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(A) The standard enthalpy of formation $(\Delta_f H^\circ)$ is defined as the enthalpy change when $1 	ext{ mole}$ of a compound is formed from its c

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$(1), (2)$

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(A) The standard enthalpy of formation $(\Delta_f H^\circ)$ is defined as the enthalpy change when $1 	ext{ mole}$ of a compound is formed from its constituent elements in their most stable standard states.$(1)$ $\frac{3}{2} O_{2(g)} ightarrow O_{3(g)}$: $O_2$ is the most stable form of oxygen. $1 	ext{ mole}$ of $O_3$ is formed. This represents $\Delta_f H^\circ$.$(2)$ $\frac{1}{8} S_{8(s)} + O_{2(g)} ightarrow SO_{2(g)}$: $S_8$ is the most stable form of sulfur and $O_2$ is the most stable form of oxygen. $1 	ext{ mole}$ of $SO_2$ is formed. This represents $\Delta_f H^\circ$.$(3)$ $2 H_{2(g)} + O_{2(g)} ightarrow 2 H_2O_{(l)}$: Here,$2 	ext{ moles}$ of $H_2O$ are formed,so this is $2 	imes \Delta_f H^\circ$,not $\Delta_f H^\circ$.$(4)$ $2 C_{(g)} + 3 H_{2(g)} ightarrow C_2H_{6(g)}$: Carbon is not in its most stable state (graphite is the standard state). Thus,this is not $\Delta_f H^\circ$.Therefore,reactions $(1)$ and $(2)$ satisfy the condition.

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