4-6 Kinetics

# 4-6 Kinetics - Half Life of a First Order Reaction Half...

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CH112 LRSVDS Kinetics 4-6 1 Half lives Half life: t 1/2 t 1/2 is the time it takes for the concentration of a reactant to drop to half of its initial value . For the reaction A ! products t 1/2 is where For 1 st order reactions: What is t 1/2 ? First order t 1/2 doesn’t depend on concentration ! n [ A ] = ! n [ A ] o " kt [ A ] = [ A ] o 2 CH112 LRSVDS Kinetics 4-6 2 Half Life of a First Order Reaction Pressure of methyl isonitrile as a function of time What is the half life according to the graph below? 1. 75 s 2. 13600 s 3. 27200 s

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CH112 LRSVDS Kinetics 4-6 3 HALF LIFE OF 2 ND ORDER REACTIONS To get t 1/2 , we let [A] = 1/2 [A] o t 1/2 = t 1/2 of a Second Order Reaction depends on the initial concentration: [A] 0 1 A [ ] = k t + 1 A [ ] 0 CH112 LRSVDS Kinetics 4-6 4 HALF LIFE OF A SECOND ORDER REACTION NO 2 (g) ! NO(g) + 1/2 O 2 (g) [NO 2 ] M 0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.01 0 100 200 300 400 500 600 Time (sec)
CH112 LRSVDS Kinetics 4-6 5 Half Life Problem It requires 100 years for a first-order reaction to go 30.0% of the way to completion. What is the half- life of the reaction? CH112 LRSVDS Kinetics 4-6 6 Temperature Dependence of Reaction Rates CH 3 " N # C : ! CH 3 " C # N : Rate = k [CH 3 " N # C : ] Measure rate at different temperatures: Rates remain first order in [CH 3 " N # C : ] What variable must change?

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CH112 LRSVDS Kinetics 4-6 7 For a reaction to occur; 1.A molecular collision must occur 2. The orientation of the molecules may determine the effectiveness of the collision. Temperature Dependence of Reaction Rates Cl + NOCl ! Cl 2 + NO Cl + NOCl ! Cl + NOCl CH112 LRSVDS Kinetics 4-6 8 As T increases what happens to reaction rate? Why? What about unimolecular reactions? Look at energy profile for a typical reaction: isonitrile isomerizes to form acetonitrile Temperature dependence of Reaction Rates
CH112 LRSVDS Kinetics 4-6 9 Activation Energy % E= Energy required to go from reactants to a transition state is Activation Energy Transition State or Activated Complex: How do molecules get the energy needed to get over the barrier? CH112 LRSVDS Kinetics 4-6 10 Temperature Dependence of Kinetic Energy The fraction of molecules with a certain value of energy is f f \$ e -Ea/RT What does increasing T do to f ?

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11 Reaction Profiles For the reaction profiles below: 1. What is % E ? 2. What is the activation energy? 3. Which reaction will be fastest? CH112 LRSVDS Kinetics 4-6 12 ! Reaction rate depends on frequency of collisions ( to concentration of reactants) ! Reactants must come together with right orientation and enough KE to overcome E a How do orientation and activation energy factor into the rate laws ? Molecular level picture:
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4-6 Kinetics - Half Life of a First Order Reaction Half...

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