Kinetics ; 13.11; December Assigned HW 13.22, 13.24, 13.26, 13.34, 13.36, 13.58, Due: Monday 6-Dec Lecture 33 1

Concentration vs. Time – 0 th Order 2 The rate of 0 th order reactions are independent of concentration Decreasing Concentration aAaAProducts Time [A] Since the reaction rate does NOT change as [A] changes, [A] decreases linearly. Slope = k

Concentration vs. Time – 1 st Order 3 The rate of 1st order reactions are dependent on concentration aAaAProducts As [A] decreases, the rate decreases Reaction Rate Rate 1 Rate 2 Rate 3 Rate 1 > Rate 2 > Rate 3

Integrated Rate Law - st Order 4 aAaAProducts

Sample Problem 5 How long does it take for the concentration of A to decrease to 1.0% of its initial value in a the reaction (k = 1.0 Ms -1 ) AProducts 1.Determine the order of the reaction. 2. Use to solve for time.

Sample Problem 6 How long does it take for the concentration of A to decrease to 1.0% of its initial value in a the reaction (k = 1.0 s -1 ) AProducts 1.Determine the order of the reaction. 2. Use to solve for time.

Half Life for a 1 st Order Reaction 7 How long does it take for the concentration of A to decrease to 50.0% of its initial value in a first order reaction. AProducts For a first order reaction, the time it takes for half of the reactants to decompose to products The half life is INDEPENDENT of CONCENTRATION for a 1 st order reaction

Half Life for a 1 st Order Reaction 8 AProducts Each t 1/2 is the same for 1 st order reactions.

Half Life for a 1 st Order Reaction 9 AProducts The half life is ONLY dependent on k for a 1 st order reaction

Half Life for a 1 st Order Reaction 10 For the reaction:C 2 H 6 2 CH 3 k = 5.5 x s -1 How long will it take for [C 2 H 6 ] to fall to 1/16 th its original concentration?

Half Life for a 1 st Order Reaction 11 For the reaction:C 2 H 6 2 CH 3 k = 5.5 x s -1 Calculate how long it will take for 1.5 M CH 3 to be produced if the reaction begins with 1 M C 2 H 6.

Concentration vs. Time – 2 nd Order 12 The rate of 2 nd order reactions are dependent on concentration aAaAProducts Bigger k faster rate

Integrated Rate Law – 2 nd Order 13 aAaAProducts A couple math steps

Integrated Rate Law Summary 14 Rate Law Integrated Rate Law 0 th Order1 st Order 2 nd Order

Rates and Temperature – the link to Thermodynamics 15 Arrhenius Equation Arrhenius parameters: Temperature independent A pre-exponential factor E a activation energy Y-int = lnA Slope = -E a /R

Rates and Temperature – the link to Thermodynamics 16 Arrhenius Equation The rate constant for the decomposition of N 2 O 5 at 45 °C is k = 5.1 x s -1. Determine the value of the rate constant at 50 °C if the activation energy is 103 kJ mol -1.

Rates and Temperature – the link to Thermodynamics 17 Arrhenius Equation Arrhenius parameters: Temperature independent A pre-exponential factor E a activation energy The reverse reaction has a much larger Energy Barrier (E a )

Activation Energy and Catalysis 18