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For the gas-phase free-radical halogenation: R-H + X2 -> R-X + HX, the following mechanism is proposed. (i) X2 -> 2X* (fast equilibrium); (ii) X* + R-H -> R* + HX (slowest step); (iii) R* + X2 -> R-X + X* (fast). Select all correct statements.
- (A) The effective rate constant for R-X formation equals k3*k4/k2
- (B) d[RX]/dt is proportional to [X2]
- (C) The overall order of the reaction is 3/2
- (D) d[RX]/dt is proportional to [X2]^(1/2)
Correct answer: (C) The overall order of the reaction is 3/2
Solution
Pre-equilibrium on step (i) gives [X*] = sqrt(K_eq[X2]). Rate = k2*sqrt(K_eq)*[X2]^(1/2)*[RH], giving overall order 3/2 and showing rate is proportional to [X2]^(1/2).
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