Question

Methane reacts with bromine as shown in the following equation.
CH₄(g) + Br₂(g) → CH₃Br(g) + HBr(g)
Using the bond energies in the Table, calculate the enthalpy change, ΔH for the reaction.

Bond	Bond energy (kJ mol-1)
C-H	412
C-Br	276
Br-Br	193
H-Br	366

Answer 1

Bonds broken                        Bonds formed
C-H                                        C-Br
Br-Br                                      H-Br
Energy absorbed                    Energy released
412 + 193                              276 + 366
    =605 kJ                                  =642kJ
                  ΔH = 605 - 642
                       = −37kJ/mol
  OR
Bonds broken                         Bonds formed
(4 × 412) + 193                     (3 ×−412) 1 −276 +−366
       =+1841                                   =−1878
                   ΔH= 1841+−1878
                       =−37kJ/mol

Answer 2

-37