Question

If the heat of formation of $NO _{2}$ is ' $x$ '
$\left[1 / 2 N _{2(g)}+ O _{2(g)} \rightarrow NO _{2(g)}\right]$ the heat of reaction
$N _{2(g)}+ O _{2(g)} \rightarrow 2 NO _{(g)}$ is $y$ and the heat of reaction
$2 NO _{(g)}+ O _{2(g)} \rightarrow 2 NO _{2(g)}$ is $z$, then

• A. $2 x+z=y$
• B. $2 y+z=x$
• C. $2 x-z=y$
• D. $2 z+x=y$

Answer 1

Answer: (C)

Given:
$2 \times\left(\frac{1}{2} N _{2}( g )+ O _{2}( g ) \longrightarrow NO _{2}( g ) ; \Delta H = x \right) $
$= N _{2}( g )+2 O _{2} \longrightarrow 2 NO _{2}( g ) ; \Delta H _{1}=2 x$
Also,
$ N _{2}+ O _{2} \longrightarrow 2 NO ( g ) ; \Delta H = y $
$2 NO + O _{2} \longrightarrow 2 NO _{2} ; \Delta H = z$
Adding, we get
$ N _{2}+2 O _{2} \longrightarrow 2 NO _{2} ; \Delta H _{2}= y + z $
$\because \Delta H _{1}=\Delta H _{2} (\because H$ is state function)
$\Rightarrow 2 x = y + z $