Question

Given the following reactions with their enthalpy changes, at ${25}^{\circ }C$,
$N_2\left(g\right)+2O_2\left(g\right)\to 2N{O}_2\left(g\right);△H=16.18kcal$
$N_2\left(g\right)+2O_2\left(g\right)\to N_2{O}_4\left(g\right);△H=2.31kcal$
Calculate the enthalpy of dimerisation of $N{O}_2$. Is $N_2{O}_4$ apt to be stable with respect to $N{O}_2$ at ${25}^{\circ }C$?
$(13.87kcal,N_2{O}_4$ stable only at low temp.)

Answer 1

$N_2\left(g\right)+2O_2\left(g\right)⟶2N{O}_2\left(g\right);\Delta H=16.18kcal$

$2N{O}_2\left(g\right)⟶N_2\left(g\right)+2O_2\left(g\right);\Delta H=-16.18kcal------\left(1\right)$

$N_2\left(g\right)+2O_2\left(g\right)⟶N_2{O}_4\left(g\right);\Delta H=2.31kcal------\left(2\right)$

Adding (1) and (2), $2N{O}_2⟶N_2{O}_4\left(g\right);\Delta H=(-16.18+2.31)kcal=-13.87kcal$

Therefore, the enthalpy of dimerisation of $N{O}_2$ is $-13.87kcal.$

Therefore, $N_2{O}_4$ is stable at low temperatures.