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.
![Average Bond Enthalpies (kJ/mol)
Single Bonds
C-H 413
C-C 348
C-N 293
C-O 358
C-F 485
C-CI 328
C-Br 276
C-I 240
C-S
259
Si-H
323
Si-Si
226
Si-C 301
Si-O 368
Multiple Bonds
C=C 614
C=C 839
C=N 615
C=N 891
C=O 799
C=0 1072
N-H
N-N
N-O
N-F
N-CI
N-Br
391
163
201
272
200
243
436
H-H
H-F 567
H-Cl 431
H-Br 366
H-I 299
N=N 418
N=N 941
O-H 463
0-0 146
O-F 190
0-CI 203
O-I
234
S-H
S-F
339
327
S-Cl
253
218
S-Br
S-S 266
s
S=O
S=S
495
523
418
F-F 155
Cl-F 253
CI-CI 242
Br-F
237
Br-Cl 218
Br-Br 193
I-CI 208
175
151
I-Br
I-I](/v2/_next/image?url=https%3A%2F%2Fcontent.bartleby.com%2Fqna-images%2Fquestion%2F18395f57-75e3-4bdb-980a-ed8850989594%2Fd56b5838-da69-461c-970a-14cc9618eca9%2Fub5o3hf_processed.jpeg&w=3840&q=75)
![Table 9.5 Bond Enthalpies (in kJ/mol)*
HCNOSFE
Η
Br
I
H
436
413
391
463
339
567
431
366
299
C
348
393
358
259
485
328
276
240
N
163
201
272
200
243
614 C=N
839
C=N
418
N=O
945
0 0
H
H
H-C + H+
Single Bonds
S
0
146
190
203
266
327
253
218
F
234
Multiple Bonds
C=C
615
C=C
891
N=N
607
N=N
498
*Data are taken from http://wiki.chemedd1.org/index.php/15.4_Bond_Enthalpies.
159
253
237
H
CI
C=0
C=0
S=O
S=S
242
218
208
Br
193
175
I
You can imagine that the reaction takes place in steps involving the breaking and
forming of bonds. Starting with the reactants, you suppose that one C-H bond and
the Cl-Cl bond break.
151
H
H-C-H+CI-CI - H-C + H+ Cl + Cl
H
H
The enthalpy change is BE(C-H) + BE(C1-CI). Now you reassemble the fragments
to give the products.
804 (in CO₂)
1076
323
418
9.11 Bom](/v2/_next/image?url=https%3A%2F%2Fcontent.bartleby.com%2Fqna-images%2Fquestion%2F18395f57-75e3-4bdb-980a-ed8850989594%2Fd56b5838-da69-461c-970a-14cc9618eca9%2Fse3hbsl_processed.jpeg&w=3840&q=75)
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