The activation energy for a simple chemical reaction $\mathrm{A} ightarrow \mathrm{B}$ is $E_{a}$ in forward…

The activation energy for a simple chemical reaction $\mathrm{A} ightarrow \mathrm{B}$ is $E_{a}$ in forward direction. The activation energy for reverse reaction
  1. is always double of $E_{a}$
  2. is negative of $E_{a}$
  3. is always less than $E_{a}$
  4. can be less than or more than $E_{a}$

Solution

The activation energy of reverse reaction will depend upon whether the forward reaction is exothermic or endothermic.
As $\Delta \mathrm{H}=E_{a}$ (forward reaction) $-E_{a}$
(backward reaction)
For exothermic reaction
$\Delta \mathrm{H}=-\mathrm{ve}$
$\therefore-\Delta \mathrm{H}=E_{a}(\mathrm{f})-E_{a}(\mathrm{~b})$
or $E_{a}(\mathrm{f})=E_{a}(\mathrm{~b})-\Delta \mathrm{H}$
$\therefore E_{a}(\mathrm{f}) < E_{a}(\mathrm{~b})$
for endothermic reaction
$\Delta \mathrm{H}=+\mathrm{ve}$
$\therefore \Delta \mathrm{H}=E_{a}(\mathrm{f})-E_{a}(\mathrm{~b})$ or $E_{a}(\mathrm{f})=\Delta \mathrm{H}+E_{a}(\mathrm{~b})$
$\therefore E_{a}(\mathrm{f})>E_{a}(\mathrm{~b})$ *

Asked in: JEE-TOPICTESTS-CHEMISTRY

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