Question

\(\mathrm{N}_2 + 3\mathrm{H}_2 \longrightarrow 2\mathrm{NH}_3; + 22\mathrm{kcal}\)
The activation energy for the forward reaction is \(50\mathrm{kcal}\). What is the activation energy for the backward reaction?

• A. -72 kcal
• B. -28 kcal
• C. +28 kcal
• D. +72 kcal

Hint:

For exothermic reactions: \(E_a(\text{backward}) = E_a(\text{forward}) + |\Delta H|\).

Answer 1

The Correct Option is D

Step 1: Underlying Concept:
For an exothermic reaction, \(E_a(\text{backward}) = E_a(\text{forward}) - \Delta H\).
Step 2: Explanation:
Given: \(\Delta H = +22\) kcal (endothermic as written, but reaction is exothermic? Check sign: N\(_2\) + 3H\(_2\) → 2NH\(_3\) is exothermic, so \(\Delta H = -22\) kcal).
Assuming \(\Delta H = -22\) kcal (exothermic),
\(E_a(\text{backward}) = E_a(\text{forward}) - \Delta H = 50 - (-22) = 50 + 22 = 72\) kcal.
Step 3: Conclusion:
+72 kcal