Heats of Combustion, Heats of Formation, Heats of Hydrogenation

Heats of Combustion, Heats of Formation, Heats of Hydrogenation and Bond Dissociation Energies

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. Germain Henri Hess (1802 - 1850) State functions are independent of path.

The “Heat” Liberated in Either Route is the Same San Francisco Chicago Denver QuickTime and a TIFF (Uncompressed) decompressor are needed to see this picture. Dallas New Haven Miami

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 H2, 1/2O2 1C, O2 -68.3 kcal/mol -94.0 kcal/mol CO2 carbon dioxide H2O liquid water

Combustion of Methane C, 2H2 Hfo -17.9 kcal/mol 1C, O2 2H2, O2 -94.0 kcal/mol Combustion of H2 2x(-68.3) -137.6 kcal/mol CH4 CO2 carbon dioxide combustion of methane Ho -213.7 kcal/mol -137.6 kcal/mol 2H2O liquid water sum -231.6 kcal/mol

Not All Compounds Are More Stable Than Their Elements: Acetylene C2H2 acetylene Hfo 54.3 kcal/mol 2C H2 Ho(comb) -310.6 kcal/mol Ho(comb) -256.3 kcal/mol 2CO2 H2O

Not All Compounds Are More Stable Than Their Elements: Cyclopropane C3H6 cyclopropane Hfo 12.7 kcal/mol 3C 3H2 Standard State Ho(comb) -499.7 kcal/mol Ho(comb) -487.0 kcal/mol 3CO2 3H2O

Combustion of Hexane Constitutional Isomers 6C 7H2 Hfo -39.9 kcal/mol Hfo -41.1 kcal/mol Hfo -44.5 kcal/mol Hcombo Hcombo Hcombo 6CO2 7H2O Ho (comb) -1,042 kcal/mol

Heats of Hydrogenation Hfo 0.0 kcal/mol standard state Hfo -1.7 kcal/mol Hfo -2.7 kcal/mol 1.0 kcal/mol Ho(H2) -27.3 kcal/mol Ho(H2) -28.3 kcal/mol Ho(H2) -30.0 kcal/mol Hfo -30.0 kcal/mol

Heats of Hydrogenation - 2 standard state Hfo -14.8 kcal/mol o Hf -16.3 kcal/mol Hfo -15.2 kcal/mol Ho(H2) -26.2 kcal/mol Ho(H2) -25.8 kcal/mol Ho(H2) -26.2 kcal/mol Hfo -41.0 kcal/mol Hfo -42.5 kcal/mol Hoisomerization -1.5 kcal/mol

Hfo of Methanol and Its Products of Oxidation 1C, 2H2, 1/2O2 1C, H2, 1/2O2 1C, H2, O2 1C, O2 -26.0 kcal/mol -57.4 kcal/mol CH2O formaldehyde(g) CH3OH methanol(l) -101.6 kcal/mol HCO2H formic acid(l) -94.0 kcal/mol CO2 carbon dioxide

Bond Dissociation Energy (BDE) . . (R H ) . Hfo (H ) DHo (RH) . Hfo (RH) Hfo (R ) . Hfo (H ) . . DHo (RH) H o (H ) H o (R ) - H o (RH)

Bond Dissociation Energy and Hybridization 140 C-H DHo (kcal/mol) 130 HCCH C-C(sp3) 120 CH2CH2 110 CH3CH3 100 90 80 C-sp 0 C-sp 1 C-sp22 Carbon Hybridization 3 3 C-sp C-sp 4

BDEs of C-C, C-X and C-H Bonds 110 DHo (kcal/mol) 100 H 90 80 Et Me Cl i-Pr t-Bu Br 70 60 I 50 0 Me 1 Me 2 Et Me i-Pr 3 Alkyl Substituent 4 t-Bu Me5

C-C Bond Strength vs. Bond Length 92 CH3CH3 DHo (kcal/mol) 90 88 86 84 82 80 78 1.52 (CH3)3CC(CH3)3 1.53 1.54 1.55 1.56 Bond Length (Angstroms) 1.57 1.58

C-C Bonds 100 90 90 89 88.6 87.5 Ho (kcal/mol) 80 70 60 50 40 35 35 30 20 35 25 20 29 40 35 32 35 21.6 12.5 10 0 1 Me-Me Hf R-R’ 2 Me-Et 3 Me-iPr Hf Radical 1 BDE 4 Me-tBu Hf Radical 2 tBu-tBu

Butane Isomers 120 101 Ho (kcal/mol) 100 101 99 97 80 60 40 20 52 52 30 52 32 30 19 52 17 32 17 13 0 1 n-Bu 1o n-Bu2 2o Hf R-H 3 o i-Bu 1 Hf H atom BDE 4 i-Bu 3o Hf R radical

Formation of C4-C18 Even Primary Radicals from Their n-Alkanes 150 Ho (kcal/mol) 100 101 101 52 101 52 101 52 101 52 101 52 101 52 101 52 52 50 19 Hf(RH) 9 BDE 0 Hf(H atom) -1 -50 -30 -40 -50 -11 -60 -100 Hf(R radical) -21 -70 -31 -80 -41 -90 -150 Hf R-H Hf R radical BDE Hf H atom -51 -100

The End