9. 8 The rate theory of unimolecular reaction Extensive reading: Levine, pp. 906-908 Section: 23. 6
9.8 The rate theory of unimolecular reaction Extensive reading: Levine, pp. 906-908 Section: 23.6
9.8 The rate theory of unimolecular reaction ()Theories for bimolecular reactions Transitional energy SCT Threshold energy, Vibrational energy? Potential energy TST ●●●●● Vibrational energy Zero point energy What kind of activation energy do these reactions relate? kalB How can a common molecule get activated? dt
(1) Theories for bimolecular reactions: SCT TST Transitional energy; Threshold energy; Vibrational energy? Potential energy; Vibrational energy; Zero point energy What kind of activation energy do these reactions relate? How can a common molecule get activated? 9.8 The rate theory of unimolecular reaction d[P] [A][B] d k t =
9.8 The rate theory of unimolecular reaction (2) Examples for possible unimolecular reactions decomposition Isomerization (1) Does unimolecular reaction need activation? (2) How does the molecule get activated?
(2) Examples for possible unimolecular reactions decomposition Isomerization + (1) Does unimolecular reaction need activation? (2) How does the molecule get activated? 9.8 The rate theory of unimolecular reaction
9.8 The rate theory of unimolecular reaction 9.8.1 Puzzling problems (1) Does unimolecular reaction need activation? It is obvious that a single molecule at ground state will not undergo any reaction (or this compound is unstable)except that it was activated by energy of some types A—)A P How do reactant molecules acquire necessary activation energy (a) Activation through collision--heat? (b) Activation by energy radiation--work?
It is obvious that a single molecule at ground state will not undergo any reaction (or this compound is unstable) except that it was activated by energy of some types. A ⎯→ A* ⎯→ P How do reactant molecules acquire necessary activation energy？ 9.8.1 Puzzling problems (1) Does unimolecular reaction need activation? (a) Activation through collision—heat? (b) Activation by energy radiation—work? 9.8 The rate theory of unimolecular reaction
9.8 The rate theory of unimolecular reaction 9.8.1 Puzzling problems (a) activation by collision: Before reaction takes place, reactants must collide with each other to acquire enough activation energy. This may lead to a conclusion that all gaseous unimolecular reaction should be second-ordered dP A+M→>A*)P dt IA=k[AJM] At early 19th century, it was found that all unimolecluar reaction, such as gaseous decomposition and isomerization, is of first-order dP klA dt
(a) Activation by collision: Before reaction takes place, reactants must collide with each other to acquire enough activation energy. This may lead to a conclusion that all gaseous unimolecular reaction should be second-ordered. At early 19th century, it was found that all unimolecluar reaction, such as gaseous decomposition and isomerization, is of first-order. A + M ⎯→ A* ⎯→ P dP [A] d k t = 9.8.1 Puzzling problems * 1 dP [A ] [A][M] d k k t = = 9.8 The rate theory of unimolecular reaction
9.8 The rate theory of unimolecular reaction 9.8.1 Puzzling problems (b) Activation by radiation: In 1919, Perrin proposed that, for unimolecular reaction, the reactant molecule was activated by absorption of infrared radiation from the container or other molecules radiation activation theon dP A radiation →A P kLA] The result is in good accordance with the early observation of kinetic characteristics of the unimolecular reaction
(b) Activation by radiation: In 1919, Perrin proposed that, for unimolecular reaction, the reactant molecule was activated by absorption of infrared radiation from the container or other molecules – radiation activation theory. A A P * 1 ⎯⎯⎯⎯→ ⎯⎯→ radiation k 1 dP [A] d k t = The result is in good accordance with the early observation of kinetic characteristics of the unimolecular reaction. 9.8.1 Puzzling problems 9.8 The rate theory of unimolecular reaction
9.8 The rate theory of unimolecular reaction 9.8.1 Puzzling problems Problems proposed by langmuir: 1) The energy of the infrared radiation of the wall of container is usually quite lc and is not sufficient for activation 2)Some reactant molecules do not have an absorption band in the wave-length region of infrared radiation 3)Latterly, it was observed that the unimolecular reaction is of second-order at low pressure and first-order at high pressure
Problems proposed by Langmuir : 1) The energy of the infrared radiation of the wall of container is usually quite low and is not sufficient for activation. 2) Some reactant molecules do not have an absorption band in the wave-length region of infrared radiation. 3) Latterly, it was observed that the unimolecular reaction is of second-order at low pressure and first-order at high pressure. 9.8.1 Puzzling problems 9.8 The rate theory of unimolecular reaction
