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Write a balanced chemical equation corresponding to the standard enthalpy of formation of $\ce{H2SO4}.$

\eqref{rxn:r1} is the given answer, \eqref{rxn:r2} is my answer:

\begin{align} \ce{1/8 S8 (s) + H2(g) + 2 O2(g) &-> H2SO4(l)} \label{rxn:r1}\tag{R1}\\ \ce{S(s) + H2(g) + 2 O2(g) &-> H2SO4(l)} \label{rxn:r2}\tag{R2} \end{align}

How can I know I have to use $\ce{S8}$ rather than simply $\ce{S}$?

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  • $\begingroup$ Your answer would have been correct if $\ce{S}$ was available as a pure substance. The trouble is that the free sulfur atom tends to join to other sulfur atoms at room temperature to make $\ce{S8}$ octogones. Nobody knows why. So $\ce{S(s)}$ does not exist. $\endgroup$
    – Maurice
    Apr 27 at 5:24
  • $\begingroup$ It is important to know what is and what isn't important. Writing S as S8, for one thing, isn't. $\endgroup$ Apr 27 at 6:25
  • $\begingroup$ You know it the same way you know to write $\ce{O2}$ instead of $\ce{O}$: from knowledge of the chemistry. And it is important to know this, since the free energy for that reaction includes breaking the covalent bonds within $\ce{S8}$, which wouldn't be the case if it were instead $\ce{S}$. $\endgroup$
    – theorist
    Apr 27 at 6:47
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    $\begingroup$ Well, IMHO, if we cannot use S(s) with implied whatever structure the solid sulfur has, we cannot use NaCl(s) nor NaHCO3(s) either, as there is no NaCl nor NaHCO3 in these solid salts. $\endgroup$
    – Poutnik
    Apr 27 at 10:27
  • $\begingroup$ See: chemistry.stackexchange.com/questions/8438/… $\endgroup$ Apr 27 at 10:29

1 Answer 1

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Here is a definition of standard enthalpy of formation from Wikipedia.

The standard enthalpy of a compound is the change of enthalpy in formation of one mole of the substance from its constituent elements, with all substances in their standard states.

So according to it, the constituent elements should be in their standard states. Now coming to question, the standard state of sulphur is rhombic sulphur and it exists as $\ce{S8}$ molecule in nature but not simply as individual atoms.

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  • $\begingroup$ Note that by IUPAC molecule definition, there are no monoatomic molecules. I do not personally like it, but I am not an authority here. $\endgroup$
    – Poutnik
    Apr 27 at 5:39
  • $\begingroup$ So, which terminology is used here? $\endgroup$
    – Infinite
    Apr 27 at 5:42
  • $\begingroup$ Atoms. Or possibly the more general term particles. $\endgroup$
    – Poutnik
    Apr 27 at 5:44

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