How can we use the Arrhenius equation?

Transcription

1 How can we use the Arrhenius equation? k = Ae Ea RT Lab H 3 CNC(g) H 3 CCN(g) 1. Experiment to determine rate law 2. Experiment to determine Ea Temperature (K) k (s -1 ) 1/T ln k E E E E

2 Example Problems I. The gas phase reaction shown below has an overall enthalpy change of -66 kj. The activation energy for the reaction is 7 kj. a.) Sketch the overall energy profile for the reaction, and label Ea, E, and the transition state. Supply a possible (and acceptable) activated complex. b.) What is the activation energy for the reverse reaction? c.) What is a possible reason for the low activation energy? Cl(g) + HBr(g) HCl(g) + Br(g) 2

3 II. The temperature dependance of the rate constant for the reaction CO(g) + NO 2 (g) CO 2 (g) + NO(g) is shown below. Calculate Ea and A. Temperature (K) k (M -1 s -1 )

4 Potential Energy III. Reaction Mechanisms Reaction Mechanism - Describes the step-by-step process of how reactants turn in to products in a chemical reaction Example Each step in the mechanism is called an elementary reaction 2NO(g) + O 2 (g) 2NO 2 (g) Proposed Mechanism Step 1 NO(g) + NO(g) N 2 O 2 (g) Step 2 N 2 O 2 (g) + O 2 (g) NO 2 (g) + NO 2 (g) Overall NO(g) + NO(g) + N 2 O 2 (g) +O 2 (g) N 2 O 2 (g) +NO 2 (g)+no 2 (g) reactants Reaction Progress products 4

5 Notes about Mechanisms 1. Must be consistent with the stoichiometry of the balanced chemical equation 2. Must be made up of elementary steps/reactions 3. Must be consistent with the experimentally determined rate law What are Elementary Steps/Reactions I. Simple Reactions in which the stoichiometric coefficients are equal to the exponents in the rate law for a reaction Molecularity Elementary Rxn Rate Law Unimolecular A products rate=k[a] Bimolecular 2A products rate=k[a] 2 Bimolecular A+B products rate=k[a][b] Termolecular* 3A products rate=k[a] 3 Termolecular* 2A+B products rate=k[a] 2 [B] Termolecular* A+B+C products rate=k[a][b][c] 2. Are reversible and may reach equilibrium 5

6 How do we relate Mechanism to Rate Law for Overall Reaction Which step is the slowest step of the proposed mechanism? 6

7 Which step is the slowest cont d 7

8 Another Example Two mechanisms have been proposed for the reaction shown below. For each mechanism give the rate law. 2NO(g) + 2H 2 (g) N 2 (g) + 2H 2 O(g) Mechanism #1 Step #1 H 2 (g) + 2NO(g) N 2 O(g) + H 2 O(g) (Slow) Step #2 N 2 O(g) + H 2 (g) N 2 (g) + H 2 O(g) (Fast) Mechanism #2 Step #1 2NO N 2 O 2 (g) (Fast) Step #2 N 2 O 2 (g) + H 2 (g) N 2 O(g) + H 2 O(g) (Slow) Step #3 N 2 O(g) + H 2 (g) N 2 (g) + H 2 O(g) (Fast) 8

9 b.) Briefly explain why mechanism #1 is not likely to be the true mechanism for the reaction. c.) In the space below, draw the reaction profile for mechanism #2. Identify the intermediates and place them where they should appear, label any transition state(s) (as T.S.), and use arrows to identify the magnitude of the activation energy for the individual steps as well as the overall reaction. The overall reaction is exothermic but the first and second steps are endothermic. 9

10 IV. Catalysis Catalyst a catalyst is a substance that speeds up the rate of a chemical reaction without undergoing a permanent chemical change Catalysts generally speed up a reaction by providing a brand new rxn mechanism with a lower activation energy Decomposition of H 2 O 2 2H 2 O 2 (aq) 2H 2 O(l) + O 2 (g) very slow Catalyzed Decomposition of H 2 O 2 H 2 O 2 (aq) + I - (aq) OI - (aq) + H 2 O(l) H 2 O 2 (aq) + OI - (aq) H 2 O(l) + I - (aq) + O 2 (g) 2H 2 O 2 (aq) 2H 2 O(l) + O 2 (g) 10

11 Potential Energy Reaction profiles for the decomposition of H 2 O 2 H 2 O 2 (aq) + I - (aq) reactants H 2 O 2 (aq) + OI - (aq)+ H 2 O(l) 2H 2 O(l) + I - (aq) + O 2 (g) products Reaction Progress Ea for uncatalyzed reaction is 76 kj/mol Ea for catalyzed reaction is 19 kj/mol This decrease in Ea increases the rate of the reaction by ~1x10 9 ps.prenhall.com/wps/media/objects/3312/ /imag1406/aaaweaz0.jpg 11

12 Types of Catalysts Homogeneous catalysts Heterogeneous catalysts catalysts that exist in the same phase as the reactants catalysts that exist in a different phase than the reactants (generally involves reactant molecules adsorbing onto a solid surface often a transition metal) Enzymes Biological catalysts 12