Title: Thermochemistry
1??????????? (Thermochemistry)
2Thermochemistry
- ??????????????????????????????????????????????????
?????????????????????????????? - Exothermic process ???????????????????????
- Endothermic process ???????????????????????
- ??????????????????????????????????????????
- Internal energy at constant volume
- Enthalpy at constant pressure
3Standard 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)
4Various Types of Enthalpy Changes
- H ???? state function ??? ?H ????????????????????
- Sublimation (sub)
- Fusion (fus)
- Vaporization (vap)
- Transition ( )
- Combustion
- compound O2 CO2 H2O
- Formation
- H ???? Extensive property
5Enthalpies of Chemical Changes
- Standard reaction enthalpy
?????????????????????????????????????????????????
???????? (????????????????????????????????????????
?) - ????????????????
- CH4(g) 2O2(g) ? CO2(g)2H2O(l) ?rH -890kJ
- Standard Enthalpy of the reaction
6Hesss 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
11Ex. 4
Calculate the standard enthalpy of combustion of
propene.
12Ex. 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)
13Enthalpy 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
19Mean Bond Enthalpy
- ????????????????????????????????????????? (-)
????????????? () - ??????????????????????????????????????????????????
?????????
20Mean 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????? - ??? (??????????????????) ???? - ???
(???????????????????)
????????????????????
??????????????????????????? ????????????????????
?????????????
23Ex. 4 ?????????????????????????????????????
24Temperature Dependence of ?H
- At constant pressure
- A substance is heated from T1 to T2
25Kirchhoffs law
Products
Reactants
26The standard reaction enthalpy change
27Ex. 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.