2Н-Br(g) (а) Н— Н(g) + Br — Br(g) (b) Н 20—С—0 +4H—0—Н 2Н—С—О— н +30—0 н т TABLE 5.4 Average Bond Enthalpies (kJ/mol) 391 413 N-H 0-H 463 155 348 163 0-0 146 614 253 C=C 201 495 Cl-F C-N 190 242 293 N-F 272 0-F Cl-CI N-CI 358 200 0-CI 203 799 N-Br 243 0-I 234 Br-F 237 C-F 485 Br-Cl 218 С—СІ 328 Н-н 436 Br –Br 193 С -Вг 276 H-F 567 431 208 C-I 240 Н—СІ I-CI 1— Вг 175 Н-Br 366 Н—І 299 I-I 151
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.
Use bond enthalpies in Table 5.4 to estimate ΔH for each of
the following reactions:
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