For which one of the following equations is $\Delta H^\circ_{react}$ equal to $\Delta H^\circ_f$ for the product :
A
$N_2(g) + O_3(g) \rightarrow N_2O_3(g)$
B
$CH_4(g) + 2Cl_2(g) \rightarrow CH_2Cl_2(l) + 2HCl(g)$
C
$Xe(g) + 2F_2(g) \rightarrow XeF_4(g)$
D
$2CO(g) + O_2(g) \rightarrow 2CO_2(g)$
The answer is C.
Solution
The standard enthalpy of formation is the enthalpy change when one mole of a compound is formed from its elements in their most stable (standard) states. $N_2(g) + O_3(g) \rightarrow N_2O_3(g)$: ozone is not the standard state of oxygen ($O_2$ is). $CH_4 + 2Cl_2 \rightarrow CH_2Cl_2 + 2HCl$: $CH_4$ is a compound, not an element, and there are two products. $2CO + O_2 \rightarrow 2CO_2$: CO is a compound, and 2 moles of product are formed. $Xe(g) + 2F_2(g) \rightarrow XeF_4(g)$: one mole of $XeF_4$ is formed from its elements in their standard states (Xe gas and $F_2$ gas). So for $Xe(g) + 2F_2(g) \rightarrow XeF_4(g)$, $\Delta H^\circ_{react} = \Delta H^\circ_f(XeF_4)$.
ConceptDefinition of standard enthalpy of formation
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