Question

In gaseous reactions important for the understanding of the upper atmosphere $H_{2}O$ and O react bimolecularly to form two OH radicals. $\Delta H$ for this reaction is 72 kJ at 500 K and $E_a$ is $77kJmo{l}^{-1}$, then $E_a$ for the bimolecular recombination of two OH radicals to form $H_{2}O$ and O is:

• A. $3kJmo{l}^{-1}$
• B. $4kJmo{l}^{-1}$
• C. $5kJmo{l}^{-1}$
• D. $7kJmo{l}^{-1}$

Answer 1

Answer: (C)

$5kJmo{l}^{-1}$

$H_{2}O+O\to 2OH;\Delta H=72kJ$ at 500 K; $E_a=77kJmo{l}^{-1}$
$2OH\to H_{2}O+O;E_{a_{B.R.}}$
For a reaction, $E_{a_{F.R.}}=\Delta H+E_{a_{B.R.}}$
$\therefore 77=72+E_{a_{B.R.}}$
$\therefore E_{a_{B.R.}}=5kJmo{l}^{-1}$.