Question

Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A): Heavy water is used for the study of reaction mechanism.
Reason (R): The rate of reaction for the cleavage of O-H bond is slower than that of O-D bond.

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

Hint:

Remember the kinetic isotope effect: bonds involving heavier isotopes are generally stronger and break more slowly than bonds with lighter isotopes. Therefore, a C-D bond is stronger than a C-H bond, and an O-D bond is stronger than an O-H bond. This difference in reaction rates is the basis for using isotopic labeling to study reaction mechanisms.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
We need to evaluate an Assertion and a Reason related to the use of heavy water (D\(_2\)O) and the relative strengths of O-H and O-D bonds.
Step 2: Detailed Explanation:
Analysis of Assertion (A):
Heavy water (D\(_2\)O) contains the isotope deuterium (D) instead of protium (H). In chemistry, isotopic labeling is a powerful technique used to trace the path of atoms through a chemical reaction. By substituting an atom with its isotope (like H with D) and analyzing the products, chemists can determine which bonds are broken and formed, thus elucidating the reaction mechanism. This is a well-established use of heavy water and other deuterated compounds. Therefore, Assertion (A) is true.
Analysis of Reason (R):
This statement compares the rates of cleavage of O-H and O-D bonds. The O-D bond is stronger and more stable than the O-H bond. This is due to the greater mass of deuterium, which leads to a lower zero-point vibrational energy for the O-D bond compared to the O-H bond. A lower zero-point energy means more energy is required to break the bond. Because the O-D bond is stronger, it breaks more slowly than the O-H bond. This phenomenon is known as the kinetic isotope effect. The Reason (R) states that the cleavage of the O-H bond is slower than that of the O-D bond, which is the opposite of what is true. Therefore, Reason (R) is false.
Step 3: Final Answer:
Assertion (A) is a true statement, but Reason (R) is a false statement. This corresponds to option (C).