14.3 Activation Barrier to Reaction Process Kinetic Molecular ...

14.3 Activation Barrier to Reaction Process Kinetic Molecular ...

14.3 Activation Barrier to Reaction Process Kinetic Molecular Theory and Collision Theory Dr. Fred Omega Garces Chemistry 201 Miramar College 1 Activation Barrier to Reaction Process 2/9/20 Speed of a Reaction Chemical savvy says that reaction occurs when molecules or atoms come in contact (or collide together). Some reactions are very fast: 2 H2 (g) + O2 (g) 2 H2O (g) Others are very slow 4 Fe (s) + 3 O2 (g) 2 Fe2O3(s) Reaction rate vs. Temp Rxn rate

In general, the rate of a reaction doubles for every 10 increase of the temperature. Temp 2 Activation Barrier to Reaction Process 2/9/20 Kinetic Molecular Theory Kinetic Molecular Model Collision Theory - explains factors which influence the speed of a chemical reaction ? Basic idea: In a reaction, there must be a chemical changes For chemical change to occur: Reactant bonds are broken Product bonds are formed Factors influence reaction rate: Rate 3 [

Collision frequency Orientation of molecules Activation Barrier to Reaction Process Activation Energy ] & Catalyst 2/9/20 Collision Frequency 1. Collision Theory - A collision must occur Collision frequency influence the rate of a reaction rate collision / time 4 particle system

2 and 2 4 collision 6 particle system 4 o and 2 8 collision 8 particle system 4 o and 4 16 collision In general: n n = number collisions Z = collision frequency = z [A] [B] 4 Activation Barrier to Reaction Process 2/9/20 Steric Factor 2. Orientation Factor - For collision to lead to products, the orientation of the molecules must be correct for bonds to break NO + NO3 2NO2 No reaction

Product formation 5 Activation Barrier to Reaction Process 2/9/20 Activation Energy KMT- According to the Kinetic Molecular Theory; any molecule in motion possesses kinetic energy (1/2 mv2). Faster the motion, the more likely reactants will fragment upon collision. Consider the Car Analogy: Cars will not disintegrate by itself. However at low speeds, collision between car will cause little to no damage. But at high speeds, there will be major damages. 6 Activation Barrier to Reaction Process 2/9/20 Eact - Activation Energy If the k.E. is large enough, then upon collision molecules will vibrate so vigorously

as to break bonds which can lead to new products. Energetically, there must be some minimum collision energy that must be exceeded before a reaction occurs. Svante Arrhenius Suggested that molecules must possess a minimum amount of K.E. in order to react. In other words, energy must be available to the reactant for the reaction to occur. SVANTE AUGUST ARRHENIUS http://www.Nobel.se/laureates/chemistry-1903-1bio.html 7 Activation Barrier to Reaction Process 2/9/20 Stuff you should know: Terminolo gy Chemical Specie Energy reactant (Er) Energy product (Ep) Energy intermediate Transition state energy

Chemical Process Energy of reaction (HHrxn) Activation Energy of the reaction (Eact) Activation Energy of the reaction forward direction (Eact f) Activation Energy of the reaction reverse direction (Eact r) Terminology Reaction progression diagram Endothermic reactions Exothermic reactions Maxwell Boltzmann Plot Activated complex Transition state Intermediate 8 Activation Barrier to Reaction Process 2/9/20 Reaction Coordinate Diagram Consider the following reaction coordinate diagram in which NOCl is decomposed to NO and Cl2. The reactant NOCl has a certain initial energy. In order for products to form, an activation barrier must be overcome, this activation barrier is a the energy of the transition state, or the activated complex. Activated Complex - The

Ea Activation Energy; intermediate The minimum energy specie that is form and conditions or produced necessary for reactant to [intermediate]. be converted to In the end, the NO Eact products. and Cl2 products formed is more energetic than NOCl, the reaction is endothermic. H (+) 9 Activation Barrier to Reaction Process 2/9/20 Reaction Coordinate Diagram: Detailed View Consider the reaction: A(react) + B(react) [A B]*

.. . 10 Activation Barrier to Reaction Process AB (prod) 2/9/20 Energy-Level Diagram: Exothermic Vs Endothermic Consider the two reaction below. Each with two steps. Rxn A is exothermic, Rxn B is endothermic. A B Note that in both examples, it is a two step reaction in which reactant converts to product. The faster step is the one with the lower Eact . 11

The valley in the reaction coordinate correspond to a stable intermediate. Activation Barrier to Reaction Process 2/9/20 Reaction Rate and Eact The rate of a reaction is directly correlated to the height of the activation barrier. The higher the Eact, the slower the reaction. Reactions with very small Eact, tend to go very fast. In the reaction coordinate diagram: Rxn A is endothermic with a high Eact. This reaction is the slowest among the three examples 12 Rxn B is exothermic with a lower Eact, and it is faster than Rxn A Rxn C is exothermic with a very low Eact. This reaction is very fast.

