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CHEM-5151_S05_Kinetics_class

CHEM-5151_S05_Kinetics_class - Kinetics and Atmospheric...

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1 Kinetics and Atmospheric Chemistry Edward Dunlea, Jose-Luis Jimenez Atmospheric Chemistry CHEM-5151/ATOC-5151 Required reading: Finlayson-Pitts and Pitts Chapter 5 Recommended reading: Jacob, Chapter 9 Other reading: Seinfeld and Pandis 3.5 General Outline of Next 3 Lectures Intro = General introduction Quick review of thermodynamics Finlayson-Pitts & Pitts, Chapter 5 A. Fundamental Principles of Gas-Phase Kinetics B. Laboratory Techniques for Determining Absolute Rate Constants for Gas-Phase Reactions C. Laboratory Techniques for Determining Relative Rate Constants for Gas-Phase Reactions D. Reactions in Solution E. Laboratory Techniques for Studying Heterogeneous Reactions F. Compilations of Kinetic Data for Atmospheric Reactions
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2 Kinetics and Atmospheric Chemistry What we’re doing here… Photochemistry already covered We will cover gas phase kinetics and heterogeneous reactions Introductions to a few techniques used for measuring kinetic parameters What kind of information do we hope to get out of “atmospheric kinetics”? Predictive ability over species emitted into atmosphere Which reactions will actually proceed to products? Which products will they form? How long will emitted species remain before they react? Competition with photolysis, wash out, etc. Pare down list of thousands of possible reactions to the ones that really matter Aiming towards idea practical predictive abilities Use look up tables to decide if reaction is likely to proceed and determine an “atmospheric lifetime” ( = amount of time required for concentration of species to decrease by 1/ e, we’ll get to this…) Intro Why do we care about gas phase reactions? Atmosphere is primarily gas phase medium Pollution emissions often in gas phase Consider the lonely OH molecule here in Boulder… Number of photons seen per second Assume 1360 W m -2 , all radiation is 500 nm, and OH molecule has cross section of 0.5 nm 2 You tell me… Number of heterogeneous collisions per second Assume ~ 10 5 particles cm -3 with average diameter of 500 nm ~ 50 collisions per second We will derive this later Number of gas phase collisions Use 630 Torr, 298 K ~ 8 x 10 9 collisions per second!! We will derive this later Intro
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3 Before we start… Where do molecules hide all of their energy? Intro Quick Refresher on Thermodynamics Gibbs free energy G = H – T S Gibbs free energy ~ energy available for chemical reaction – Negative G = spontaneous reaction H = change in enthalpy from reactants to products • Lower H leads to lower G S = change in entropy from reactants to products • Higher S leads to lower G No need to reinvent the wheel, you can look up H and S values Idea is be able to calculate which reactions will occur From there, we can spend time figuring out how fast they occur Intro
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4 Enthalpy Enthalpy = internal energy + pressure-volume state Hrxn > 0
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