First slide

Hess law of constant summation

Question

Consider,
$N_{2} + 2 O_{2} \to 2 {NO}_{2} + X kJ; 2 NO + O_{2} \to 2 {NO}_{2} - Y kJ; Enthalpy of formation of NO in kJ will be$

Moderate

Solution

$eq.(1) \Rightarrow \,\,\,\,{N_{2(g)}} + {\text{ }}2{O_{2(g)}} \to {\text{ }}2N{O_2}{\text{;}}\,\,\,\Delta {{\text{H}}_1}{\text{ = - }}X{\text{ }}KJ$

$\underline {\frac{1}{{eq.(2)}} \Rightarrow \,\,\,\,\,2N{O_2}_{(g)} \to 2N{O_{(g)}} + {O_{2(g)}};\,\,\Delta {{\text{H}}_2}{\text{ = + Y }}KJ\,\,\,\,}$
$On\,addition:\,\,\,\,{N_{2(g)}} + {\text{ }}{O_{2(g)}} \to 2N{O_{(g)}}\,;\,\,\,\,\,\Delta {\text{H}} = Y - X$
$But\,\,\,\Delta {{\text{H}}_f}\,is\,always\,for\,the\,formation\,of\,one\,mole\,\,of\,\,a\,compound$
$\,\,\,\,\therefore \,\,\Delta {{\text{H}}_f}\left( {NO} \right) = \frac{1}{2}(Y - X)$

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