Spontaneous Processes and Entropy

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Presentation transcript:

Spontaneous Processes and Entropy Thermodynamics lets us predict whether a process will occur but gives no information about the amount of time required for the process. A spontaneous process is one that occurs without outside intervention. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. Entropy The driving force for a spontaneous process is an increase in the entropy of the universe. Entropy, S, can be viewed as a measure of randomness, or disorder. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Positional Entropy A gas expands into a vacuum because the expanded state has the highest positional probability of states available to the system. Therefore, Ssolid < Sliquid << Sgas Copyright©2000 by Houghton Mifflin Company. All rights reserved.

The Second Law of Thermodynamics . . . in any spontaneous process there is always an increase in the entropy of the universe. Suniv > 0 for a spontaneous process. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. Free Energy G = H  TS (from the standpoint of the system) A process (at constant T, P) is spontaneous in the direction in which free energy decreases: G means +Suniv Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Effect of H and S on Spontaneity Copyright©2000 by Houghton Mifflin Company. All rights reserved.

The Third Law of Thermodynamics . . . the entropy of a perfect crystal at 0 K is zero. Because S is explicitly known (= 0) at 0 K, S values at other temps can be calculated. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Free Energy Change and Chemical Reactions G = standard free energy change that occurs if reactants in their standard state are converted to products in their standard state. G = npGf(products)  nrGf(reactants) Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Free Energy and Pressure G = G + RT ln(Q) Q = reaction quotient from the law of mass action. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Free Energy and Equilibrium G = RT ln(K) K = equilibrium constant This is so because G = 0 and Q = K at equilibrium. Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Temperature Dependence of K y = mx + b (H and S  independent of temperature over a small temperature range) Copyright©2000 by Houghton Mifflin Company. All rights reserved.

Reversible v. Irreversible Processes Reversible: The universe is exactly the same as it was before the cyclic process. Irreversible: The universe is different after the cyclic process. All real processes are irreversible -- (some work is changed to heat). Copyright©2000 by Houghton Mifflin Company. All rights reserved.