Question

Assertion (A) : The molecularity of the given reaction is 2.
\( 2\text{HI} \longrightarrow \text{H}_2 + \text{I}_2 \)
Reason (R) : Two molecules of the reactants are involved in simultaneous collision between them.

• A. Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
• B. Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
• C. Assertion (A) is true, but Reason (R) is false.
• D. Assertion (A) is false, but Reason (R) is true.

Hint:

Molecularity is only applicable to elementary reactions. For complex reactions, molecularity has no overall meaning; instead, we look at the molecularity of the rate-determining (slowest) step.

Answer 1

The Correct Option is A

Step 1: Recall the meaning of molecularity.
Molecularity tells us how many reactant species must physically come together and collide in a single elementary step for that step to occur. It is a theoretical number read straight off the balanced equation of an elementary reaction.
Step 2: Look at the given elementary step.
The decomposition $2HI \rightarrow H_2 + I_2$ is a one-step, elementary reaction, and its equation shows two molecules of HI as reactants.
Step 3: Connect the coefficient to the collision picture.
Since exactly two HI molecules must collide together at the same instant for the H-I bonds to break and new H-H and I-I bonds to form, this is a bimolecular step, so the molecularity is 2. This is precisely the simultaneous two-molecule collision the Reason describes.
Step 4: Decide the option.
The Assertion is true, the Reason is true, and the Reason correctly explains why the molecularity is 2. \[ \boxed{\text{Option (A): Both A and R are true, R is the correct explanation}} \]