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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
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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