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

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