JEE Main · 2025 · Shift-IhardCK-131

The reaction A2 + B2 - 2AB follows the mechanism: The overall order of the reaction is:

Chemical Kinetics · Class 12 · JEE Main Previous Year Question

Question

The reaction \ceA2+B2−>2AB\ce{A2 + B2 -> 2AB} follows the mechanism: image The overall order of the reaction is:

Options
  1. a

    1.5

    ✓
  2. b

    3

  3. c

    2.5

  4. d

    2

Correct Answera

1.5

Detailed Solution

Step 1 — Rate from slow step

Rate =k2[\ceA][\ceB2]= k_2[\ce{A}][\ce{B2}]

Step 2 — Eliminate intermediate [A] using pre-equilibrium

Keq=[\ceA]2[\ceA2]K_{eq} = \frac{[\ce{A}]^2}{[\ce{A2}]} → [\ceA]=Keq⋅[\ceA2]1/2[\ce{A}] = \sqrt{K_{eq}} \cdot [\ce{A2}]^{1/2}

Step 3 — Substitute

Rate =k2Keq⋅[\ceA2]1/2[\ceB2]1= k_2 \sqrt{K_{eq}} \cdot [\ce{A2}]^{1/2} [\ce{B2}]^1

Overall order =12+1=1.5= \frac{1}{2} + 1 = 1.5 → Option (a)

Key Points to Remember:

  • Rate = slow step rate; eliminate intermediates via equilibrium
  • [\ceA]∝[\ceA2]1/2[\ce{A}] \propto [\ce{A2}]^{1/2} from fast equilibrium
  • Overall order = 1/2 + 1 = 1.5

Practice this question with progress tracking

Want timed practice with adaptive difficulty? Solve this question (and hundreds more from Chemical Kinetics) inside The Crucible, our adaptive practice platform.

Practice in The Crucible →Browse Chemical Kinetics chapter →

More JEE Main Chemical Kinetics PYQs

2025medium

Drug X becomes ineffective after 50% decomposition. The original concentration of drug in a bottle was 16 mg/mL which becomes 4 mg/mL in 12 months. The expiry time of the drug in months is ________.…

2021easy

Isotope(s) of hydrogen which emits low energy - particles with t1/2 value 12 years is/are

2025hard

Consider the following statements related to temperature dependence of rate constants. Identify the correct statements: A. The Arrhenius equation holds true only for an elementary homogeneous…

2025hard

Reactant A converts to product D through the given mechanism (with the net evolution of heat): A - B slow; H = +ve B - C fast; H = -ve C - D fast; H = -ve Which of the following represents the above…

2025hard

| [A]0 / mol L-1 | t1/2 / min | |---|---| | 0.100 | 200 | | 0.025 | 100 | For reaction R P, t1/2 is related to [A]0 as above. Given: 2 = 0.30. Which of the following is true? (A) The order of the…

View all 51 Chemical Kinetics PYQs →
The reaction A2 + B2 - 2AB follows the mechanism: The overall order of the reaction is: (JEE Main 2025) | Canvas Classes