JEE Main 2021 · 26 August · evening · Single correct · +4 / −1
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.
Choose the most appropriate answer from the options given below :
- (a) Both (A) and (R) are true but (R) is not the true explanation of (A).
- (b) Both (A) and (R) are true and (R) is the true explanation of (A).
- (c) (A) is false but (R) is true.
- (d) (A) is true but (R) is false.
Answer and solution
Answer (d) (A) is true but (R) is false.
Solution
Chemistry ma’am · 4 steps
The assertion is fine, but the reason has the speeds backwards: O–D breaks slower, not O–H.
- Check the assertion. Heavy water, D₂O, is used to study reaction mechanisms. Deuterium acts as a label: you swap H for D and follow where it ends up, or watch how the rate changes. So (A) is true.
- Which bond is stronger. D is twice as heavy as H, so the O–D bond vibrates more slowly and sits lower in energy. That makes the O–D bond slightly stronger than the O–H bond, and it needs more energy to break.
- Check the reason. A stronger bond breaks more slowly. So O–D cleavage is slower than O–H cleavage. This rate difference is called the kinetic isotope effect. (R) says the opposite, so (R) is false.
- So. (A) is true and (R) is false, which is option (d).
The idea to keep. Heavier isotope means a stronger bond and slower cleavage. In an Assertion–Reason question, check each statement's direction word by word before you think about whether R explains A.
Answer from the NTA final answer key. Solution by PrepWiser.