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Thermochemistry

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H ???? Extensive property. Gas. Liquid. Solid. Enthalpy, H. Enthalpies of Chemical Changes. Standard reaction enthalpy : ?????????????? ... – PowerPoint PPT presentation

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Title: 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
7
??????????????????????
1. ????????????????????????? ????????????????????
????? DH ( ???? - ???? - ???? )
2. ???????????????????????????????????
???????????????????? DH ??????????????????????????
? ? ????
8
??????????? 1 Hess Law
  • ?????????????????????????? ??? B2H6(g)
    ???298 K

9
??????????? 2 Hess Law
  • ??????? ???????????????? ethane ??? hydrogen
    ???? methane ?????????????????

10
  • 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
11
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)
13
Enthalpy of Formation
  • ???????? HessLaw ????????????????????????????????
    ????????????????????????????????
  • ????????????????????????????????????????? ???????
    ???????????????? ?????????????????????????????????
    ???????????????????????????????????????????
  • ??????????????????????????????????????????????
    ????????????????????????(basis) ???????????
  • Enthalpy of Formation ???????????????????????????
    ??????? ??????????????????????????????????????????
    ???????????????????

14
???????????????????????????????????????? ?????????
?????? ????? ????????????????? ???? ?????
???? C(s) Na(s) He(g) S(s)
O2(g) Ca(s) N2(g)
H2(g) Cl2(g) I2(s) Br2(l)
15
?????????????????? ??????????????? ???
16
??????????? 6 Heat or Reaction
  • ????????? ???

17
??????????? 7 1st Law
  • ????????????????????????????????? 25 c? ???

18
??????????? 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

19
Mean Bond Enthalpy
  • ????????????????????????????????????????? (-)
    ????????????? ()
  • ??????????????????????????????????????????????????
    ?????????

20
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
21
???????????????? ????????????????????????
1. ??????????????? (reactants) ??????????
???????????????????? (??)
????????????? ??? ????????????????
2. ?????????????????????????????? (??) ????
?????????????????????????????? (products)
????????????? ??? ????????????????????????????
22
????? - ??? (??????????????????) ???? - ???
(???????????????????)
????????????????????
??????????????????????????? ????????????????????
?????????????
23
Ex. 4 ?????????????????????????????????????
24
Temperature Dependence of ?H
  • At constant pressure
  • A substance is heated from T1 to T2

25
Kirchhoffs law
Products
Reactants
26
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.
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