Assertion (A): In the hydrolysis of butyl chloride, \( \ce{C4H9Cl + H2O -> C4H9OH + HCl} \), the instantaneous rate at t = 600 s is lower than the instantaneous rate at t = 250 s.
Reason (R): The average rate of a reaction is constant for the time interval for which it is calculated.
In the light of the above statements, choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Both (A) and (R) are true but (R) is not the correct explanation of (A).
The instantaneous rate is the rate at one moment, found from the slope of the tangent to the concentration–time curve. The average rate is the change in concentration divided by the time taken over an interval.
Assertion (A): In the hydrolysis of butyl chloride, the instantaneous rate at t = 600 s is lower than the instantaneous rate at t = 250 s. — True. The tangents give 1.22 × 10−4 mol L−1 s−1 at 250 s and only 5.12 × 10−5 mol L−1 s−1 at 600 s.
Reason (R): The average rate of a reaction is constant for the time interval for which it is calculated. — True. This is why an average rate cannot be used to find the rate at a particular instant.
Why R does not explain A?
R tells us why the instantaneous rate has to be found separately; it says nothing about why the rate at 600 s is smaller.
The correct reason is that the concentration of butyl chloride decreases as the reaction goes on, and the rate falls as the reactant concentration falls.
NCERT (2026-27 edition):
3.1 Rate of a Chemical Reaction – “…average rate cannot be used to predict the rate of a reaction at a particular instant as it would be constant for the time interval for which it is calculated.”
“…So, rinst at 600 s … = 5.12 × 10–5 mol L–1s–1 At t = 250 s rinst = 1.22 × 10–4 mol L–1s–1”
3.2.2 Rate Expression and Rate Constant – “…rate of a reaction decreases with the passage of time as the concentration of reactants decrease.”