A student attempts to determine the standard enthalpy change, \(\Delta H^\circ_{rxn}\), for the industrial dehydrogenation of propane to propene:

\[\text{C}_3\text{H}_8\text{(g)} \rightarrow \text{C}_3\text{H}_6\text{(g)} + \text{H}_2\text{(g)}\]

The student looks up the standard enthalpies of combustion for the relevant species, as summarized in the table below.

Reaction	Chemical Equation	Enthalpy Change
(1)	\(\text{C}_3\text{H}_8\text{(g)} + 5\,\text{O}_2\text{(g)} \rightarrow 3\,\text{CO}_2\text{(g)} + 4\,\text{H}_2\text{O(l)}\)	\(\Delta H_1\)
(2)	\(\text{C}_3\text{H}_6\text{(g)} + \dfrac{9}{2}\,\text{O}_2\text{(g)} \rightarrow 3\,\text{CO}_2\text{(g)} + 3\,\text{H}_2\text{O(l)}\)	\(\Delta H_2\)
(3)	\(\text{H}_2\text{(g)} + \dfrac{1}{2}\,\text{O}_2\text{(g)} \rightarrow \text{H}_2\text{O(l)}\)	\(\Delta H_3\)

Which of the following mathematical expressions correctly determines \(\Delta H^\circ_{rxn}\) for the dehydrogenation reaction?