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

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Presentation on theme: "Title Heats of Combustion, Heats of Formation, Heats of Hydrogenation and Bond Dissociation Energies."— Presentation transcript:

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

2 Hess’s Law Germain Henri Hess ( ) 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.

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

4 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

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

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

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

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

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

10 Heats of Hydrogenation  H o (H2) = kcal/mol  H f o = kcal/mol  H o (H2) = kcal/mol 1.0 kcal/mol  H o (H2) = 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

11 Heats of Hydrogenation-2 standard state  H f o = kcal/mol  H f o = kcal/mol  H f o = kcal/mol  H f o = kcal/mol  H o (H2) = kcal/mol  H o (H2) = kcal/mol  H f o = kcal/mol  H o (H2) = kcal/mol  H o isomerization = -1.5 kcal/mol

12 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) kcal/mol CH 2 O formaldehyde(g) kcal/mol CO 2 carbon dioxide kcal/mol HCO 2 H formic acid(l) kcal/mol

13 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)

14 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

15 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

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

17 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

18 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 )

19 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

20 The End


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