Question

Assertion (A): The molecularity of the given reaction is 2
$2HI \longrightarrow H_2 + I_2$
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 determined by counting the stoichiometric coefficients of reactants in a single-step elementary reaction.

Answer 1

The Correct Option is A

Step 1: Concept
Molecularity represents the total number of reacting species taking part in an elementary chemical reaction.

Step 2: Meaning
These species must collide simultaneously to bring about the chemical change.

Step 3: Analysis
The given elementary reaction involves two molecules of hydrogen iodide: \[ 2HI \rightarrow H_2+I_2. \] For this elementary step to occur, two $HI$ molecules must collide simultaneously.

• Molecularity is defined as the number of reacting species participating in a single elementary reaction step.

• The given elementary reaction is: \[ 2HI \rightarrow H_2+I_2. \]

• Since two molecules of hydrogen iodide participate directly in the elementary step, the reaction requires the simultaneous collision of two molecules.

• Therefore, the molecularity of the reaction is: \[ \boxed{2}. \]

• A reaction involving two colliding molecules is called a \[ \boxed{\text{bimolecular reaction}}. \]

• Molecularity is always a whole positive integer because it represents the actual number of reacting particles involved in a single elementary step.

• Unlike reaction order, molecularity cannot be fractional or zero.

Step 4: Conclusion
Therefore, the molecularity is exactly 2 (a bimolecular reaction). The Reason accurately describes the physical requirement for the Assertion to be true.

Final Answer: (A)