title heats of combustion, heats of formation, heats of hydrogenation and bond dissociation energies
Post on 17-Dec-2015
220 Views
Preview:
TRANSCRIPT
Heats of Combustion,Heats of Formation,
Heats of Hydrogenationand
Bond Dissociation Energies
Germain Henri Hess
(1802 - 1850)
Hess’s Law (1840)
• The total heat liberated in a series of chemical reactions is equal to the sum of the heats liberated in the individual steps.
• The heat liberated, Ho, (enthalpy) is a state function.
•State functions are independent of path.
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
New Haven
ChicagoDenver
SanFrancisco
Dallas
Miami
The “Heat” Liberated in Either Route is the Same
The Standard State is the zero point for chemical reactions.
A temperature of 298oK and 1 atmosphere pressure
Carbon as graphite
Hydrogen as a gas
Oxygen as a gas
Chlorine as a gas
Bromine as a liquid
Ho = 0 kcal/mol
Combustion of Graphite and Hydrogen at 298oK
1C, O2
CO2
carbon dioxide
-94.0 kcal/mol
H2, 1/2O2
H2O liquid water
-68.3 kcal/mol
Combustion of Methane
2H2, O2
2H2O liquid water
Combustion of H2
2x(-68.3)=-137.6 kcal/mol
combustion of methaneHo = -213.7 kcal/mol
C, 2H2
CH4
Hfo = -17.9 kcal/mol
-94.0 kcal/mol
1C, O2
CO2
carbon dioxide
sum = -231.6 kcal/mol
-137.6 kcal/mol
Hfo of Methanol and Its Products of Oxidation
1C, 2H2, 1/2O21C, H2, 1/2O21C, H2, O2 1C, O2
CH3OHmethanol(l)
-57.4 kcal/mol
CH2Oformaldehyde(g)
-26.0 kcal/mol
CO2
carbon dioxide
-94.0 kcal/mol
HCO2H formic acid(l)
-101.6 kcal/mol
Not All Compounds Are More Stable Than Their Elements
acetylene
C2H2
2CO2 + H2O
Ho(comb)
= -310.6 kcal/mol
Hfo = +54.3
kcal/mol2C + H2
Ho(comb)
= -256.3 kcal/mol
Combustion of Hexane Constitutional Isomers
Hfo = -39.9
kcal/molHf
o = -41.1 kcal/mol
Hfo = -44.5
kcal/mol
6C + 7H2
6CO2 + 7H2O Ho (comb)
= -1,042 kcal/mol
Hcombo
Hcombo
Hcombo
Heats of Hydrogenation
Ho(H2)
= -27.3 kcal/mol
Hfo = -30.0
kcal/mol
Ho(H2)
= -28.3 kcal/mol
1.0 kcal/mol
Ho(H2)
= -30.0 kcal/mol
Hfo = 0.0
kcal/molstandard state
Hfo = -1.7
kcal/molHf
o = -2.7 kcal/mol
DHo (RH)
(R. + H
.)
Hfo
(RH)
Hfo
(R.)
Hfo
(H.)
Bond Dissociation Energy (BDE)
Hfo
(H.)
DHo (RH) = Hfo (H.) + Hf
o (R.) - Hf
o (RH)
80
90
100
110
120
130
140
0 1 2 3 4C-spC-sp C-sp2 C-sp3 C-sp
DHo (kcal/mol)
C-H
C-C(sp3)
HCCH
CH2CH2
Carbon Hybridization
Bond Dissociation Energy and Hybridization
CH3CH3
50
60
70
80
90
100
110
0 1 2 3 4 5Me Me Mei-Pr t-Bu MeEt
I
Br
Cl
t-Bu
H
Et Me
i-Pr
DHo (kcal/mol)
Alkyl Substituent
BDEs of C-C, C-X and C-H Bonds
78
80
82
84
86
88
90
92
1.52 1.53 1.54 1.55 1.56 1.57 1.58
Bond Length (Angstroms)
DHo (kcal/mol)
CH3CH3
(CH3)3CC(CH3)3
C-C Bond Strength vs. Bond Length
2025
32
40
90 89 88.6 87.5
35 35 35 353529
21.6
12.5
0
10
20
30
40
50
60
70
80
90
100
1 2 3 4
C-C Bonds
BDE
Hf R-R’ Hf Radical 1 Hf Radical 2
Ho (kcal/mol)
Me-Me Me-Et Me-iPr Me-tBu tBu-tBu
Butane Isomers
30 30 32 32
101 99 10197
52 52 52 52
19 17 1713
0
20
40
60
80
100
120
1 2 3 4
BDE
Hf R-H Hf H atom Hf R radical
n-Bu 1o n-Bu 2o i-Bu 1o i-Bu 3o
Ho (kcal/mol)
Formation of C4-C18 Even Primary Radicals from Their n-Alkanes
-30-40
-50-60
-70-80
-90-100
101 101 101 101 101 101 101 101
52 52 52 52 52 52 52 52
199
-1-11
-21-31
-41-51
-150
-100
-50
0
50
100
150
Hf(RH)
BDE
Hf(H atom)
Hf(R radical)
Ho (kcal/mol)
Hf R radical Hf H atom
Hf R-H BDE
The End
top related