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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.

Index

10.1 Introduction
10.2 First Law of Thermodynamics
10.3 Enthalpy
10.4 Second Law of Thermodynamics
10.5 Gibb's Free Energy
10.6 Absolute Entropies

Chapter 11





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