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.
Step 1 (Definition of Molecularity): [0.25 Mark]
Molecularity is defined as the number of reacting species (atoms, ions, or molecules) taking part in an elementary reaction, which must collide simultaneously in order to bring about a chemical reaction.
Step 2 (Analysis of the Reaction): [0.5 Mark]
For the given elementary reaction:
$$2\text{HI} \longrightarrow \text{H}_2 + \text{I}_2$$The stoichiometric coefficient of the reactant $\text{HI}$ is $2$. Since this is an elementary reaction, the number of molecules colliding simultaneously is equal to the stoichiometric coefficient. Therefore, the molecularity is $2$ (bimolecular).
Step 3 (Final Concluded Answer): [0.25 Mark]
Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A).
💡 EL10 Concept Insight: Think of molecularity like a dance; if two people need to bump into each other at the exact same time to start the dance, the "molecularity" is two because it takes two participants to make the collision happen.