Calculate the average bond energy of the sulfur-oxygen bonds in so2 (in kj/mol), given the following information. sf4(g) + 2 h2o(g) → so2(g) + 4 hf(g) δh = –123 kj bond dissociation energies: s–f 327 kj/mol f–f 154 kj/mol h–f 565 kj/mol h–o 467 kj/mol

520 kJ·mol⁻¹  

Explanation:

You calculate the energy required to break all the bonds in the reactants. Then you subtract the energy to break all the bonds in the products.

                     SF₄(g) + 2H₂O(g) ⟶ SO₂(g) + 4HF(g); ΔH = –123 kJ

Bonds:           4S-F   +   4O-H          2S=O     4H-F

D/kJ·mol⁻¹:     327          467               x          565

The formula relating ΔᵣH and bond dissociation energies (D) is

(Note: This is an exception to the rule. All other thermochemical reactions are “products – reactants”. With bond energies, it’s “reactants – products”. The reason comes from the way we define bond energies.)

Σ(Dreactants) = 4 × 327 + 4 × 467 =  3176 kJ

Σ(Dproducts) = 2x +4 × 565 = (2x + 2260) kJ

-123  =3176 - (2x + 2260)

-123 = 3176 - 2x - 2260

-123 = 916 - 2x

  2x = 1039

    x = 520 kJ·mol⁻¹

The average bond energy of the S=O bonds in SO₂ is 520  kJ/mol.