Question

Selected bond dissociation enthalpies are given in this table.

Bond Dissociation enthalpy (kJ/mol)
C−C 348
C=C 614
C≡C 839
C−Cl 328
C−H 413
Cl−Cl 242
H−H 436
H−Cl 431
Equilibrium 1: C2H4(g)+Cl2(g)⇌C2H4Cl2(g)
Use bond energies to estimate the enthalpy change for Equilibrium 1.

Answer 1

Bonds broken:

1 C=C bond (2 x 614 kJ/mol) = 1228 kJ/mol

1 Cl-Cl bond (1 x 242 kJ/mol) = 242 kJ/mol

Bonds formed:

1 C-C bond (1 x 348 kJ/mol) = 348 kJ/mol

2 C-Cl bonds (2 x 328 kJ/mol) = 656 kJ/mol

ΔH = (energy required to break bonds) - (energy released when new bonds form)

ΔH = (1228 kJ/mol + 242 kJ/mol) - (348 kJ/mol + 656 kJ/mol)

ΔH = 466 kJ/mol

Therefore, the estimated enthalpy change for Equilibrium 1 is 466 kJ/mol.