Question

Consider the recombination reaction: 2 Br(g) Br2 (8) The mechanism involves two elementary steps. The first step results in formation of an energized Bra molecule, the excess energy carried by Bris then transferred to a molecule M for subsequent formation of Bry in the second step First step: 2 Br (8) Bry* () + M ) Brz* () Brz () +M() Second step: By the steady-state approximation and work out an expression for d[Brydr in term of k, k., ky and other species. Show all of your work. (5 marks) Assume that [M] is high, show that the rate-determining step of the reaction is the first step (2 marks) Assume that (M) is low, show that the rate-determining step of the reaction is the second step. (2 marks) Table below shows the rate constant (koha) observed at two temperatures for the reaction: 2 Br (8) Brz () T/K 367 349 kode/moldm's 1.07 x 10° 1.15 x 10° (d) Determine the activation energy (in kJ moll) of the reaction. (3 marks) Based on the information in the table, decide whether the first step or the second step is rate-determining in the reaction. (1 mark) (1) Do you think that M is a catalyst in the reaction? Explain you answer. (3 marks)

Answer 1

(all (9) 28 -- 1012 Brala) + mug) ke? Bez+Mg dB) = ky Br, * -k, [Br] - Asy-Br is inter midiate sex d[Bry] = k1 [Br]2-K-y [Br. * - k, [Br,"] [m] _ Apply SSA, d [Brent0 dt K,[br]{-ky[Br] - ke [Brz* ICM] =0 K, [br]? = K-y [by * ] + k_ [ Bry*] [m] K, [br]² = (Br, x] {ktk [m]} : [ Bright ] = K[br]? - K KIMI So, putting value of Brt in ① buls Kkm[Br]? - Kr[br]? मा

=k_ik, Ebr]? - K,Ka, Br J² - K, K₂ [m] [Br]² KitK₂ [m] H Bit -K, Kz [m] [Br]² K-1 tk 2 [m] (b) now, if m is present in very high conco, K-,+K2Cm] a ke[m] d [B2] = -K, B/2 CMI [Br]? ISEEN : : -k,[Br]? This signifies that it depends only upon &,. So, rate determining stepp is first step- (c) Now, if [m) is low, K-,+Ke[m) K- d (Be) = -K, K₂ [m] [B2] {ifk, aku? 546 --K2CM] [Br]2 Now, I [be] depends upon Kz. So; rate determining step is and stepp.