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Gen. Chem q from DAT DESTROER #136

biostudent101

Full Member
10+ Year Member
Dec 24, 2008
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  1. Pre-Dental
    hi,

    consider the below reaction:

    C2H5 + Br2--- C2H5Br+ HBr.

    the values for the Bond energies are given. when solving for bond dissociation, you do reactant- product. why are the C-C and C-H bond not included in the calculation.
    Why would this be wrong [(c-c)+(c-H)+(Br-Br)] - [(C-C)+ (C-H)+ (H-Br)]?

    Thanks
     

    nze82

    Full Member
    May 15, 2009
    1,922
    3
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    1. Dental Student
      hi,

      consider the below reaction:

      C2H5 + Br2--- C2H5Br+ HBr.

      the values for the Bond energies are given. when solving for bond dissociation, you do reactant- product. why are the C-C and C-H bond not included in the calculation.
      Why would this be wrong [(c-c)+(c-H)+(Br-Br)] - [(C-C)+ (C-H)+ (H-Br)]?

      Thanks
      Delta H = Sum(Bonds broken) - Sum(Bonds formed)

      In the above reaction, one of the C-H bonds and the Br-Br bond are broken, while a C-Br bond and a H-Br bond are formed. So, you must only consider the energy of these bonds (Notice that C-C bonds remain unaffected).

      Delta H = Sum(C-H + Br-Br) - Sum(C-Br + H-Br)

       
      Last edited:

      alanan84

      D1
      10+ Year Member
      5+ Year Member
      Mar 17, 2008
      1,387
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      1. Dental Student
        Delta H = Sum(Bonds broken) - Sum(Bonds formed)

        In the above reaction, one of the C-H bonds is broken, while a C-Br bond and a H-Br bond are formed. So, you must only consider the energy of these bonds (Notice that C-C bonds remain unaffected).

        Delta H = Sum(C-H) - Sum(C-Br + H-Br)


        Don't you break a Br-Br bond as well?
         
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