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

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

  2. 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, DHo, (enthalpy) is a state function. • State functions are independent of path. Hess’s Law Germain Henri Hess (1802 - 1850)

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

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

  5. H2, 1/2O2 1C, O2 -68.3 kcal/mol -94.0 kcal/mol CO2 carbon dioxide H2O liquid water Combustion of Graphite and Hydrogen at 298oK Combustion of C and H

  6. 1C, O2 2H2, O2 C, 2H2 DHfo = -17.9 kcal/mol -94.0 kcal/mol Combustion of H2 2x(-68.3)=-137.6 kcal/mol CH4 CO2 carbon dioxide combustion of methane DHo = -213.7 kcal/mol -137.6 kcal/mol 2H2O liquid water sum = -231.6 kcal/mol Combustion of Methane Combustion of Methane

  7. C2H2 acetylene 2C + H2 DHo(comb)= -256.3 kcal/mol 2CO2 + H2O Not All Compounds Are More Stable Than Their Elements: Acetylene Acetylene DHfo = +54.3 kcal/mol DHo(comb)= -310.6 kcal/mol

  8. Not All Compounds Are More Stable Than Their Elements: Cyclopropane C3H6 cyclopropane DHfo = +12.7 kcal/mol 3C + 3H2 Standard State DHo(comb)= -499.7 kcal/mol DHo(comb)= -487.0 kcal/mol 3CO2 + 3H2O Cyclopropane

  9. 6C + 7H2 DHcombo DHcombo DHcombo 6CO2 + 7H2O DHo (comb)= -1,042 kcal/mol Combustion of Hexane Constitutional Isomers Alkane isomers DHfo = -39.9 kcal/mol DHfo = -41.1 kcal/mol DHfo = -44.5 kcal/mol

  10. DHfo = 0.0 kcal/mol standard state 1.0 kcal/mol DHo(H2)= -27.3 kcal/mol DHo(H2)= -30.0 kcal/mol DHo(H2)= -28.3 kcal/mol DHfo = -30.0 kcal/mol Heats of Hydrogenation Heats of Hydrogenation DHfo = -1.7 kcal/mol DHfo = -2.7 kcal/mol

  11. DHfo = -14.8 kcal/mol DHfo = -15.2 kcal/mol DHfo = -16.3 kcal/mol DHo(H2)= -26.2 kcal/mol DHo(H2)= -25.8 kcal/mol DHo(H2)= -26.2 kcal/mol DHfo = -41.0 kcal/mol DHfo = -42.5 kcal/mol DHoisomerization= -1.5 kcal/mol Heats of Hydrogenation - 2 Heats of Hydrogenation-2 standard state

  12. 1C, 2H2, 1/2O2 1C, H2, O2 1C, H2, 1/2O2 1C, O2 -26.0 kcal/mol -57.4 kcal/mol CH2O formaldehyde(g) -94.0 kcal/mol CH3OH methanol(l) -101.6 kcal/mol HCO2H formic acid(l) CO2 carbon dioxide DHfo of Methanol and Its Products of Oxidation Methanol Combustion Products

  13. (R. + H.) DHfo (H.) DHo (RH) DHfo (H.) BDE Bond Dissociation Energy (BDE) DHfo (R.) DHfo (RH) DHo (RH) = DHfo (H.) + DHfo (R.) - DHfo (RH)

  14. Bond Dissociation Energy and Hybridization C-H HCCH C-C(sp3) CH2CH2 DHo(kcal/mol) CH3CH3 C-sp C-sp C-sp2 C-sp3 C-sp Carbon Hybridization

  15. BDEs of C-C, C-X and C-H Bonds H Et Me DHo(kcal/mol) Cl i-Pr t-Bu Br I Me Me Me Et i-Pr t-Bu Me Alkyl Substituent

  16. C-C Bond Strength vs. Bond Length CH3CH3 DHo(kcal/mol) (CH3)3CC(CH3)3 Bond Length (Angstroms)

  17. Me-Me Me-Et Me-iPr Me-tBu tBu-tBu Hf R-R’ Hf Radical 1 Hf Radical 2 BDE C-C Bonds Ho(kcal/mol)

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

  19. Formation of C4-C18 Even Primary Radicals from Their n-Alkanes Ho(kcal/mol) Hf R-H BDE Hf R radical Hf H atom

  20. The End The End

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