A hypothetical reaction A2+B2?2AB follows the mechanism as given below:A2?A+A (fast) A+B2?AB+B (slow) A+B?AB (fast) The order of the overall reaction is (1) 2 (2) 1 (3) 1.5 (4) 0

Answer: (3)

Since (A) is the intermediate reactive substance whose concentration is determined from the equilibrium step.

Let us consider the slow step of the reaction

A + B2 → AB + B (slow)

r = k[A][B2]…………….(1)

From the equilibrium step

A2⇌ A + A (fast)

keq = [A]2/[A2]

∴ [A] = keq[A2]1/2

Substitute the value of [A] in equation (1)

r = k[ keq[A2]1/2][B2]

r = k. keq1/2.[A2]1/2. B2

Thus, the order of the reaction is

⇒ 1/2 +1

⇒ 3/2

⇒ 1.5

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