their isomers)  (Read 327 times)

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Offline CanerMom76

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Hello, fellow chemists! For the past few days I have been searching the web for data I can use for a science investigation. Unfortunately, this has proven rather difficult. My research question is "how does the position of the hydroxyl group in alcohols affect the enthalpy of combustion". The data that I need are the enthalpies of combustion values for the first 10 alcohols as well as their isomers (i.e. all the values from methanol to 5-Decanol). So far, the most useful source has been NIST (https://webbook.nist.gov/chemistry/name-ser/), but it does not give the enthalpies of combustion values for isomers that come after 3-pentanol.
My hope is that someone in this subreddit has access to a resource containing the data I'm looking for who would be willing to share it (be it a textbook, database, research paper etc.). It would be greatly appreciated! :)

Offline CanerMom76

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I would actually be willing to put a $50 (US) bounty on this as well! Provided I have received useful data with all the enthalpy of combustion values that I need (2-hexanol, 3-hexanol, 2-heptanol, 3-heptanol, 4-heptanol, 2-octanol, 3-octanol, 4-octanol, 2-nonanol, 3-nonanol, 4-nonanol, 5-nonanol, 2-decanol, 3-decanol, 4-decanol, 5-decanol). :)

Offline Orcio_Dojek

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@CanerMom76 If you can't find data, you can also calculate it on the base of standard enthalpy of formation (easier to find).
« Last Edit: March 10, 2021, 05:26:09 PM by Orcio_Dojek »

Offline CanerMom76

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That sounds like a good solution to circumvent my problem, but I'm afraid I'm not familiar with that method. Is there a formula/law you could refer me to? Thank you for your response :)

Offline Orcio_Dojek

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Standard enthalpy of combustion (for example CH3-CH2-CH2OH):

CH3-CH2-CH2OH (liquid) + 4,5 O2  :rarrow: 3 CO2 (g) + 4 H2O (liquid)

Standard enthalpy of formation (ΔHf):

3 C (graphite) + 4 H2 + 1/2 O2 :rarrow: CH3-CH2-CH2OH (liquid)

So..

Standard enthalpy of combustion (ΔHc) of C3H7OH will be:

3 ΔHf CO2 + 4 ΔHf H2O - ΔHf C3H7OH - 4,5 ΔHf O2


All you need to do is to find standard enthalpy formation of CO2, H2O, O2 and respective alcohol. Different alcohol - different stoichiometry, but the same rule. It is called a Hess's Law.
« Last Edit: March 11, 2021, 02:01:57 PM by Orcio_Dojek »

Offline Enthalpy

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The enthalpy of formation is the standard way of computing, because there are fewer of them. Combustion is one reaction among many, tables won't give all the reactions, but from the enthalpies of formation of the reactants and products you deduce the reaction, here a combustion.

For instance the handbook of chemistry and physics has a tables of the heats of formation. Search also for Janaf, Argonne...  Whatever the table, double and triple-check that all compounds are in the desired state, generally they aren't. And beware that most tables are just meaningless values estimated by software (inaccurate and for a gas at zero kelvin), totally insufficient for thermochemistry.

Beware two heats of combustion exist, depending on whether the produced water is liquid or gaseous. You must check each and every time.

Note also that the position of the hydroxyl has the same influence of the heats of formation and combustion (possibly with a sign), because isomers follow the same combustion reaction. That is, if an isomer has 1 J more heat of formation, the combustion produces 1 J more too.
« Last Edit: March 11, 2021, 08:25:17 PM by Enthalpy »

Offline Enthalpy

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I suppose the question is academic?

Alcohols aren't good fuels. Their oxygen atom has already "burned" 1H and 1/4 C, which won't produce any heat more. Worse, the oxygen increases the fuel mass.

Wherever fuel performance is critical (rockets, aeroplanes...), hydrocarbons and sometimes amines replace alcohols.

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