The number of molecules with the minimum kinetic energy large enough to initiate a reaction is related to temperature. Activation Barrier to Reaction Process 2/9/20 Maxwell-Boltzman Distribution Fraction of molecules with given speed Recall from the gas laws that an energy profile for molecules can be describe by the Maxwell-Boltzmann distribution diagram. Maxwell-Boltzmann Simulator Molecules with sufficient energy to overcome the activation energy, T1 and T2 Energ y Eact

The increase of temperature yields to more molecules exceeding the Eact , the minimum energy required to overcome the activation barrier for a reaction to occur. 13 As the temperature goes up, the population of molecules with more energy also increases. Activation Barrier to Reaction Process Eact e -Ea/RT 2/9/20 Boltzman Distribution and Rxn Coord. Diagram The energy of a collection of reactants and the minimum energy necessary to overcome the activation barrier An energy-level diagram of the fraction of collision reaching the activated state. In either direction, the fraction of collisions exceeding Eact is greater at higher temperatures. In general, in an exothermic

reaction at any temperature, the fraction of collision exceeding Eact(f) is greater than the fraction exceeding Eact(r). This is accounted for by the purple shade in the reaction coordinate diagram shown. Once the temperature is raised for an exothermic reaction, however the percentage increase of the product molecules overcoming the activation barrier is higher than that of the reactant and therefore Keq decreases and the reaction shifts to the left. This is accounted for by the red shade in the reaction coordinate diagram shown. The % increase is greater for the product than the reactant as shown in the reaction coordinate diagram. 14 Activation Barrier to Reaction Process 2/9/20 Boltzman Distribution: Exothermic Increasing the temperature for an exothermic reaction result in a greater proportion of molecules of the product exceeding the activation energy, therefore the rate rev becomes more significant than the rate for. The temperature increase is more significant for the product. Temp increase

R + Energy P Exo: Rate r > Rate 15 f Activation Barrier to Reaction Process 2/9/20 Boltzman Distribution: Endothermic Similarly, increasing the temperature for an endothermic reaction result in a greater proportion of molecules of the reactant exceeding the activation energy, therefore the rate for becomes more significant than the rate rev. The temperature increase is more significant for the reactant. Endo: Rate f > Rate r 16 Activation Barrier to Reaction Process 2/9/20

Maxwell-Boltzman Distribution and Equilibrium How does the Maxwell-Boltzman plot explain why the reaction favors the reactant for an exothermic reaction and why it favors the product for an endothermic reaction. In general, a reaction is faster when the temperature is raised. 17 Activation Barrier to Reaction Process 2/9/20 Arrhenius Equation Savant Arrhenius (1889) Expressed the three factors responsible for a reaction into an equation (Arrhenius Equation). Factors influence reaction rate: Rate [ Collision

frequency Orientation of molecules Rate. = A e -{Ea/RT} Rate = K Where, K - rate constant = A e K=Ae 18 ln K = ]

Energy A - pre-exponential term therefore, Activation Eact = e -{Ea/RT} [Conc(react)] = k [Rct] x [A]x [C]y -{Ea/RT} -{Ea/RT} ln A - Ea /RT ln K = - Ea/R (1/T) + ln A y m x

b Activation Barrier to Reaction Process 2/9/20 Graphic Representation: Arrhenius Equation ln K = - Ea ln A R T where, Ea - Activation Energy R = 8.314 J/molK T = Absolute Temp A = Collision freq. factor 1 + plot of ln K vs 1/T yields m = -Ea/R m yields Eact ln [A] ln [K]

1/T Note that as the magnitude of Eact (), the K becomes more negative, or the rate constant is smaller. ln K - Ea (minus Ea) The reaction rate decreases as the energy barrier increases For a graph with 2 points: Eact large, K also small Eact small, K also large 19 Activation Barrier to Reaction Process k' Ea 1 1 ln = k R T T' 2/9/20 Recall; Catalysis Factor Rate [Collision

frequency Orientation of molecules Activation Energy ] Increase rate by making collision effective via Decrease Eact for the reaction Increase the steric factor Catalyst: A catalyst can serve this purpose in which an external chemical changes the reaction pathway without being consumed. A catalyzed reaction Uncatalyzed reaction proceeds by a different set of steps Catalyzed which involves reaction lowering the Eact. Results: Reaction proceeds at a faster forward rate.

20 Activation Barrier to Reaction Process 2/9/20 Heterogeneous Catalysis Catalysis in action: Heterogeneous In the combustion of gasoline, the reaction CxHx + N2 + O2 2NO + CO + CO2 + H2O In addition : O2 2NO2 (g) (Smog) Clean air Act `75 Require minimum emission. Possible Mechanism: 2NO 21 + 2CO

N2 + 2CO2 Activation Barrier to Reaction Process 2/9/20 Homogeneous Catalysis Decomposition of Acetic Acid: RCOOR RCOOR 22 + H2O RCOOH + ROH + H2O -H++ RCOOH + ROH Note: The initial and final state (energies) are the same since the energies of compounds are state function. The catalyzed reaction has a lower overall

Eact. The decomposition is catalyzed by acid. Activation Barrier to Reaction Process 2/9/20 Catalysis Review Addition of catalyst Lowers the Eact for the forward and the reverse reaction. Endothermic a) Uncatalyzed b) Catalyzed a b a b Exothermic a) Uncatalyzed b) Catalyzed 23

Activation Barrier to Reaction Process 2/9/20 In Class Exercise Consider the following Reaction Coordinate Diagram. Determine the following: i) Steps in mechanism: ii) Ereactant iii) Eproduct iv) Eact (forward)1 v) Eact (forward)2 vi) Eact (forward)overall vii) Eact (reverse)1 viii) Eact (reverse)2 ix) Eact (reverse) overall 24 x) E1, E2, Eint, Hrxn Activation Barrier to Reaction Process 2/9/20

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