For which of the following reactions is the value of Delta H rxn equal to Delta H of formation?

I. 2Ca(s) + O2(g) --> 2CaO(s)
II. C2H2(g) + H2(g) --> C2H4(g)
III. S(s) + O3(g) --> SO3(g)
IV. 3Mg(s) + N2(g) --> Mg3N2(s).

According to my answer key, the only one is IV. I cannot figure out how to tell the difference. Is there a trend that I am missing?

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  1. The heat of formation must be an equation for the formation of the compound (1 mol) on the right from elements on the left.
    I is OK BUT it is for two moles.
    II is no go since C2H2 is not an element.
    III is no go since O3 is not a standard state of oxygen (O2 is the standard state).
    IV is from elements on the left, in their standard states, and forms 1 mol of the product in its standard state on the right..

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