Yes. Yes. Yes. Experimental data: the concentration of a reactant or product measured as a function of time. Graph of conc. vs.

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1 Experimental data: the concentration of a reactant or product measured as a function of time Graph of conc. vs. time Is graph a straigh t line? No Graph of ln[conc.] vs. time Yes System is zero order Is graph a straigh t line? No Graph of 1/ [conc.] vs. time Yes System is 1 st order Is graph a straight line? No System is beyond scope of this book Yes System is 2 nd order

2 Chapter 13.4 Mechanisms I: Temperature and Energetics Objectives Describe the effect of changing temperature on the rate of reaction. Use the collision theory to relate collision frequency, activation energy, and steric factor to the rate of reaction. Relate temperature, activation energy, and rate constant through the Arrhenius equation.

3 Influence of Temperature on the rate constant Reactions proceed at faster rates at higher temperatures

4 Collision Theory The reaction rate is proportional to the collision frequency, Z, the number of molecular collisions per second. Z depends on the temperature and the concentrations of the colliding molecules. Not all molecular collisions result in the formation of products.

5 Collision Theory Activation energy (E a ): the minimum collision energy required for reaction to occur. Activated complex: the highest energy arrangement of atoms that occurs in the course of the reaction.

6 Activated Complex Energy NO + O 3 [activated complex] NO 2 + O 2 NO + O 3 [NO-O 3 ]* E a NO 2 + O 2 Reaction Coordinate

7 Influence of Temperature on Kinetics collision frequency, Z, the number of molecular collisions per second. The fraction of collisions with energy in excess of E a is given by: fr = e E a/rt The reaction rate is proportional to the rate of collisions times the fraction of collisions with energy in excess of E a. Hence rate = Z x f r

8 Orientation of Reactants NO + O 3 [ON-O 3 ]* NO 2 + O 2 NO + O 3 [NO-O 3 ] NO + O 3

9 The Steric Factor The steric factor, p, is a number between 0 and 1 that is needed to account for factors like orientation. The reaction rate is proportional to the steric factor times the collision frequency times the fraction of collisions with energy in excess of E a : rate = p x Z x fr

10 The Arrhenius Equation rate = p x Z x f r rate = pz o [colliding species]e E a/rt Combine p and Z o into a term A: rate = A[colliding species]e E a/rt Experiments show that rate = k [colliding species] so k = Ae E a/rt The Arrhenius equation is exponentially dependent on temperature.

11 The Arrhenius Equation k = Ae -E a/rt Take the natural log of both sides of this equation: lnk = lna - E a /R (1/T) A plot of lnk vs. 1/T gives a straight line with a slope of E a /R and an intercept of ln A. ln k 1 = E a k 2 R 1 T 1 1 T 2

12 Measuring Activation Energy 2NO 2 2NO + O 2 given the following data: k (M -1 s -1 ) T (K) ln k 1/T (K -1 ) x x x x x 10-3

13 Plot of ln(k) vs. 1/T ln k slope = x 10 4 slope = -E a /R E a = (1.37 x 10 4 ) * (8.314) E a = 1.14 x 10 5 J = 114 kj /T (K -1 )

14 In Class Example The rate constant of a reaction exactly doubles when the temperature is changed from 25 o C to 36 o C. Calculate the activation energy for this reaction.

15 Chapter 13.5 Catalysis Objectives Define catalysis and identify heterogeneous, homogeneous, and enzymatic catalysts. Draw energy-level diagrams for catalyzed and uncatalyzed reactions. October 27 th, 2016

16 Energy Catalyst A catalyst is a substance that increases the reaction rate but is not consumed in the reaction. A catalyst provides an alternate reaction path with a lower activation energy. Uncatalyzed pathway Reactants Catalyzed pathway Products Reaction

17 Homogeneous Catalysis A homogeneous catalyst is one that is present in the same phase as the reactants. Bromide ion is a homogeneous catalyst for the decomposition of hydrogen peroxide. Step 1: H 2 O 2 (aq) + 2Br - (aq) + 2H + (aq) Br 2 (aq) + 2H 2 O(l) Step 2: H 2 O 2 (aq) + Br 2 (aq) 2Br - (aq) + 2H + (aq) + O 2 (g) Overall reaction: 2H 2 O 2 (aq) 2H 2 O (l) + O 2 (g)

18 Heterogeneous Catalysis A heterogeneous catalyst is one that is present in a different phase from the reactants. The gas phase reaction of hydrogen with many organic compounds is catalyzed by solid platinum.

19 Enzyme Catalysis Enzymes are large molecules (macromolecules) that catalyze specific biochemical reactions. Enzymes can increase the rates of reactions by factors as large as Enzymes are specific for the reactions they catalyze. Enzymes are active under mild reaction conditions.

20 Hydrogenase Enzyme