Question

1. to which one of the following reactions occurring at 25°c does the symbol δh°fhno₃(aq) refer?

a. h(g) + n(g) + o₃(g) → hno₃ (aq)
b. ½ h₂(g) + ½ n₂ (g) + ³/₂ o₂ (g) → hno₃(aq)
c. hno₃ (aq) → ½ h₂ (g) + ½ n₂ (g) + ³/₂ o₂(g)
d. hno₃ (aq) → h(g) + n(g) + 3 o(g)
e. h₂(g) + n₂(g) + o₃(g) →hno₃(aq)

Explanation:

The standard enthalpy of formation ($\Delta H^\circ_f$) is defined as the enthalpy change when one mole of a compound is formed from its elements in their standard states.

• In option A, the elements ($H$, $N$, $O_3$) are not in their standard states at $25^\circ C$. The standard state of hydrogen is $H_2(g)$, nitrogen is $N_2(g)$, and oxygen is $O_2(g)$.
• In option B, $\frac{1}{2}H_2(g)+\frac{1}{2}N_2(g)+\frac{3}{2}O_2(g)\to HNO_3(aq)$ shows the formation of $HNO_3(aq)$ from its elements ($H_2$, $N_2$, $O_2$) in their standard states.
• In option C, it is the decomposition reaction of $HNO_3(aq)$ instead of formation.
• In option D, the elements ($H$, $N$, $O$) are not in their standard states at $25^\circ C$.
• In option E, $O_3$ is not the standard state of oxygen (standard state is $O_2$).

Answer:

B. $\frac{1}{2}H_2(g)+\frac{1}{2}N_2(g)+\frac{3}{2}O_2(g)\to HNO_3(aq)$