Question

Consider the three statements about reaction energy diagrams and the relative magnitudes of the activation energy, $E_a$, and the enthalpy of reaction, $△H$.
One or more of the statement is true.
Identify the correct statement or combination of statements from the four choices below.

	Statement
I	For an endothermic reaction, the magnitude of $E_a$ is always greater than $△H$.
II	For an exothermic reaction, the magnitude of $E_a$ is always greater than $△H$.
III	For an exothermic reaction, adding a catalyst will decrease the magnitude of $△H$.

• A. Statement I only is true.
• B. Statement II only is true.
• C. Both statements I and II are true.
• D. Both statements I and III are true.

Answer 1

The correct option is A Statement I only is true.

From transition state theory, we have

$\Delta H=E_a-nRT$ or $E_a=\Delta H+nRT$

Thus for Endothermic Reaction, $\Delta H$ is positive.

Thus $E_a>\Delta H$ always.

For exothermic reaction, $\Delta H$ is negative.

$E_a<\Delta H$

$⟶$ On adding a catalyst, $E_a$ of the reaction alters as $\Delta E$decrease, so, $\Delta H$ will increase in case of Exothermic Reaction.

For Exothermic;

$nRT=E_a+\Delta H$

Thus on decrement of $E_a$, $\Delta H$ magnitude increase.

So, Statement $1$ is true only.