Enthalpy of Formation

[[DDS] Samer]

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For which of the following reactions is ΔHrxn =ΔHf ?
a) Na+(aq) + Cl-(aq) → NaCl(s)
b) C(graphite) + 1/2O2(g) → CO(g)

c) Na(s) + Cl(g) → NaCl(s)
d) 1/2O2(g) + H2O(l) →H2O2(l)

A rxn for which ΔHrxn =ΔHf *must be a formation rxn.* A formation rxn forms 1 mole of a single
product from individual elements (eliminates choice D as H2O is a
compound not an element) in their standard states (eliminates choices A and C as
the standard state of sodium is Na(s), the standard state of chlorine is Cl2(g)).
Only choice B is left to satisfy the definition of a formation rxn.

I do not understand why C is wrong.

 

[[DDS] Samer]

Nevermind, I figured it out. Cl(g) is not in its standard state. If it were correct it would be written as Cl2(g).