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Standard enthalpies of formation of different compounds are given
in Table 14.
Table 14 Enthalpy of formation at 250C & 1 atm in KCal/mol
|
Compound |
DHf |
Type of reaction |
Heat flow |
|
H2O (l) |
- 68.32 |
exothermic |
released |
|
HCl (g) |
- 22.06 |
exothermic |
released |
|
HI (g) |
+ 6.2 |
endothermic |
absorbed |
|
C2H4 (g) |
+ 12.5 |
endothermic |
absorbed |
|
C6H6 (l) |
+ 11.72 |
endothermic |
absorbed |
|
NaCl (s) |
- 92.23 |
exothermic |
released |
|
BaCO3 (s) |
- 291.3 |
exothermic |
released |
|
AgCl (s) |
- 30.36 |
exothermic |
released |
|
ZnO (s) |
- 83.17 |
exothermic |
released |
|
ZnCl2 (s) |
- 99.2 |
exothermic |
released |
Example : Calculate the enthalpy change for the following reaction and classify it as exothermic or endothermic.
ZnCl2 (s) + H2O (l) ZnO (s) + 2 HCl (g)
DH = [ -83.17 + 2 (-22.06) ] - [ -99.2 + (-68.32) ]
= [ - 83.17 - 44.12 ] - [ -167.52 ]
= -127.29 + 167.52
= + 40.23
The reaction is endothermic.
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