JEE Main 2025 · 23 January · evening · Single correct · +4 / −1

Which of the following graphs most appropriately represents a zero order reaction?

  1. (a)
    Option (a)
  2. (b)
    Option (b)
  3. (c)
    Option (c)
  4. (d)
    Option (d)
Answer and solution

Answer (b)

Solution

Chemistry ma’am · 3 steps

Zero order means a constant rate, so the reactant runs down at a steady pace: a straight falling line.

  1. Rate law. For a zero order reaction, rate = k[R]⁰ = k. The rate does not depend on concentration, so it stays the same for the whole reaction.

    [R] = [R]₀ - kt

  2. Read the graph. This has the form y = c + mx with slope -k. A plot of reactant concentration against time is a straight line falling from [R]₀.
    OptionGraph shownZero order?
    (a)Rate falls as a curve with timeNo, rate must stay constant
    (b)Concentration falls in a straight lineYes, slope = -k
    (c)Rate falls in a straight lineNo, rate must stay constant
    (d)Concentration stays flatNo, the reactant is not being used
  3. So. Only graph (b) shows reactant concentration falling linearly with time, so (b) represents the zero order reaction.

The idea to keep. For zero order, the rate against time is flat and the concentration against time is a straight line with slope -k. Always check which quantity sits on the y-axis.

Answer from the NTA final answer key. Solution by PrepWiser.

Study Chemical Kinetics

Asked about 150 times in JEE Main since 2020. This question is from 2025.