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. - PowerPoint PPT PresentationTRANSCRIPT
ThermodynamicsThermodynamics
Chapter 19Chapter 19
Spontaneous Processes and Entropy
Spontaneous Processes and Entropy
ThermodynamicsThermodynamics lets us predict lets us predict whether a whether a process will occur process will occur but gives no information but gives no information about the amount of time required for the about the amount of time required for the process.process.
A A spontaneousspontaneous process is one that process is one that occurs occurs without outside interventionwithout outside intervention..
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En
erg
y
Reaction progress
Reactants
Products
Domain of kinetics(the reaction pathway)
Domain ofthermodynamics
(the initial andfinal states)
EntropyEntropy
The driving force for a spontaneous The driving force for a spontaneous process is an process is an increase in the entropy of the increase in the entropy of the universeuniverse..
Entropy, Entropy, SS, can be viewed as a measure of , can be viewed as a measure of randomness, or disorder.randomness, or disorder.
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Positional EntropyPositional Entropy
A gas expands into a vacuum because the A gas expands into a vacuum because the expanded state has the highest expanded state has the highest positional positional probability probability of states available to the of states available to the system.system.
Therefore, Therefore,
SSsolidsolid < < SSliquidliquid << << SSgasgas
The Second Law of Thermodynamics
The Second Law of Thermodynamics
. . .. . . in any spontaneous process there is in any spontaneous process there is always an always an increase in the entropy of the increase in the entropy of the universeuniverse..
SSunivuniv > 0 > 0
for a spontaneous process.for a spontaneous process.
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Table 16.3 Interplay of Ssys and Ssurr in Determining the Sign of Suniv
Signs of Entropy Changes
Process Spontaneous?
Yes
No (reaction will occur in opposite direction)
Yes, if Ssys has a larger magnitude than Ssurr
Yes, if Ssurr has a larger magnitude than Ssys
Ssys Ssurr Suniv
Free EnergyFree Energy
GG = = HH TTSS (from the standpoint of the system)(from the standpoint of the system)
A process (at constant A process (at constant TT, , PP) is spontaneous in ) is spontaneous in the direction in which free energy decreases:the direction in which free energy decreases:
GG meansmeans ++SSunivuniv
Effect of H and S on Spontaneity
Effect of H and S on Spontaneity
H S Result
+ spontaneous at all temps
+ + spontaneous at high temps
spontaneous at low temps
+ not spontaneous at any temp
The Third Law of ThermodynamicsThe Third Law of Thermodynamics
. . . the entropy of a perfect crystal at 0 K is zero.. . . the entropy of a perfect crystal at 0 K is zero.
Because Because SS is explicitly known (= 0) at 0 K, is explicitly known (= 0) at 0 K, SS values at other temps can be calculated. values at other temps can be calculated.
Free Energy Change and Chemical Reactions
Free Energy Change and Chemical Reactions
GG = = standard free energy change standard free energy change that that occurs if reactants in their standard occurs if reactants in their standard state are converted to products in state are converted to products in their standard state.their standard state.
GG = = nnppGGff(products)(products) nnrrGGff(reactants)(reactants)
Entropy and EnthalpyEntropy and Enthalpy
Substance Hof (kJ/mol) So (J/K mol)
SO2 -297 248
SO3 -396 257
O2 0 205
Free Energy and PressureFree Energy and Pressure
GG = = GG + + RTRT ln( ln(QQ))
QQ = reaction quotient from the law = reaction quotient from the law of mass action.of mass action.
Free Energy and Equilibrium
Free Energy and Equilibrium
GG = = RTRT ln( ln(KK))
KK = equilibrium constant = equilibrium constant
This is so because This is so because GG = 0 and = 0 and QQ = = KK at at equilibrium.equilibrium.
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A
B
A
B
(a) (b)
C
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0Fraction of A reacted
0.5 1.0
G
Equilibriumoccurs here
0Fraction of A reacted
0.5 1.0
G
Equilibriumoccurs here
0Fraction of A reacted
0.5 1.0
Equilibriumoccurs here
(a) (b) (c)
G
Temperature Dependence of K
Temperature Dependence of K
yy = = mxmx + + bb
((HH and and SS independent of temperature independent of temperature over a small temperature range) over a small temperature range)
ln(K) = -Ho/R*(1/T) + So/R
Reversible v. Irreversible Processes
Reversible v. Irreversible Processes
ReversibleReversible: The universe is : The universe is exactly the exactly the same same as it was before the cyclic process.as it was before the cyclic process.
IrreversibleIrreversible: The universe is : The universe is differentdifferent after after the cyclic process.the cyclic process.
All real processes are irreversible All real processes are irreversible -- (some -- (some work is changed to heat).work is changed to heat).
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