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Title Heats of Combustion, Heats of Formation, Heats of Hydrogenation and Bond Dissociation Energies
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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.
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New Haven Chicago Denver San Francisco Dallas Miami The “Heat” Liberated in Either Route is the Same US Map
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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)
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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
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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
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Bond Length (Angstroms) DH o ( kcal/mol ) CH 3 (CH 3 ) 3 CC(CH 3 ) 3 C-C Bond Strength vs. Bond Length
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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
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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 )
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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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The End
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