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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.

Thermodynamics

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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

• 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).