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

Order of a reaction

The Order of reaction refers to the relationship between the rate of a chemical reaction and the concentration of the species taking part in it. In order to obtain the reaction order, the rate expression (or the rate equation) of the reaction in question must be obtained.

  • Reaction order represents the number of species whose concentration directly affects the rate of reaction.
  • It can be obtained by adding all the exponents of the concentration terms in the rate expression.
  • The order of reaction does not depend on the stoichiometric coefficients corresponding to each species in the balanced reaction.
  • The reaction order of a chemical reaction is always defined with the help of reactant concentrations and not with product concentrations.
  • The value of the order of reaction can be in the form of an integer or a fraction. It can even have a value of zero.

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