Given the average bond energies listed below, estimate D H for the following reaction:
CH₄ (g) + 2 O₂ (g) →CO₂ (g) + 2 H₂O (g) D H _rxn = ??
Average Bond Energies:
C - H = 413 kJ/mol O - O = 498 kJ/mol C=O = 799 kJ/mole (for CO₂) H - O = 467 kJ/mol

Question

Quizplus · Accepted Answer

To calculate D H, we need to determine the energy required to break the bonds in the reactants and the energy released when the bonds in the products are formed. Then we subtract the energy required to break the bonds from the energy released when the bonds are formed. Reactants: CH₄ (g): 1 C-H bond = 413 kJ/mol 2 O₂ (g): 2 O=O bonds = 2 × 498 kJ/mol Products: CO₂ (g): 1 C=O bond = 799 kJ/mol 2 H₂O (g): 2 H-O bonds = 2 × 467 kJ/mol 1 O=O bond = 498 kJ/mol (because there is one left over from the oxygen molecule) Energy required to break bonds in reactants: 1 C-H bond + 2 O=O bonds = (1 × 413 kJ/mol) + (2 × 498 kJ/mol) = 1409 kJ/mol Energy released when bonds are formed in products: 1 C=O bond + 2 H-O bonds + 1 O=O bond = (1 × 799 kJ/mol) + (2 × 467 kJ/mol) + (1 × 498 kJ/mol) = 2231 kJ/mol DH = energy released - energy required = 2231 kJ/mol - 1409 kJ/mol = -822 kJ/mol ≈ -818 kJ/mol Therefore, the best choice is B.

Given the Average Bond Energies Listed Below, Estimate D H