Using bond energies from the table below, estimate ΔH for the reaction below. The two oxygens in the reactant are each bound to carbon. One of the hydrogens is bound to carbon. HCOOH(g) ↔ H2(g) + CO2(g) Bond Energies, kJ/mol Single Bonds H C N O S F Cl Br I H 432 C 411 346 N 386 305 167 O 459 358 201 142 S 363 272 --- --- 286 F 565 485 283 190 284 155 Cl 428 327 313 218 255 249 240 Br 362 285 243 201 217 249 216 190 I 295 213 --- 201 --- 278 208 175 149 Multiple Bonds C=C 602 C=N 615 C=O 799 CºC 835 CºN 887 CºO 1072 N=N 418 N=O 607 S=O (in SO2) 532 NºN 942 O2 494 S=O (in SO3) 469
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.
Using bond energies from the table below, estimate ΔH for the reaction below. The two oxygens in the reactant are each bound to carbon. One of the hydrogens is bound to carbon.
HCOOH(g) ↔ H2(g) + CO2(g)
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Bond Energies, kJ/mol |
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Single Bonds |
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|
H |
C |
N |
O |
S |
F |
Cl |
Br |
I |
|
H |
432 |
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|
C |
411 |
346 |
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|
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N |
386 |
305 |
167 |
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|
O |
459 |
358 |
201 |
142 |
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|
|
|
|
S |
363 |
272 |
--- |
--- |
286 |
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|
F |
565 |
485 |
283 |
190 |
284 |
155 |
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|
|
Cl |
428 |
327 |
313 |
218 |
255 |
249 |
240 |
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|
Br |
362 |
285 |
243 |
201 |
217 |
249 |
216 |
190 |
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|
I |
295 |
213 |
--- |
201 |
--- |
278 |
208 |
175 |
149 |
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Multiple Bonds |
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C=C |
602 |
C=N |
615 |
C=O |
799 |
|
CºC |
835 |
CºN |
887 |
CºO |
1072 |
|
N=N |
418 |
N=O |
607 |
S=O (in SO2) |
532 |
|
NºN |
942 |
O2 |
494 |
S=O (in SO3) |
469 |
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