By definition the heat of combustion for compound is equal to the energy that is released on complete oxidation of one mole that substance to its highest oxidation state. In case of sulphur, online I found that ∆H for $\ce{S8 + O2->SO2}$ is heat of combustion. Clearly + 4 is not the highest oxidation state of sulphur, then why is ∆H for $\ce{S8 + O2->SO3}$ not considered as heat of combustion

  • $\begingroup$ Exceptions. Chemistry has lots of exceptions when making sweeping statements. $\endgroup$ – MaxW May 11 at 20:50
  • 4
    $\begingroup$ Because you get SO2 not SO3 in combustion, as simple as that. $\endgroup$ – Mithoron May 11 at 21:00

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