Question:

For an elementary chemical reaction, \(\mathrm{A_2} \underset{K_{-1}}{\overset{K_1}{\rightleftharpoons}} 2\mathrm{A}\), the expression for \(\frac{d[A]}{dt}\) is

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Each forward event makes 2 A and each backward event consumes 2 A, so both rate terms are multiplied by 2.
Updated On: Nov 5, 2026
  • \(2K_1[A_2] - K_{-1}[A]^2\)
  • \(K_1[A_2] - K_{-1}[A]^2\)
  • \(K_1[A_2] + K_{-1}[A]^2\)
  • \(2K_1[A_2] - 2K_{-1}[A]^2\)
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The Correct Option is D

Solution and Explanation

Forward (elementary): \(\mathrm{A_2} \to 2\mathrm{A}\), rate \(= K_1[A_2]\); 2 mol of A are formed per event, so A is formed at \(2K_1[A_2]\).
Backward (elementary): \(2\mathrm{A} \to \mathrm{A_2}\), rate \(= K_{-1}[A]^2\); 2 mol of A are consumed per event, so A is used up at \(2K_{-1}[A]^2\).
\(\frac{d[A]}{dt} = 2K_1[A_2] - 2K_{-1}[A]^2\).
Answer: (4) \(2K_1[A_2] - 2K_{-1}[A]^2\)
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