Question

The enthalpies of combustion of carbon and carbon monoxide are $ -393.5 $ and $ -283\, kJ\, mol^{−1} $ respectively. The enthalpy of formation of carbon monoxide per mole is

• A. $ 110.5\,kJ $
• B. $ 676.5\,kJ $
• C. $ -676.5\,kJ $
• D. $ -110.5\,kJ $

Answer 1

Answer: (D)

$\ce{I : C(s) +O2(g) -> CO2(g) \,;\Delta H=-393.5\,kJ}$
$\ce{II : CO(g) +\frac{1}{2} +O2(g) -> CO2(g)\,; \Delta H=-283.0\,kJ}$
$\ce{I}$ and $\ce{II}$ gives
$\ce{III : C(s) + \frac{1}{2} O2(g) -> CO(g) ; \,\Delta H=-110.5\,kJ}$
This equation III also represents formation of $1\, mole$ of $CO$ and thus enthalpy change is the heat of formation of $CO(g)$.