Page 109 - Essentials of physical chemistry

The First Law of Thermodynamics 71

TABLE 4.1
Selected Values of Heats of Combustion,
DH 0 (1 atm, 298) a
comb
Compound DH 0 (kJ=mol)
comb
C (graphite) 393.5
CO 283.0
285.8
H 2
NH 3 382.8
667.1
H 2 NNH 2
N 2 O 82.1
890.8
CH 4
HCCH 1301.1
1411.2
H 2 CCH 2
1560.7
H 3 CCH 3
3267.6
C 6 H 6
CH 3 OH 726.1
CH 3 CH 2 OH 1366.8
1460.4
CH 3 OCH 3
CH 2 O 570.7
HCOOH 254.6
H 3 CCOOH 874.2
HCN 671.5
709.2
H 3 CNO 2
1085.6
H 3 CNH 2
0
Hg (liq)
90.79
HgO (red)
Source: Lide, D.R., CRC Handbook of Chemistry and Physics, 90th Edn.,
CRC Press, Boca Raton, FL, 2009–2010, pp. 5–68.
a
Note these values are at 1 atm not 1 bar (1 atm ¼ 1.01325 bar).

HESS’S LAW OF HEAT SUMMATION
Now we come to a simple principle that is very important in thermochemistry, Hess’s law. Since
energy (enthalpy) can be treated quantitatively we can balance the energy of a reaction as well as the
mass. Not only that, we can add and subtract the enthalpies of several reactions. Suppose we want to
find out the energy change for the hydrogenation of acetylene to form ethane, we can use the heats
of combustion to find the enthalpy change for the hydrogenation step.

5 0
HCCH þ O 2 ! 2CO 2 þ H 2 O; DH comb ¼ 1301:1 kJ (þ1)
2

1 0
H 2 þ O 2 ! H 2 O; DH comb ¼ 285:8 kJ (þ2)
2

7 0
H 3 CCH 3 þ O 2 ! 2CO 2 þ 3H 2 O; DH comb ¼ 1560:7 kJ ( 1)
2
_______________________________________________________________________________
0
HCCH þ 2H 2 ! H 3 CCH 3 ; DH ¼ [( 1301:1) þ 2( 285:8) ( 1560:7)]kJ ¼ 312:0 kJ
rxn