Question

Which of the following reaction defines $\Delta H_{f}^{o}$ ?

• A. $C_{(\text {diamond) }}+O_{2}(g) \longrightarrow CO _{2}(g)$
• B. $\frac{1}{2} H _{2}(g)+\frac{1}{2} F _{2}(g) \rightarrow HF(g)$
• C. $N _{2}(g)+3 H _{2}(g) \longrightarrow 2 NH _{3}(g)$
• D. $CO (g)+\frac{1}{2} O _{2}(g) \longrightarrow CO _{2}(g)$

Answer 1

Answer: (B)

$\frac{1}{2} H _{2}(g)+\frac{1}{2} F _{2}(g) \rightarrow HF(g)$

Answer 2

$\Delta H_{f}^{o}$ is the enthalpy change when 1 mol of the substance is formed from its elements in the standard state. Reaction [a] does not represent $\Delta H_{f}^{o}$ because standard state of carbon is graphite and not diamond.