Thermodynamics

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# Thermodynamics - PowerPoint PPT Presentation

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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## PowerPoint Slideshow about ' Thermodynamics' - vaughan-graves

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Presentation Transcript

### 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
• 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
• 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
• . . . in any spontaneous process there is always an increase in the entropy of the universe.
• Suniv > 0

for a spontaneous process.

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 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
• 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
• G = G + RT ln(Q)
• Q = reaction quotient from the law of mass action.
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
• 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: 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).