3_LectureWeb - Announcements Help Room(83 Chemistry...

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Announcements Help Room (83 Chemistry) Schedule Ë Monday 9 AM–12 PM, 2–5 PM Ë Tuesday 9 AM–12 PM Ë Wednesday 9–10 AM, 4–5 PM Ë Thursday 10 AM–12 PM, 3–5 PM Ë Friday 1–2 PM, 3–5 PM TAs Ë Krista Cruse, Steve DiFranco, Steve Halpin, Troy Knight, Lisa LaGoo, Cate Dedeian, Sarah Meisinger, and Sean Preshlock Learning Objectives 1/15/08 You should 1. understand the role played by collisions in chemical reactions. 2. be able to identify the reactants, products, transition state, and intermediates (if any) on the reaction potential and determine if the overall reaction is exothermic or endothermic. 3. understand the role played by temperature in overcoming barriers to reaction and how increasing or decreasing temperature affects the rates of chemical reactions. 4. be able to provide a physical interpretation of the activation energy E a and the pre-exponen - tial factor A from the Arrhenius equation based on Collision Theory and Activated Com - plex Theory. 5. know how to apply the Arrhenius equation to quantitatively predict how a rate constant k will change with temperature. 6. be able to use experimental data (rate constants measured as a function of temperature) to find E a and A either graphically or by using the result from simultaneously solving equations for data collected at two temperatures. 2 7. understand that catalysts enhance the rates of chemical reactions without being consumed by providing pathways with lower activation barriers for conversion of reactants to products. 3 Chemical Reactions and Collisions (Collision Theory) What happens in a chemical reaction? Atoms are rearranged as bonds are broken and new bonds are formed. Chemical reactions require collisions. Why? Ë Collisions bring reactants together. Note that in order for a reaction to occur the reac - tants must be oriented properly with respect to each other. Ë Collisions provide the energy required to overcome barriers to reaction. Recall that energy is required to break bonds. Ë Even so called “unimolecular” reactions involving decomposition of a single molecule into atoms and/or smaller molecules require collisions to provide the energy needed to break bonds. 4
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Potential Energy Profile Along the Reaction Coordinate (Reaction Profile) 5 Key Terms Associated with the Reaction Potential Reaction coordinate: pathway between reactants and products used to describe the progress of the reaction Transition state: local maximum along the reaction potential corresponding to the point at which the reactants have collided to form the activated complex. The transition state is an unstable point along the reaction coordinate from which the activated complex can either separate and return to reactants or proceed to form products.
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