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

2. 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.

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 -94.0 kcal/mol H 2, 1/2O 2 H 2 O liquid water -68.3 kcal/mol

6. 2H 2, O 2 2H 2 O liquid water 1C, O 2 CO 2 carbon dioxide Combustion of Methane Combustion of H 2 2x(-68.3)=-137.6 kcal/mol -94.0 kcal/mol C, 2H 2 CH 4 sum = -231.6 kcal/mol combustion of methane  H o = -213.7 kcal/mol  H f o = -17.9 kcal/mol -137.6 kcal/mol

7. 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

8. Acetylene Not All Compounds Are More Stable Than Their Elements 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

9. 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

10. 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

11. 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)

12. The End