Physical chemistr Reaction Kinetics(5) Xuan Cheng Xiamen University
1 Reaction Kinetics (5) Xuan Cheng Xiamen University Physical Chemistry
Ch ical chemistr Reaction Kinetic Key words ■ Pyrolysis ■高温分解 ■ Acetaldehyde 乙醛 ethane ■甲烷 ■( Polymerization 聚合 Monomer ■单体 ■ intiator ■引发剂 ■ Relaxation ■迟豫
2 Key Words ◼ Pyrolysis ◼ Acetaldehyde ◼ Methane ◼ Polymerization ◼ Monomer ◼ Initiator ◼ Relaxation ◼ 高温分解 ◼ 乙醛 ◼ 甲烷 ◼ 聚合 ◼ 单体 ◼ 引发剂 ◼ 迟豫 Physical Chemistry Reaction Kinetics
Ch ical chemistr Reaction Kinetic Rice-Herzfeld mechanisms a chain reaction can lead to a simple rate law yrolysis of acetaldehyde CH3CHO(8)-4 >CH4(8)+CO(g) dt-kICH3CHOJ/2 d[Ch4 Simple rate laws can follow from quite complex chain mechanisms The Rice -Herzfeld mechanism for the pyrolysis of acetaldehyde is (a)Initiation: CH3CHO ka、CH3+CHO r=kalch3chol (b) Propagation ChaCHO +oCHa kB CHA+CHaCO. r=kb[CH3 CHOJ. 1 (c) Retardation:CH3CO、k。CH3+CO r=kclCH3. Termination: CH3 +oCH kd CH3CH
3 Physical Chemistry Rice-Herzfeld Mechanisms Simple rate laws can follow from quite complex chain mechanisms. (a) Initiation: CH CHO a CH CHO k ⎯⎯→• +• 3 3 [ ] r = ka CH3 CHO (b) Propagation: CH CHO +•CH ⎯⎯→CH +CH CO• b k 3 3 4 3 [ ][ ] CH3 CHO CH3 r kb = • (c) Retardation: CH CO c CH CO k 3 • ⎯⎯→• 3 + [ ] 3 r = k CH CO• c (d) Termination: CH3 CH3 CH3 CH3 d k • +• ⎯⎯→ 2 3 r k [ CH ] d = • The Rice-Herzfeld mechanism for the pyrolysis of acetaldehyde is A chain reaction can lead to a simple rate law. Pyrolysis of acetaldehyde ( ) ( ) ( ) CH3 CHO g ⎯⎯→CH4 g +CO g 3/ 2 3 4 [ ] [ ] k CH CHO dt d CH = Reaction Kinetics
Ch ical chemistr Reaction Kinetic Rice-Herzfeld mechanisms CH3CHO- Ma oCH3+oCHO r=kalch3chol ChaCHO CHa ks ,CHa+CHaCO. r=kblCH3] CH3COdk foCH+co r=kCH3CO。] QCH3+CH3M、CH3C3 r=ka[CH312 The net rates of the formation of the two intermediates are docH ka[CH3CHO]-k6ICH3EHOCH3]+k[](o CH31=0 d[CH3 CO. kCH3eHOH3-kCO。=0 dt The sum of the two equation is kalch3 cho]-kaloCH3]-=0
4 Physical Chemistry Rice-Herzfeld Mechanisms The net rates of the formation of the two intermediates are [ ] [ ][ ] [ ] [ ] 0 [ ] 2 3 3 3 3 3 3 = − • + • − • = • k CH CHO k CH CHO CH k CH CO k CH dt d CH a b c d [ ][ ] [ ] 0 [ ] 3 3 3 3 = • − • = • k CH CHO CH k CH CO dt d CH CO b c [ ] [ ] 0 2 ka CH3 CHO − kd •CH3 = CH CHO a CH CHO k ⎯⎯→• +• 3 3 [ ] r = ka CH3 CHO CH CHO +•CH ⎯⎯→CH +CH CO• b k 3 3 4 3 [ ][ ] CH3 CHO CH3 r kb = • CH CO c CH CO k 3 • ⎯⎯→• 3 + [ ] 3 r = k CH CO• c CH3 CH3 CH3 CH3 d k • +• ⎯⎯→ 2 3 r k [ CH ] d = • The sum of the two equation is Reaction Kinetics
Ch ical chemistr Reaction Kinetic Rice-Herzfeld mechanisms kalch3choj-kalCh 1/2 lCH [CH3CHOI The rate of formation of Cha is d[CHAzkbCH3CHOJLoCH31=h6/h,)1/2 [CH3CHOI 3/2 in agreement with the three-halves order observed experimentall However, the true mechanism is more complicated thanr-h mechanism Other products(acetone, CH3COCH3, and propanaldehyde, CH CH2 CHO) can be formed Prob.17.81
5 Physical Chemistry The rate of formation of CH4 is 1/ 2 3 1/ 2 3 [ ] [CH CHO] k k CH d a • = 3/ 2 3 1/ 2 3 3 4 [ ][ ] [ ] [ ] CH CHO k k k b CH CHO CH k dt d CH d a b = • = Rice-Herzfeld Mechanisms [ ] [ ] 0 2 ka CH3 CHO − kd •CH3 = in agreement with the three-halves order observed experimentally. However, the true mechanism is more complicated than R-H mechanism. Other products (acetone, CH3COCH3 , and propanaldehyde, CH3CH2CHO) can be formed. Prob. 17.81 Reaction Kinetics