Given the following bond energies: estimate ΔH for the reaction H₂O₂ + CH₃OH → H₂CO + 2H₂O.

Question

Quizplus · Accepted Answer

According to the law of conservation of energy, the energy in the bonds broken equals the energy in the bonds formed plus any excess energy released or absorbed as heat. To calculate the ΔH for the given reaction, we need to break the bonds in the reactants and form the bonds in the products and then subtract the energy released or absorbed. Reactants: H₂O₂: H-S = 339 kJ/mol, S-O = 497 kJ/mol, O-S = 271 kJ/mol CH₃OH: C-H = 414 kJ/mol, H-O = 463 kJ/mol, O-H = 467 kJ/mol Total energy needed to break bonds in reactants = (2x339) + 2x497 + 2x271 + 414 + 463 + 467 = 5560 kJ/mol Products: H₂CO: H-S = 339 kJ/mol, S-C = 259 kJ/mol, O-C = 360 kJ/mol 2H₂O: 2xS-O = 2x497 kJ/mol Total energy released in forming bonds in products = (2x339) + 259 + 360 + (2x2x497) = 4449 kJ/mol ΔH = (energy needed to break bonds in reactants) - (energy released in forming bonds in products) ΔH = 5560 kJ/mol - 4449 kJ/mol = -1111 kJ/mol = -1111/2 = -555.5 kJ/mol The negative sign indicates that the reaction is exothermic, meaning that it releases heat. However, the given answer choices are in kJ, not kJ/mol. Multiplying by the appropriate number of moles on both sides of the equation, we get -1111 kJ/mol * (1/2 mol) * (-1) = 555.5 kJ, which is closest to answer choice C (-291 kJ).

Given the Following Bond Energies: Estimate ΔH for the Reaction