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.

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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