# Title Heats of Combustion, Heats of Formation, Heats of Hydrogenation and Bond Dissociation Energies.

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

Hess’s Law 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,  H o, (enthalpy) is a state function. State functions are independent of path.

New Haven Chicago Denver San Francisco Dallas Miami The “Heat” Liberated in Either Route is the Same US Map

Standard State The Standard State is the zero point for chemical reactions. A temperature of 298 o K and 1 atmosphere pressure Carbon as graphite Hydrogen as a gas Oxygen as a gas Chlorine as a gas Bromine as a liquid  H o = 0 kcal/mol

Combustion of C and H Combustion of Graphite and Hydrogen at 298 o K 1C, O 2 CO 2 carbon dioxide -94.0 kcal/mol H 2, 1/2O 2 H 2 O liquid water -68.3 kcal/mol

Combustion of Methane 2H 2, O 2 2H 2 O liquid water Combustion of H 2 2x(-68.3)=-137.6 kcal/mol combustion of methane  H o = -213.7 kcal/mol C, 2H 2 CH 4  H f o = -17.9 kcal/mol -94.0 kcal/mol 1C, O 2 CO 2 carbon dioxide sum = -231.6 kcal/mol -137.6 kcal/mol

Acetylene Not All Compounds Are More Stable Than Their Elements: Acetylene acetyleneC2H2C2H2 2CO 2 + H 2 O  H o (comb) = -310.6 kcal/mol  H f o = +54.3 kcal/mol 2C + H 2  H o (comb) = -256.3 kcal/mol

Cyclopropane Not All Compounds Are More Stable Than Their Elements: Cyclopropane cyclopropaneC3H6C3H6  H o (comb) = -499.7 kcal/mol  H f o = +12.7 kcal/mol 3CO 2 + 3H 2 O 3C + 3H 2  H o (comb) = -487.0 kcal/mol Standard State

Alkane isomers Combustion of Hexane Constitutional Isomers  H f o = -39.9 kcal/mol  H f o = -41.1 kcal/mol  H f o = -44.5 kcal/mol 6C + 7H 2 6CO 2 + 7H 2 O  H o (comb) = -1,042 kcal/mol  H comb o

Heats of Hydrogenation  H o (H2) = -27.3 kcal/mol  H f o = -30.0 kcal/mol  H o (H2) = -28.3 kcal/mol 1.0 kcal/mol  H o (H2) = -30.0 kcal/mol  H f o = 0.0 kcal/mol standard state  H f o = -1.7 kcal/mol  H f o = -2.7 kcal/mol

Heats of Hydrogenation-2 standard state  H f o = -16.3 kcal/mol  H f o = -15.2 kcal/mol  H f o = -14.8 kcal/mol  H f o = -41.0 kcal/mol  H o (H2) = -25.8 kcal/mol  H o (H2) = -26.2 kcal/mol  H f o = -42.5 kcal/mol  H o (H2) = -26.2 kcal/mol  H o isomerization = -1.5 kcal/mol

Methanol Combustion Products  H f o of Methanol and Its Products of Oxidation 1C, 2H 2, 1/2O 2 1C, H 2, 1/2O 2 1C, H 2, O 2 1C, O 2 CH 3 OH methanol(l) -57.4 kcal/mol CH 2 O formaldehyde(g) -26.0 kcal/mol CO 2 carbon dioxide -94.0 kcal/mol HCO 2 H formic acid(l) -101.6 kcal/mol

DH o (RH) (R. + H. ) BDE  H f o (RH)  H f o (R. )  H f o (H. ) Bond Dissociation Energy (BDE)  H f o (H. ) DH o (RH) =  H f o (H. ) +  H f o (R. ) -  H f o (RH)

C-sp C-sp 2 C-sp 3 C-sp DH o ( kcal/mol ) C-H C-C(sp 3 ) HCCH CH 2 Carbon Hybridization Bond Dissociation Energy and Hybridization CH 3

Me i-Prt-BuMeEt I Br Cl t-Bu H Et Me i-Pr DH o ( kcal/mol ) Alkyl Substituent BDEs of C-C, C-X and C-H Bonds

Bond Length (Angstroms) DH o ( kcal/mol ) CH 3 (CH 3 ) 3 CC(CH 3 ) 3 C-C Bond Strength vs. Bond Length

C-C Bonds BDE  H f R-R’  H f Radical 1  H f Radical 2  H o ( kcal/mol ) Me-MeMe-EtMe-iPrMe-tButBu-tBu

Butane Isomers BDE  H f R-H  H f H atom  H f R radical n-Bu 1 o n-Bu 2 o i-Bu 1 o i-Bu 3 o  H o ( kcal/mol )

Formation of C 4 -C 18 Even Primary Radicals from Their n-Alkanes  H o ( kcal/mol )  H f R radical  H f H atom  H f R-H BDE

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