[Flyndaran]

I'm designing hopefully realistic anaerobic aliens.  Unlike earth life, they store long term energy as triacetin (3 acetic acid molecules plus glycerol as a triglyceride).

But I'm having issues with the differences between Gibbs free energy and Delta H enthalpies.  The numbers are starting to blur together in a giant mess.  What would be the bottom line on how much energy they could get out of that reaction assuming near 100% efficiency as organisms can always find some use for waste heat?

As for the trying to solve myself, ugh but here it goes.

Acetic acid enthalpy -483.5, Methane -74.9, Carbon Dioxide -393.5  The Delta H = -15.1 kJ/mol  But the listed Gibbs = -36 kJ/mol.

I'm not trying to be exact with my fiction, but having a larger factor than 2 is a bit much.  What am I screwing up with my layman's understanding?

Thank you for reading all that mess of a question even if you can't help.

[Archer]

If you don't want the idear to be too far fetched you may wish to look at organisms endogenous to Earth which are anaerobics

http://en.m.wikipedia.org/wiki/Anaerobic_respiration


See if you can apply this to triacetin.

[Flyndaran]

It's real world biology that gave me the idea to make my aliens truly alien rather than just animal mimics.
Methanogens can metabolize fatty acids, and triacetin is merely a triglyceride of acetic acid as the constituent fatty acid.  It's just that no known organism makes triacetin, even though humans can apparently consume it quite safely.

Trying to break the problem down.
I have acetic acid which is stated to be equivalent to acetate in biochemistry pH areas.
So how much energy can I get from that part?
C2H3O2{-} + H{+} = CH4 + CO2

Next is the glycerol part.
4 C3H8O3 = 7 CH4 + 5 CO2 + 2 H2O