Question

$N_2\left(g\right)+3H_2\left(g\right)\to 2N{H}_3\left(g\right);\Delta H=-22kcal$
Activation energy, $E_a$ for the given reaction is $70kcal$. Find the activation energy for $2N{H}_3\left(g\right)\to N_2\left(g\right)+3H_2\left(g\right)$ in $kcal$.

Answer 1

The given reaction is an exothermic reaction since $\Delta H=-ve$
$E_{a_F}$ =Activation energy of the forward reaction
$E_{a_B}$= Activation energy of the backward reaction

Enthalpy change can be expressed also in terms of activation energy,

$\Delta H=E_{a_F}-E_{a_B}$
$\Rightarrow -22=70-E_{a_B}$
$\Rightarrow E_{a_B}=92\text{kcal}$