Question

For the given heat of reaction,
(i) $C ( s )+ O _{2}( g ) \rightarrow CO _{2}( g )+97\, kcal$
(ii) $CO _{2}( g )+ C ( s ) \rightarrow 2 CO ( g )-39\, kcal$
the heat of combustion of $CO ( g )$ is

• A. 68 kcal
• B. -68 kcal
• C. +48 kcal
• D. None

Answer 1

Answer: (B)

Substracting equation from equation (i), we get;
$C ( s )+ O _{2}( g ) \rightarrow CO _{2}( g )+97\, kcal$
$CO _{2}( g )+ C ( s ) \rightarrow 2 CO ( g )-39\, kcal$
we get
$2 CO ( g )+ O _{2} \rightarrow 2 CO _{2}( g )+136\, kcal$
or, $CO ( g )+1 / 2 O _{2}( g ) \rightarrow CO _{2}( g )+68\, kcal$
Heat of combustion $(\Delta H )=-68\, kcal$