Question

Carbon and carbon monoxide bum in oxygen to form carbon dioxide according to the following reactions $ C+O_{2} \rightarrow CO_{2}; $ $ \Delta H=-394\,kJ\,mol^{-1} $ $ 2CO+O_{2}\rightarrow 2CO_{2}; $ $ \Delta H=-569\,kJ\,mol^{-1} $
The heat of formation of $ 1\,mole $ of monoxide is, thus

• A. $-219.0 \,kJ \,mol^{-1}$
• B. $-109.5\,kJ \,mol^{-1}$
• C. $-175.0\,kJ \,mol^{-1}$
• D. $-87.5\,kJ \,mol^{-1}$

Answer 1

Answer: (B)

Given, reactions are $C + O _{2} \rightarrow CO _{2} $;
$ \Delta H=-394 \,J / mol$ ...(i)
$2 CO + O _{2} \rightarrow 2 CO _{2} $;
$ \Delta H =-569 \,kJ / mol$...(ii)
We obtained reaction $C+\frac{1}{2} O_{2} \rightarrow C O$;
$\Delta H=$ ? Divide Eq. (ii) by $2$ ,
take its reciprocal and add the obtained equation in Eq. (i)
$CO _{2} \rightarrow CO +\frac{1}{2} O _{2}$,
$ \Delta H =+284.5 \,kJ / mol$
$\frac{ C + O _{2} \rightarrow CO _{2},\Delta H =-394\, kJ / mol }{ C +\frac{1}{2} O _{2} \rightarrow CO , \Delta H =-109.5 JJ / mol }$
$ \Delta H( CO )=-109.5\, kJ / mol$