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Unformatted text preview: 1 Quiz #4 Next Week (Feb 12-16) Covers sections 14.5-14.7 During lectures next week, we will complete ALL OF Chapter 15 and parts of Chapter 16! The Second Midquarter Exam will cover Chapters 14, 15, and 16.1-16.3 Week six, a continuation of Chapter 14 Chemical Kinetics 14.1 Factors that Affect Reaction Rates 14.2 Reaction Rates 14.3 Concentration and Rate 14.4 The Change of Concentration with Time 14.5 Temperature and Rate The Collision Model Activation Energy The Orientation Factor The Arrhenius Equation and Activation Energies 14.5 Reaction Mechanisms Elementary Steps; Multistep Mechanisms Rate Laws for Elementary Steps Rate Laws for Multistep Mechanisms 14.7 Catalysis Homogeneous and Heterogeneous Catalysis Enzymes Note the DRAMATIC effect of temperature on k Activation Energy Activation Energy Consider the rearrangement of acetonitrile: In H 3 C-N C, the C-N C bond bends until the C-N bond breaks and the N C portion is perpendicular to the H 3 C portion. This structure is called the activated complex or transition state. The energy required for the above twist and break is the activation energy, E a . Once the C-N bond is broken, the N C portion can continue to rotate forming a C-C N bond. H 3 C N C C N H 3 C H 3 C C N Activation Energy Activation Energy The change in energy for the reaction is the difference in energy between CH 3 NC and CH 3 CN. The activation energy is the difference in energy between reactants, CH 3 NC and transition state. The rate depends on E a . Notice that if a forward reaction is exothermic (CH 3 NC CH 3 CN), then the reverse reaction is endothermic (CH 3 CN CH 3 NC). 2 Activation Energy Activation Energy Consider the reaction between Cl and NOCl: If the Cl collides with the Cl of NOCl then the products are Cl 2 and NO. If the Cl collided with the O of NOCl then no products are formed. We need to quantify this effect. Activation Energy Activation Energy The Arrhenius Equation The Arrhenius Equation Arrhenius discovered most reaction-rate data obeyed the Arrhenius equation : k is the rate constant, E a is the activation energy, R is the gas constant (8.314 J/K-mol) and T is the temperature in K. A is called the frequency factor, and is a measure of the probability of a favorable collision. Both A and E a are specific to a given reaction. RT a E Ae k = The Arrhenius Equation The Arrhenius Equation If we have a lot of data, we can determine E a and A graphically by rearranging the Arrhenius equation: If we do not have a lot of data, then we can use A T E k a ln R ln + = = 1 2 1 2 1 1 R ln T T E k k a Look Familiar???...
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This note was uploaded on 07/17/2008 for the course CHEM 122 taught by Professor Zellmer during the Winter '07 term at Ohio State.
- Winter '07