山东大学：《物理化学》课程教学资源（讲义资料）10.3 Chain reaction

§10.3 Chain reaction Outside class reading Levine: p. 564-569

§ 10.3 Chain Reaction Outside class reading: Levine: p. 564-569

§10.3 Chain Reaction 10.3.1 basic concepts of chain reaction: Chain reactions are those reactions in which some intermediates are consumed and regenerated in a cycle of reactions the net result of which is to carry forward the overall reaction Cl,→2Cl chain-initiating step 少C+H2>HCl+H H+Cl→HC+Cl chain-propagating steps 2Cl·—→>CN2 chain-terminating step

10.3.1 basic concepts of chain reaction: chain-initiating step chain-propagating steps chain-terminating step 1 2 3 4 2 2 2 2 Cl 2Cl Cl H HCl H H Cl HCl+Cl 2Cl Cl k k k k ⎯⎯→   + ⎯⎯→ +   + ⎯⎯→   ⎯⎯→ § 10.3 Chain Reaction Chain reactions are those reactions in which some intermediates are consumed and regenerated in a cycle of reactions the net result of which is to carry forward the overall reaction

§10.3 Chain Reaction 10.3.1 basic concepts of chain reaction: (1) Chain-initiating step: a reaction in which a molecule is decomposed forming some activated species such as free atoms, radicals, carbonium ions which act as chain initiating species also called chain carriers, by thermal collision or radiation In some cases. a molecule named as initiator. which can readily decompose and produces radicals, is introduced into the system chain-propagating steps Cl·+H>HC1+H H●+Cl,kHCl+Cl Owing to the participation of active intermediate, the activation energy of chain reactions usually ranging between 0-40 kJ mol-I is much lower than that of other kinds of reactions of 40-400 kJ mol-. Therefore, chain reactions can proceed rapidly

Owing to the participation of active intermediate, the activation energy of chain reactions usually ranging between 0 ~ 40 kJ mol-1 is much lower than that of other kinds of reactions of 40 ~ 400 kJ mol-1 . Therefore, chain reactions can proceed rapidly. chain-propagating steps 2 3 2 2 Cl +H HCl+H H +Cl HCl+Cl k k • ⎯⎯→ • • ⎯⎯→ • 10.3.1 basic concepts of chain reaction: （1）Chain-initiating step: a reaction in which a molecule is decomposed forming some activated species such as free atoms, radicals, carbonium ions which act as chain initiating species also called chain carriers, by thermal collision or radiation. In some cases, a molecule named as initiator, which can readily decompose and produces radicals, is introduced into the system. § 10.3 Chain Reaction

§10.3 Chain Reaction 10.3.2 Basic characteristics of chain reaction: 1)with active radicals participating; 2)With low activation energy of the chain propagation Examples for Chain reactions: 3)wide existence 1)H2+Cl2→;H2+O2→); 2)Pyrolysis of many organic molecules. 3)polymerization; 4)depletion of ozone 5)photochemical fog

1) with active radicals participating; 2) With low activation energy of the chain propagation 3) wide existence Examples for Chain reactions: 1) H2 + Cl2 →; H2 + O2 →; 2) Pyrolysis of many organic molecules. 3) polymerization； 4) depletion of ozone; 5) photochemical fog 10.3.2 Basic characteristics of chain reaction: § 10.3 Chain Reaction

§10.3 Chain Reaction 10.3.3 Brief history of chain reactions ZETTSCHRIFT FUR ELEKTROCHEMIE [Bd. 19, 1913 Herr Prof. Dr. M. Bodenstein- Hannover PHOTOCHEMISCHE KINETIK DES CHLORKNALLGASES Es ist danach bier dt ks lOl ILl ad daher k2JoICll+ks lel(clal Die Geschwindigkeit mit der das Elektron verbraucht wird, ist gemaS lla und llb k4le1IO21+kolelICll Im stationaren Zustand, der sich praktisch unmittelbar nach Beginn der Belichtung ein- stellt, ist Max Ernst august bodenstein dem Absolutwert nach, und deswegen German physical chemist ICl,I 1913, He was first to postulate a chain lel= k,, reaction mechanism and that explosions are 1. M. Bodenstein, W. Dux, Photochemische Kinetik des Chlorknallgases. Z. branched chain reactions Phys. Chemie(1913)85,297-328 2. M. Bodenstein, Eine Theorie der photochemischen Reaktions ges https://en.wikipediaorg/wiki/maxBodenstein chwindigkeiten, Z. Phys. Chem. (1913)85, 390-421

1913, He was first to postulate a chain reaction mechanism and that explosions are branched chain reactions. Max Ernst August Bodenstein German physical chemist 10.3.3 Brief history of chain reactions https://en.wikipedia.org/wiki/Max_Bodenstein § 10.3 Chain Reaction 1. M. Bodenstein, W. Dux, Photochemische Kinetik des Chlorknallgases. Z. Phys. Chemie (1913) 85, 297-328 2. M. Bodenstein, Eine Theorie der photochemischen Reaktions ges chwindigkeiten, Z. Phys. Chem. (1913) 85, 390–421

