Chemical dynamics

Chemical dynamics - Chemical Kinetics review Rate laws of...

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Chemical Dynamics 1 Chemical Kinetics review Rate laws of reactions: A + 2 B 3 C + D dt ] B [ d 2 1 dt ] A [ d dt ] C [ d 3 1 dt ] D [ d Rate action Re = = = υ initial initial initial m initial n inital initial ] B ln[ m ] A ln[ n k ln dt ] D [ d ln ] B [ ] A [ k dt ] D [ d initial Rate + + = = = Initial rate method: m n ] B [ ] A [ k dt ] D [ d Rate = = Rate constant Integrated equations: t k ] A [ ] A [ ln 1 0 t = t k ] A [ 1 ] A [ 1 2 0 t = [A] = [A] 0 - kt first order second order zeroth order 1 2 1 k 2 ln t = 0 2 2 1 ] A [ k 1 t = 2 1 0 t k 2 ] A [ = half life Molecularity : order for an elementary step.
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Chemical Dynamics 2 Temperature dependence of rate constant: = = = 1 2 A 1 2 A RT E T 1 T 1 R E k k ln T 1 R E A ln k ln Ae k A Arrhenius: Arrhenius-type: T 1 R E T ln n A ln k ln e T A k A RT E n A + = = Steady state: When an intermediate is at equilibrium. Example: A B (k f , k r ) B C (k’) 0 ] B [ ' k ] B [ k ] A [ k dt ] B [ d r f = =
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Chemical Dynamics 3 Unimolecular reactions Chain reactions Explosions Chemical dynamics (Reaction mechanisms) Photochemistry Monitoring fast reactions Fluorescence Phosphorescence Quenching Flash photolysis Flow techniques Relaxation TOPICS IN CHEMICAL DYNAMICS/KINETICS Modeling reaction mechanisms
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Chemical Dynamics 4 Mechanisms for first-order reactions Many reactions in the gas phase are first order. But, reactions must involve a bimolecular collision, so that the molecule can have enough energy to react. How can they then be first order? Lindeman’s (1922) proposed mechanism: k 2 A + A A* + A bimolecular collision where an excited A is formed This is the activation step . k -2 A* + A A + A some of the activated A may become deactivated k 1 A* P some of the activated A may form the products when releasing its energy P may be one or more species, but it is not involved in the mechanism (e.g., it may be a solid and comes out from the gas phase).
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Chemical Dynamics 5 k 2 A + A A* + A k -2 k 1 A* P ------------------------ A P 2 1 2 2 1 1 2 2 2 1 2 2 2 1 ] A [ k ] A [ k k k dt ] P [ d k ] A [ k ] A [ k *] A [ 0 *] A [ k ] A *][ A [ k ] A [ k dt *] A [ d : * A on state Steady *] A [ k dt ] P [ d : law Rate + = + = = = = We need another assumption: k -2 [A] >> k 1 ] A [ k k k ] A [ ] A [ k k k dt ] P [ d 2 2 1 2 2 2 1 = = The mechanism has to be tested. For example, by reducing the amount of A (pressure of A), and seeing if it starts deviating from first-order kinetics, since then k -2 [A] will not be >> k 1 . ] A [ 1 k 1 k k k ] A [ k k k ] A [ k k ] A [ k k 1 k ] A [ k ] A [ k k k ] A [ k dt ] P [ d 2 2 1 2 2 1 1 2 1 2 effec 1 2 2 1 effec effec + = + = + = = to test if the reaction is first order on A
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Chemical Dynamics 6 ] A [ 1 k 1 k k k k 1 2 2 1 2 effec + = A plot of 1/k effec versus 1/[A] will show when is it that the reaction follows Lindeman’s mechanism.
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Chemical dynamics - Chemical Kinetics review Rate laws of...

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