Question

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

• A. $ {{C}_{(diamond)}}+{{O}_{2}}(g)\xrightarrow{{}}C{{O}_{2}}(g) $
• B. $ \frac{1}{2}{{H}_{2}}(g)+\frac{1}{2}{{F}_{2}}(g)\xrightarrow{{}}HF(g) $
• C. $ {{N}_{2}}(g)+3{{H}_{2}}(g)\xrightarrow{{}}2N{{H}_{3}}(g) $
• D. $ CO(g)+\frac{1}{2}{{O}_{2}}(g)\xrightarrow{{}}C{{O}_{2}}(g) $

Answer 1

Answer: (B)

$ \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.