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

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Presentation transcript:

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

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.

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 kcal/mol H 2, 1/2O 2 H 2 O liquid water kcal/mol

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)= kcal/mol kcal/mol C, 2H 2 CH 4 sum = kcal/mol combustion of methane  H o = kcal/mol  H f o = kcal/mol 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) kcal/mol CH 2 O formaldehyde(g) kcal/mol CO 2 carbon dioxide kcal/mol HCO 2 H formic acid(l) kcal/mol

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

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

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

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)

The End