Question

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

• A. $ C_{\text{(diamond)}}+O_{2}(g)\rightarrow CO_{2}(g) $
• B. $ \frac{1}{2}H_{2}(g)+\frac{1}{2}F_{2}(g)\rightarrow HF(g) $
• C. $ N_{2}(g)+3H_{2}(g)\rightarrow 2NH_{3}(g) $
• D. $CO (g)+\frac{1}{2} O _{2}(g) \longrightarrow CO _{2}(g)$

Answer 1

Answer: (B)

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