Question

for which of the following reactions does $delta h_{rxn}^{o}=delta h_{f}^{o}$? check all that apply.
$\square p_{4}\langle white\
angle+5o_{2}(g)\to p_{4}o_{10}(s)$
$\square s\langle rhombic\
angle+3f_{2}(g)\to sf_{6}(g)$
$\square c\langle graphite\
angle+o_{2}(g)\to co_{2}(g)$
$\square o(g)+o_{2}(g)\to o_{3}(g)$
$\square$ none of the above

Explanation:

$\Delta H^\circ_{\text{rxn}} = \Delta H^\circ_f$ only if the reaction is the formation of 1 mole of a compound from its elements in their standard states.

• $\text{P}_4$(white) and $\text{O}_2$(g) are standard states; product is 1 mole of $\text{P}_4\text{O}_{10}$(s): valid.
• $\text{S}$(rhombic) and $\text{F}_2$(g) are standard states; product is 1 mole of $\text{SF}_6$(g): valid.
• $\text{C}$(graphite) and $\text{O}_2$(g) are standard states; product is 1 mole of $\text{CO}_2$(g): valid.
• $\text{O}$(g) is not the standard state of oxygen ($\text{O}_2$(g) is): invalid.

Answer:

$\text{P}_4(\text{white}) + 5\text{O}_2(g)
ightarrow \text{P}_4\text{O}_{10}(s)$, $\text{S}(\text{rhombic}) + 3\text{F}_2(g)
ightarrow \text{SF}_6(g)$, $\text{C}(\text{graphite}) + \text{O}_2(g)
ightarrow \text{CO}_2(g)$