Using the table of bond dissociation energies, the ΔH for the following gas-phase reaction is ________ kJ.
A)291
B)2017
C)-57
D)-356
E)-291

Question

Using the table of bond dissociation energies, the ΔH for the following gas-phase reaction is ________ kJ.  
A)291 
B)2017 
C)-57 
D)-356 
E)-291

Quizplus · Accepted Answer

The ΔH for the reaction is equal to the sum of the bond dissociation energies of the bonds broken minus the sum of the bond dissociation energies of the bonds formed.

Breaking bonds:
4C-H bonds: 4 x 413 kJ/mol = 1652 kJ/mol
2C-C bonds: 2 x 347 kJ/mol = 694 kJ/mol
1C=C bond: 839 kJ/mol

Forming bonds:
3C=C bonds: 3 x 603 kJ/mol = 1809 kJ/mol
4H-H bonds: 4 x 436 kJ/mol = 1744 kJ/mol

ΔH = (1652 kJ/mol + 694 kJ/mol + 839 kJ/mol) - (1809 kJ/mol + 1744 kJ/mol) 
ΔH = -57 kJ/mol

Therefore, the best choice is C, -57.

Using the Table of Bond Dissociation Energies, the ΔH for the Following