Thermochemistry

1
??????????? (Thermochemistry)
2
Thermochemistry

• ??????????????????????????????????????????????????
  ??????????????????????????????
• Exothermic process ???????????????????????
• Endothermic process ???????????????????????
• ??????????????????????????????????????????
• Internal energy at constant volume
• Enthalpy at constant pressure

3
Standard Enthalpy Changes
????????????????? enthalpy ??? standard state
(?????????????????????? 1 ???????? ?????????????
?)
The conventional temperature is 25C (298.15 K)
Process at constant pressure
DH gt O ???????????????????? (Endothermic
reaction) DH lt O ????????????????????
(Exothermic reaction)
4
Various Types of Enthalpy Changes

• H ???? state function ??? ?H ????????????????????
• Sublimation (sub)
• Fusion (fus)
• Vaporization (vap)
• Transition ( )
• Combustion
• compound O2 CO2 H2O
• Formation
• H ???? Extensive property

5
Enthalpies of Chemical Changes

• Standard reaction enthalpy
  ?????????????????????????????????????????????????
  ???????? (????????????????????????????????????????
  ?)
• ????????????????
• CH4(g) 2O2(g) ? CO2(g)2H2O(l) ?rH -890kJ
• Standard Enthalpy of the reaction

6
Hesss Law

• ??????????

• ??????????????????????????????????????????????????
  ?????????????????????

???????????????????? ????????? ?H ??? C(s) ½
O2(g) ? CO(g) C(s)O2(g) ? CO2(g) ?H -393.5
kJ CO(g) ½ O2(g) ? CO2(g) ?H -283.0 kJ step
1- step 2 -398.5 283.0 -110.5 kJ
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??????????????????????
1. ????????????????????????? ????????????????????
????? DH ( ???? - ???? - ???? )
2. ???????????????????????????????????
???????????????????? DH ??????????????????????????
? ? ????
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??????????? 1 Hess Law

• ?????????????????????????? ??? B2H6(g)
  ???298 K

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??????????? 2 Hess Law

• ??????? ???????????????? ethane ??? hydrogen
  ???? methane ?????????????????

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• Calculate of C(s) ½ O2(g) ? CO(g) from
• C(s)O2(g) ? CO2(g)
• CO(g) ½ O2(g) ? CO2(g)

Ex. 3
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Ex. 4
Calculate the standard enthalpy of combustion of
propene.
12
Ex. 5
CH4(g) 2 O2(g) gt CO2(g) 2 H2O(l)
CH4(g) O2(g) gt CH2O(g) H2O(g)
CH2O(g) O2 (g) gt CO2(g) H2O(g)
H2O(l) gt H2O(g)
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Enthalpy of Formation

• ???????? HessLaw ????????????????????????????????
  ????????????????????????????????
• ????????????????????????????????????????? ???????
  ???????????????? ?????????????????????????????????
  ???????????????????????????????????????????
• ??????????????????????????????????????????????
  ????????????????????????(basis) ???????????
• Enthalpy of Formation ???????????????????????????
  ??????? ??????????????????????????????????????????
  ???????????????????

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???????????????????????????????????????? ?????????
?????? ????? ????????????????? ???? ?????
???? C(s) Na(s) He(g) S(s)
O2(g) Ca(s) N2(g)
H2(g) Cl2(g) I2(s) Br2(l)
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?????????????????? ??????????????? ???
16
??????????? 6 Heat or Reaction

• ????????? ???

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??????????? 7 1st Law

• ????????????????????????????????? 25 c? ???

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??????????? 8 1st Law

• ????? ?????H2O 1.00 mol 25.0 c? 1.00 atm
  ??????????? 30.0 c? 1.00 atm
• ???? state function ???????
  ???????????????????????????? (????????????????????
  ???????????????)
• Reversible heating from 25 c? to 30 c? at fixed
  pressure

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Mean Bond Enthalpy

• ????????????????????????????????????????? (-)
  ????????????? ()
• ??????????????????????????????????????????????????
  ?????????

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Mean Bond Enthalpy
C H C H
DH o298 413 kJ
C C C C
DH o298 348 kJ
C C C C
DH o298 614 kJ
C C C C
DH o298 839 kJ
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???????????????? ????????????????????????
1. ??????????????? (reactants) ??????????
???????????????????? (??)
????????????? ??? ????????????????
2. ?????????????????????????????? (??) ????
?????????????????????????????? (products)
????????????? ??? ????????????????????????????
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????? - ??? (??????????????????) ???? - ???
(???????????????????)
????????????????????
??????????????????????????? ????????????????????
?????????????
23
Ex. 4 ?????????????????????????????????????
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Temperature Dependence of ?H

• At constant pressure
• A substance is heated from T1 to T2

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Kirchhoffs law
Products
Reactants
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The standard reaction enthalpy change
27
Ex. 1 The standard enthalpy of formation of
gaseous H2O at 298 K is -241.82 kJmol-1.
Estimate its value at 100 ?C given the following
values of the molar heat capacities at constant
pressure H2O (g) 33.58 JK-1mol-1 H2(g) 28.84
JK-1mol-1 O2(g) 29.37 JK-1mol-1. Assume that
the heat capacities are independent of
temperature.