9.8 The rate theory of unimolecular reaction 9.8.2 Christiansen and Lindemann's consideration (1)Special consideration In 1921. Christiansen postulated that the activation of molecules in unimolecular reaction is also through intermolecular collision For this theory, it is easy to explain the second-ordered feature of unimolecular reaction at low pressure but it is impossible to gave any reasonable explanation to its first-ordered feature at high pressure In 1922 Lindemann and Christiansen postulated that the activated molecules react long after the collision. There is a time lag between activation and reaction. During the stay of activated molecules, some of them may lose their energy due to the further collision( deactivation). Only part of the activated molecules form product
In 1921, Christiansen postulated that the activation of molecules in unimolecular reaction is also through intermolecular collision. For this theory, it is easy to explain the second-ordered feature of unimolecular reaction at low pressure but it is impossible to gave any reasonable explanation to its first-ordered feature at high pressure. In 1922 Lindemann and Christiansen postulated that the activated molecules react long after the collision. There is a time lag between activation and reaction. During the stay of activated molecules, some of them may lose their energy due to the further collision (deactivation). Only part of the activated molecules form product. 9.8.2 Christiansen and Lindemann’s consideration (1) Special consideration 9.8 The rate theory of unimolecular reaction
9.8 The rate theory of unimolecular reaction 9.8.1 Puzzling problems Professor Svante Arrhenius: The arguments in favour of this new and extremely interesting theory of the influence of light upon the composi- Volume 1 1922 Previous Article Next Aricle tion or form of substances, have been so ably put forward by the different From the journal Transactions of the Faraday Society Dr. Irving Langmuir: I think that Professor Perrin, by bringing forward again this radiation hypothesis in such clear form has done us a great service. His original paper of two or three years ago on this subject, and particularly his pointing out the impossibility of assuming that collisions have anything to do with unimolecular reactions, have more than Discussion on"the radiation theory of chemical anything else stimulated our discussion of this aspect of the mechanism of chemical action action EA Lndemann Professor Syante Aurhenys Professor, Iving Langmuir, DL. N R Dhar. Dr Dr N.R. Dhar(communication received from Allahabad): Some years Perrin. Professor and W. C. McC Lewis, Professor ago i showed that tropical sunlight can accelerate markedly several chemical reactions of very different types.(About seventy different reactions were Professor F. A. Lindemann In view of the short time at my dis. investigated. The conclusion drawn from the paper was that light might osal I will confine my remarks to the criticism of the radiation theor of be looked upon as a general accelerator of chemical reactions. chemical reaction velocity, which I myself have published. If Professor Perrin's premises are absolutely correct his his argument in favour of the radiation theory is no doubt sound. Briefly, he says it is experi mentally established within certain limits that reaction velocity is not changed 、 Professor. Perrin(m的,角 communicated):() Professor Lowry hat the neces he interve ention of a catalyst has been if the concentration, and therefore the number of collisions per second, is demonstrated for a great number of reactions which were in the first instance diminished. He assumes that he may extrapolate to such low concentrations regarded as unimolecular. But this holds also for multimolecular reactions. that practically no collisions occur and that the reaction velocity will then still be constant. He concludes, therefore, that reaction velocity cannot be due to collisions but can only be due to some outside agency, which must Professor W. C. McC. Lewis The only point I would like to deal obviously be radiation. The argument is convincing if it is true that reaction with is one which was brought out by Dr. Langmuir, namely that the velocity is really independent of the number of collisions; but surely our quantum theory is equally involved in a variety of phenomena which evidence of this is quite inadequate at low pressures. include radiation and chemical change, and consequently there is no reason
9.8.1 Puzzling problems 9.8 The rate theory of unimolecular reaction
9.8 The rate theory of unimolecular reaction 9.8.2 Christiansen and Lindemann's consideration 1921, Lindemann proposed a mechanism for unimolecular chemical reactions and showed that the first step is a bimolecular activation N. >NO+No No+No-No.+no Activation No.+No-No.+No. deactivation N,O—>NO,+NO Frederick Alexander Lindemann https://en.wikipediaorg/wiki/frederick_lindeMann,1stviscount_cherwel
Frederick Alexander Lindemann In 1921, Lindemann proposed a mechanism for unimolecular chemical reactions and showed that the first step is a bimolecular activation. 9.8.2 Christiansen and Lindemann’s consideration N O NO NO 2 5 2 3 ⎯⎯→ + * N O N O N O N O 2 5 2 5 2 5 2 5 + ⎯⎯→ + * N O NO NO 2 5 2 3 ⎯⎯→ + * N O N O N O N O 2 5 2 5 2 5 2 5 + ⎯⎯→ + Activation Deactivation 9.8 The rate theory of unimolecular reaction https://en.wikipedia.org/wiki/Frederick_Lindemann,_1st_Viscount_Cherwell