102
Chapter 4
range of concentrations of the substrate and the end product of the pathway, and this allows the pathway
to function at most conditions the cells mav experience in practice._________________________________
4.1.2 Combustion- A Change in Reference State
When an organic compound is burnt in oxygen the reaction products are always C02, H20 and
sometimes N2
for nitrogen containing compounds. The thermodynamic data for combustion are
found in the same way as data for free energy of formation, enthalpy of formation and entropy of
formation, except that in chemical thermodynamics CO
2
and H20 are defined to have zero free
energy and enthalpy of combustion at standard conditions 25°C (or 20°C in some tables) and 1 atm,
with liquid water and with CO
2
in gas form. The physical state of the compound to be combusted is
taken as its natural state at the reference temperature unless otherwise specified in the large tables
of heats of combustion that are available.
Thus for combustion of H2 with
Vi02
the heat of combustion -A
Hc
is 68,280 cal mole'1
= 4.186 *
68.28 = 286 kJ mole'1
as derived from Example 4.1 with liquid water as the product. Similarly we
have: -A
Gc
= 56661 *
4.186 = 237 kJ mole'1
. (-A
Hc)
is larger than (-A
Gc)
due to the decrease in
entropy when the “ordered” H20 molecule is formed from the “less ordered” O
2
and H2. Normally
the relative difference between A
Gc
and
A H C
is much smaller than is the case for formation of
H20, and the two quantities have approximately the same values. As will be seen in Example 4.6
there can, however, be very large differences between A
Gc
and A
Hc.
Example 4.4 Calculation of
A G C
for ethanol combustion at 25°C, 1 atm.
Ethanol combustion to the final combustion products of liquid H20 and gaseous C 02 occurs by the
stoichiometry (1)
- C 2Hs0 H - 3 0 2
+2(C 02)g+3(H20), =0
(1)
The following data are given for fonnation of C02 from C^ ,,. and 0 2 and formation of ethanol according
to
-(2C + 3H2
+iO 2) + C2H5OH = 0
( 2 )
Compound
A H }
AG}
decal mole'1)
fkcal mole'1)
(cal mole'1
K'1)
c a
-94.05
-94.26
51.06
C,H,OH
___=6&36
-41.79
38.4