Ch_322a_10.08

Ch_322a_10.08 - Relative Reactivities of Halogens towards...

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Relative Reactivities of Halogens towards Methane The observed order of reactivity in the halogenation of methane is F 2 > Cl 2 > Br 2 >>> I 2 (no reaction) Overview of Energetics in the Halogenation Reactions (1) X 2 2X . (2) X . + CH 4 HX + CH 3 . (3) CH 3 . + X 2 CH 3 X + X . F Cl Br I ! H o (kcal/mol) +38 +58 +46 +36 -32 +1 +16 +33 -70 -26 -24 -20
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Some Observations about the Standard Enthalpy Changes Based on the earlier observation that the E a generally exceeds the ! H o for a reaction, and that E a = ! H o for a bond dissociation process, the following conclusions are reached: (1) Step 1 cannot control the reactivity order because the ! H o is lowest for I 2 which does not even react. X 2 2X . +38 +58 +46 +36 F Cl Br I ! H o (kcal/mol) (2) Step 3 is highly exothermic for all the halogens, so the expected values of E a should be small and energetically favorable for all the halogens.
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This note was uploaded on 07/22/2009 for the course CHEM 322AL taught by Professor Jung during the Summer '07 term at USC.

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Ch_322a_10.08 - Relative Reactivities of Halogens towards...

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