Lecture 12 - Read:chapter6...

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Read: chapter 6 Homework: 11, 13, 17 19, 21, 27, 29, 39, 43, 47, 51, 55 L Cl Last Class Velocity Distribution: Maxwell Boltzmann distribution Intermolecular Collisions shape, change with T, three “averages” 2 collisions /sec 4 N RT d V M π = For O 2 at 298K: 4.14 x 10 9 s 1 ! Real Gas Behavior 2 ( ) n P a V nb nRT + container volume 1 V = observed pressure corrected pressure = P ideal corrected volume = V ideal
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CHAPTER 6: Chemical Equilibrium In learning about stoichiometry, reactions assumed to proceed completely from reactants Æ products But actually some products are formed and some reactants remain at equilibrium Most of what we do this chapter: reactions of gases Some discussion of reactions of other things Also applies to phases, solutions, and other things we will discuss Reaction Proceeding to Equilibrium 2 NO ( ) Æ N O ( ) 2 NO 2 (g) N 2 O 4 (g) AT EQUILIBRIUM 2
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Why does Reaction Equilibrium Occur H 2 O (g) + CO (g) Æ H 2 (g) + CO 2 (g) Reaction occurs only upon collision of reacting molecules Energy of collisions: breaking bonds in reactants and allowing rearrangements into products Number of such collisions will depend on molecular concentrations As reactant concentration decreases, so too does collision probability Forward rate decreases; At same time reverse rate increases 3
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The Characteristics of Chemical Equilibrium Two possible reasons reactant and product concentrations remain stable: 1. System is at chemical equilibrium (thermodynamics) 2. Forward and reverse reactions are proceeding VERY slowly (kinetics) Haber reaction proceeds at 500 C, yet is infinitely slow at room temperature. N 2 (g) + 3 H 2 (g) 2 NH 3 (g) 500 o C Stoichiometry sets relative rates
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Pushing a Reaction away from Equilibrium H O (g) + CO (g) Æ H (g) + CO (g) H 2 O (g) + CO (g) H 2 (g) + CO 2 (g) Push from equilibrium by adding more reactant Will more product be present? YES There is no one set of concentrations that defines equilibrium What Affects the Equilibrium Position? Initial concentrations Relative bond energies of the reactants and products Relative “degree of organization” ENTROPY of the reactants and products We will learn about this when we study thermodynamics
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