# Gen. Chem q from DAT DESTROER #136

#### biostudent101

10+ Year Member
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

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
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?