§10.3 Chain Reaction 10.3.3 Brief history of chain reactions In 1918, after studying photochemistry, Nernst proposed the atomic chain reaction theory. It stated that when a reaction in which free atoms are formed that can decompose target molecules into more free atoms would result in a chain reaction Walther Hermann Nernst

Walther Hermann Nernst 10.3.3 Brief history of chain reactions § 10.3 Chain Reaction In 1918, after studying photochemistry, Nernst proposed the atomic chain reaction theory. It stated that when a reaction in which free atoms are formed that can decompose target molecules into more free atoms would result in a chain reaction

§10.3 Chain Reaction 10.3.4 rate equation of chain reaction Polymerization initiated by an initiator XX-2X● X·+RCH=CH2→> XRCH-CH2 XRCH-CH,·+RCH=CH2→XRCH-CH2-RCH-CH, k2 XR(CHCH2)n1CH-CH2·+RCH=CH2→>XR(CH-CH2)nCH-CH2· XR(CHCH2)nCH-CH2°+X·→>XR(CH-CH2)nCH-CH2X XR(CH-CH2)n+ XR(CH-CH2)m.+ XR(CH-CH),(CH-CH2)mRY 4 k2 k R / IR-CH-CH2JX-XP Just likes that of H2+Ch>

10.3.4 rate equation of chain reaction Polymerization initiated by an initiator X-X ⎯→2X • X • + RCH=CH2 → XRCH-CH2 • XRCH-CH2 • + RCH=CH2 → XRCH-CH2 -RCH-CH2 • …… XR(CH-CH2 )n-1CH-CH2 • + RCH=CH2 → XR(CH-CH2 )nCH-CH2 • XR(CH-CH2 )nCH-CH2 • +X • → XR(CH-CH2 )nCH-CH2X XR(CH-CH2 )n • + XR(CH-CH2 )m • → XR(CH-CH2 )n (CH-CH2 ) mRX k2 k3 1 1 2 1 2 2 2 3 [R-CH=CH ][X-X] k r k k   =     Just likes that of H2+Cl2 → k1 § 10.3 Chain Reaction

§10.3 Chain Reaction 10.3.4 rate equation of chain reaction For h+C1->2HCI =104-106 For H2+ Br 2HBr④=001 Cl,A→2Cl Brk,2Br Cl+H,k2>HCl+H. Br+H-k>HBr+H H+C1,k)HCI+Br.H+Br2-ks>HBr+Br 2Ck→Cl H+HBr-4→H2+Br 2Br5→Br

For H2+ Cl2⎯→ 2HCl  = 104 ~ 106 For H2+ Br2 ⎯→ 2HBr  = 0.01 10.3.4 rate equation of chain reaction § 10.3 Chain Reaction 1 2 3 4 2 2 2 2 Cl 2Cl Cl H HCl H H Cl HCl+Br 2Cl Cl k k k k ⎯⎯→   + ⎯⎯→ +   + ⎯⎯→   ⎯⎯→ 1 2 3 4 5 2 2 2 2 2 Br 2Br Br H HBr H H Br HBr+Br H +HBr H +Br 2Br Br k k k k k ⎯⎯→   + ⎯⎯→ +   + ⎯⎯→   ⎯⎯→   ⎯⎯→

§10.3 Chain Reaction 10.3.5 Branched chain reactions and explosion k=Aexp CGTV/ rT 好看视频 If an exothermic reaction is constrained to take place in a limited volume and the heat cannot be dissipated, temperature will increase. The higher temperature will, in turn accelerate the reaction and produce more heat,国庆年焰火表演 which causes a still higher temperature. This will finally result in a catastrophe-an explosion, which is named as thermal explosion

10.3.5 Branched chain reactions and explosion exp E a k A RT   = −    If an exothermic reaction is constrained to take place in a limited volume and the heat cannot be dissipated, temperature will increase. The higher temperature will, in turn, accelerate the reaction and produce more heat, which causes a still higher temperature. This will finally result in a catastrophe-an explosion, which is named as thermal explosion. § 10.3 Chain Reaction

§10.3 Chain Reaction 10.3.6 Branched chain reactions and explosion Explosion No Explosion B P1 p P The most extensively studied explosion reaction is the formation of water explosion limit 2H2(g)+O2(g)—>2H2O(g) region of explosion; region of normal reaction

The most extensively studied explosion reaction is the formation of water. 2 H2 (g) + O2 (g) ⎯→ 2 H2O (g) 10.3.6 Branched chain reactions and explosion § 10.3 Chain Reaction explosion limit; region of explosion; region of normal reaction