TABLE 8.3 Average Bond Enthalpies (kJ/mol) Single Bonds C-H 413 N-H 391 О—н 463 F-F 155 348 N-N 163 0-0 146 293 201 0-F 190 CI-F 253 C-0 358 N-F 272 0-CI 203 Cl-CI 242 C-F 485 N-CI 200 0-I 234 C-CI 328 N-Br 243 Br-F 237 C-Br 276 S-H 339 Br-CI 218 C-I 240 Н-Н 436 S-F 327 Br-Br 193 C-S 259 Н—F 567 S-CI 253 Н—СІ 431 S-Br 218 I-CI 208 Si-H 323 Н—Br 366 S-S 266 I-Br 175 Si-Si 226 Н-I 299 I-I 151 Si-C 301 Si-O 368 Si-CI 464 Multiple Bonds 614 N=N 418 0=0 495 839 N=N 941 615 607 523 C=N 891 S=S 418 799 1072
Thermochemistry
Thermochemistry can be considered as a branch of thermodynamics that deals with the connections between warmth, work, and various types of energy, formed because of different synthetic and actual cycles. Thermochemistry describes the energy changes that occur as a result of reactions or chemical changes in a substance.
Exergonic Reaction
The term exergonic is derived from the Greek word in which ‘ergon’ means work and exergonic means ‘work outside’. Exergonic reactions releases work energy. Exergonic reactions are different from exothermic reactions, the one that releases only heat energy during the course of the reaction. So, exothermic reaction is one type of exergonic reaction. Exergonic reaction releases work energy in different forms like heat, light or sound. For example, a glow stick releases light making that an exergonic reaction and not an exothermic reaction since no heat is released. Even endothermic reactions at very high temperature are exergonic.
(a) Compare the bond enthalpies (Table 8.3) of the carbon–
carbon single, double, and triple bonds to deduce an average
π -bond contribution to the enthalpy. What fraction of
a single bond does this quantity represent? (b) Make a similar
comparison of nitrogen–nitrogen bonds. What do you
observe? (c) Write Lewis structures of N2H4, N2H2, and N2,
and determine the hybridization around nitrogen in each
case. (d) Propose a reason for the large difference in your
observations of parts (a) and (b).
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