Chapter 102 of 2363 min read
Bond enthalpy and thermochemistry
Why this is asked: Bond enthalpies used in ΔH = Σ(bonds broken) − Σ(bonds formed) are average values for a polyatomic molecule — each successive C–H bond broken in methane actually takes a different amount of energy, and the tabulated figure is a mean. That's why a bond-enthalpy calculation only ever gives an approximate ΔH, not the exact value Hess's law gives from standard enthalpies of formation.
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