atomization: the process of breaking a molecule into

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Presentation transcript:

.. .. . atomization: the process of breaking a molecule into its individual atoms H H–C–Cl .. C (g) + 3 H (g) + Cl (g) .. . (g) methylchloride (chloromethane) -- DH for a given bond (e.g., the C–H bond) varies little between compounds. CH3Cl was once used as a refrigerant, but it is toxic and so is now used primarily in the production of silicone polymers. It is also used as an organic solvent. e.g., C–H bonds in CH4 vs. those in CH3CH2CH3 have about the same DH

-- Typical values of bond enthalpies for specific bonds have been tabulated. (p. 326) -- To find bond enthalpy for atomization, add up bond enthalpies for each bond broken. Calculate the bond enthalpy for the atomization of dichloromethane. H H–C–Cl Cl (i.e., break 2 C–H bonds and 2 C–Cl bonds) DHatm = 2 (413 kJ/mol) + 2 (328 kJ/mol) = 1482 kJ/mol

You can approximate reaction enthalpy using Hess’s law and tabulated bond enthalpies. DHrxn S(DHbroken bonds) – S(DHformed bonds) Approximate the reaction enthalpy for the combustion of propane. C3H8 + 5 O2 4 H2O + 3 CO2 H–C–C–C–H H O=O .. H–O H .. O=C=O .. Broken: 2 C–C: 2 (348) –2023 kJ 8 C–H: 8 (413) 6475 kJ 5 O=O: 5 (495) (Actual = –2220 kJ) Formed: 8 O–H: 8 (463) 8498 kJ 6 C=O: 6 (799)

bond length: the center-to-center distance between two bonded atoms -- fairly constant for a given bond (e.g., the C–H bond), no matter the compound e.g., C–H bonds in CH4 are about the same length as those in CH3CH2CH3 -- Average bond lengths have been tabulated for many bonds. (p. 329) -- As the number of bonds between two atoms increases, bond length… and bond enthalpy… e.g., C–C C=C C–C 1.54 A 348 kJ/mol 1.34 A 614 kJ/mol 1.20 A 839 kJ/mol