810.2 Mechanism assumption and Approximate treatment 10.2.1 Approximation treatment of complex reaction mechanism Es ist danach hier +-=+?=kC4 When an overall reaction is subdivided und daher k2 Jo[Cll+k, leI(Clal into elementary steps, Bodenstein's quasi- oe s8I-kIO1IOal+k,lelICll Die Geschwindigkeit mit der das Elektron steady state approximation neglects the verbraucht wird, ist gemaB lla und llb variations in the concentrations of reaction dt Im stationaren Zustand, der sich praktisch intermediates by y assuming that the unmittelbar nach Beginn der Belichtung ein- stellt, ist concentration of these intermediates would d dt remain quasi-constant. These reactive dem Absolutwert nach, und deswegen ka Jo Cla-tkslelcI intermediates can be radicals. carbonium k4O2]+k。C2 woraus folgt ions. molecules in the excited state. etc Ju[,I
When an overall reaction is subdivided into elementary steps, Bodenstein's quasisteady state approximation neglects the variations in the concentrations of reaction intermediates by assuming that the concentration of these intermediates would remain quasi-constant. These reactive intermediates can be radicals, carbonium ions, molecules in the excited state, etc. 10.2.1 Approximation treatment of complex reaction mechanism §10.2 Mechanism assumption and Approximate treatment
810.2 Mechanism assumption and Approximate treatment 10.2. 1 Approximation treatment of complex reaction mechanism (1)Steady-state approximation (2)Rate-determining step(r d S) approximation For reaction: H+ Br-2 HBr For reaction: d HBr kh,Br 1/2 HBI 2NO,+F→>2NO3F 1+m B The experimental rate equation is: In 1919.. A. Christiansen. K. F r=k[nO,e Herzfeld and m Polanyi independently proposed the mechanism consisting of No+E-k>NOF+F five elementary reactions F+no-k >NOF
In 1919, J. A. Christiansen, K. F. Herzfeld and M. Polanyi independently proposed the mechanism consisting of five elementary reactions: (1) Steady-state approximation For reaction: H2 + Br2 → 2 HBr 1/ 2 2 2 2 [HBr] [H ][Br ] [HBr] 1 Br d k dt m = + 10.2.1 Approximation treatment of complex reaction mechanism (2) Rate-determining step (r. d. s.) approximation 2 2 2 2NO F 2NO F + → For reaction: 2 2 r k = [NO ][F ] The experimental rate equation is: §10.2 Mechanism assumption and Approximate treatment 1 2 2 2 2 2 2 NO +F NO F F F NO NO F k k ⎯⎯→ + + ⎯⎯→
810.2 Mechanism assumption and Approximate treatment 10.2.1 Approximation treatment of complex reaction mechanism (3)Pre-equilibrium approximation Azo reaction of aniline under catalysis of HBr H+HNO, +C> N2+2H,O r=k[HI[HNO,IBr Its mechanism is proposed as 1)H+HNO,HNo (rapid equilibrium 2)HNO,+Br->NOBr+H,O (r.d. s 3)NOBr+CHNH->CHN,+H,O+Br(rapid reaction) deduce the rate equation according to this mechanism
(3) Pre-equilibrium approximation Azo reaction of aniline under catalysis of HBr. + + Br H +HNO +C H NH C H N +2H O 2 6 5 2 6 5 2 2 − ⎯⎯→ Its mechanism is proposed as: deduce the rate equation according to this mechanism + 2 r k[H ][HNO ][Br ] − = 10.2.1 Approximation treatment of complex reaction mechanism §10.2 Mechanism assumption and Approximate treatment
810.2 Mechanism assumption and Approximate treatment 10.2.1 Approximation treatment of complex reaction mechanism (4)Apparent activation energy (eaapp and e of elementary reactions △ k r=k,[H[HNO, Br EE a22 E a, app k E r=kh THNO,Br E.+E..-E Whether or not the activation energy app a,+ combination of elementary step is consistent to For some reaction with certain reaction order the eapp of overall reaction may be expressed as a the apparent activation energy of the overall activation energy combination of some reaction is the other important criterion for elementary steps examination on reaction mechanism
(4) Apparent activation energy (Ea,app) and Ea of elementary reactions + 2 2 [H ][HNO ][Br ] k r k k + − − = Ea,app = Ea,2 + Ea,+ − Ea,− + 2 r k[H ][HNO ][Br ] − = 10.2.1 Approximation treatment of complex reaction mechanism For some reaction with certain reaction order, the Eapp of overall reaction may be expressed as a activation energy combination of some elementary steps. Whether or not the activation energy combination of elementary step is consistent to the apparent activation energy of the overall reaction is the other important criterion for examination on reaction mechanism. §10.2 Mechanism assumption and Approximate treatment