CHAPTER 4. THERMODYNAMICS: THE FIRST LAW

4-16
Ionization gaseous atom to H ion
gaseous positive ion
Electron gain gaseous atom to H eg
Gaseous negative ion
Bond dissociation gaseous molecule to H dis
dissociated gaseous species
G. Bond Enthalpies
One consequence of the fact that bond formation in many molecules is highly localized
between neighboring atoms is that bond enthalpies are fairly insensitive to the other (non-neighbor)
atoms of the molecule. As a result of this, average bond enthalpies that have been determined for
bonds between various atoms may be used to estimate enthalpies of reactions. Average bond
enthalpies for bonds of H, C, N, and O atoms with other common elements are given in Table 4-1.
Table 4-1. Bond enthalpies of common elements (kJ/mol)
H! C! C= C/ N! N= N/ O! O=
H 436 413 391 463
C 413 348 615 812 292 615 891 351 728
N 391 292 615 891 161 418 946
O 463 351 728 139 485
F 563 441 270 185
Si 295 290 369
P 320
S 339 259 477
Cl 432 328 200 203
Br 366 276
I 299 240
Bond enthalpies, such as those given in the above table, may be determined from enthalpies
of formation. For example, to obtain the C!H bond enthalpy, One can use the following information
for the CH molecule: CH ! C + 4H
4 4(g) (g) (g)
o
H (CH
f
4(g)
) = !74.85 kJ/mol