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Thermodynamics. Chapter 19. 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. Entropy.

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Thermodynamics

Thermodynamics

Chapter 19


Spontaneous processes and entropy
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.


Entropy
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.


Positional entropy
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


The second law of thermodynamics
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.


Free energy
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:

  • Gmeans+Suniv


Effect of h and s on spontaneity
Effect of H and S on Spontaneity


The third law of thermodynamics
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.


Free energy change and chemical reactions
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)



Free energy and pressure
Free Energy and Pressure

  • G = G + RT ln(Q)

  • Q = reaction quotient from the law of mass action.


Free energy and equilibrium
Free Energy and Equilibrium

  • G = RT ln(K)

  • K = equilibrium constant

  • This is so because G = 0 and Q = K at equilibrium.


Temperature dependence of k
Temperature Dependence of K

  • y = mx + b

  • (H and S independent of temperature over a small temperature range)

ln(K) = -Ho/R*(1/T) + So/R


Reversible v irreversible processes
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).



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