Thermodynamics and Kinetics

Thermodynamics and Kinetics - C H A P T E R 24...

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261 C H A P T E R 24 THERMODYNAMICS AND KINETICS Thermodynamics Free Energy Adding Free-Energy Changes Coupling Free Energies Thermodynamic Cycles D G 5D H 2 T D S Driving Force Kinetics Velocity Transition State Theory Rate Constants Rate Constants and Mechanism The beauty of thermodynamics is that it can tell you whether or not a chemical reaction can occur and how much energy you can get out of it when it does. The beast of thermodynamics is that no one who really understands it can (or will) explain it to those of us who don’t. THERMODYNAMICS Alarms sound, eyes go blank, and a sigh can be heard.
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262 Basic Concepts in Biochemistry Take a generic chemical reaction: A 1 B P 1 Q What will happen if we mix A, B, P, and Q together? There’s some gray area here in that the answer depends somewhat on what we mean by happen . First, it depends on direction. A more appropriate way to ask the question is, Will the reaction happen in the direction written, that is, left to right? Second, it depends on the actual concentrations of A, B, P, and Q that you start with. Third, it really depends on the relationship between the initial concentrations of A, B, P, and Q and the equilibrium concentrations that will exist when the reaction finally comes to equilib- rium. Finally, when A, B, P, and Q are mixed, they will take off toward the equilibrium position, whatever that is, but thermodynamics doesn’t tell you how long it might take for the reaction to actually get to equi- librium. The How fast ? is kinetics. So the real answer is that when we mix A, B, P, and Q, the reaction will happen in the direction that takes you to equilibrium. When the reaction is actually at equilibrium, the con- centrations of A, B, P, and Q will be equal to their equilibrium concen- trations. The equilibrium constant for a reaction is just the ratio of the prod- ucts to the reactants at equilibrium: K eq 5 } [ [ A P] ] e e q q [ [ Q B ] ] e e q q } If the initial ratio of products to reactants, } [ [ A P] ] [ [ Q B ] ] } is different from the equilibrium ratio, the chemical reaction will proceed until the real product/reactant ratio equals the equilibrium product/sub- strate ratio, and then it stops at equilibrium. If ([P][Q]/[A][B]) , ([P] eq [Q] eq /[A] eq [B] eq ), the reaction goes in the direction that increases P and Q and decreases A and B so that the prod- uct/substrate ratio increases to the equilibrium value. This is the same as saying that if the concentration of products is lower than their equilib- rium values, the reaction goes in the direction that makes more products, or to the right. If ([P][Q]/[A][B]) . ([P] eq [Q] eq /[A] eq [B] eq ), the reaction goes in the direction that decreases P and Q and increases A and B so that the product/
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24 Thermodynamics and Kinetics 263 reactant ratio decreases to the equilibrium value. The reaction goes to the left.
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This note was uploaded on 04/09/2008 for the course BIO SCI 98 taught by Professor Goulding during the Spring '08 term at UC Irvine.

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Thermodynamics and Kinetics - C H A P T E R 24...

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