For the hydrolysis of butyl chloride, the curve is plotted between the concentration of butyl chloride against time (s). Arrange the value of slope of the tangent drawn to the curve at the below given times in increasing order.
A. at t = 450 s.
B. at t = 350 s.
C. at t = 150 s.
D. at t = 250 s.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → A, B, D, C
Note: The given answer is as per the official NTA Answer Key. However, the correct answer should be Option (4), as explained below.
When the concentration of a reactant is plotted against time, the curve falls, because the reactant is used up. The slope of the tangent at any instant gives the rate of change of concentration at that instant:
\( \displaystyle \text{slope}=\frac{d[R]}{dt},\quad \text{instantaneous rate}=-\frac{d[R]}{dt} \)
Sign: [R] decreases as t increases, so every slope is negative.
Steepness: the reaction is fastest at the start, when the concentration is highest. As butyl chloride is used up, the curve becomes flatter with time.
Rate: the rate is the size of the slope without its sign, and it decreases with time.
All four slopes are negative, so the steepest one (150 s) has the most negative, that is, the smallest value, and the flattest one (450 s) is the closest to zero, the largest value:
value of slope: C (150 s) < D (250 s) < B (350 s) < A (450 s)
Option (1) A, B, D, C — The answer given by NTA, but incorrect. It is the increasing order of the rates (steepness of the curve), not of the slope values.
Option (4) C, D, B, A — Correct as per NCERT. It is the increasing order of the signed slope values.