Consider the following reaction.H 2 (g)88n2H(g) H^0 rxnHHXH436 kJ/mol HXH bondsThe bond energy of the hydrogen–hydrogen bond is 436 kJ/mol of bonds. In other words,
436 kJ of energy must be absorbed for every mole of HXH bonds that are broken. This
endothermic reaction (H^0 rxnis positive) can be writtenH 2 (g)436 kJ88n2H(g)Some average bond energies are listed in Tables 15-2 and 15-3. We see from Table
15-3 that for any combination of elements, a triple bond is stronger than a double bond,
which in turn is stronger than a single bond. Bond energies for double and triple bonds
are notsimply two or three times those for the corresponding single bonds. A single bond
is a bond, whereas double and triple bonds involve a combination of and bonding.
The bond energy measures the difficulty of overcoming the orbital overlap, and we should
not expect the strength of a bond to be the same as that of a bond between the same
two atoms.
We should keep in mind that each of the values listed is the average bond energy from
a variety of compounds. The average CXH bond energyis 413 kJ/mol of bonds. AverageWe have discussed these changes in
terms of absorption or release of heat.
Another way of breaking bonds is by
absorption of light energy (Chapter 5).
Bond energies can be determined from
the energies of the photons that cause
bond dissociation.
610 CHAPTER 15: Chemical Thermodynamics
TABLE 15-2 Some Average Single Bond Energies (kJ/mol of bonds)HCNOFSiP SClBrI436 413 391 463 565 318 322 347 432 366 299 H
346 305 358 485 272 339 285 213 C
163 201 283 192 N
146 452 335 218 201 201 O
155 565 490 284 253 249 278 F
222 293 381 310 234 Si
201 326 184 P
226 255 S
242 216 208 Cl
193 175 Br
151 ITABLE 15-3 Comparison of Some Average
Single and Multiple Bond
Energies (kJ/mol of bonds)Single Bonds Double Bonds Triple BondsCXC 346 CUC 602 CmC 835
NXN 163 NUN 418 NmN 945
OXO 146 OUO 498
CXN 305 CUN 615 CmN 887
CXO 358 CUO 732* CmO 1072*Except in CO 2 , where it is 799 kJ/mol.