Chapter 19
Overview
• Spontaneous Processes
• Entropy
• Second Law of Thermo.
• Standard Molar Entropy
• Gibbs Free Energy
• Free Energy & Temp. & Equil. Const.
•Thermodynamics and Entropy
•Gibbs Free Energy DG = DH  - TDS
•Thermo. and the Equilibrium Constant
DG = -RT ln Keq
•Thermodynamics and Time
Spontaneous Processes
• Energy is conserved in all processes
• Some processes are spontaneous
– occur with no outside intervention
• Some reactions are reversible
• In an irreversible rxn the forward reaction is
spontaneous, its reverse reaction will be nonspontaneous
• Dependent on temp, energy & rxn direction
Thermo. and Entropy:
•Thermodynamics: Energy Transfer
Difference btw product/reactant energy
Spontaneous product favored
Nonspontaneous  reactant favored
•Kinetics: Speed of Reactions
Can be spontaneous (prod. favored) but slow
•Entropy: Measure of Disorder
Disorder is more probable than order
Increase in entropy  increase in disorder
Is a state function independent of pathway
A
A
B
B
A
A
B
B
The more molecules, the smaller the probability of an
ordered system
In General, Entropy
(DS = qrev / T)
of
•gases > liquids > solids
•more complex molecules > simpler ones
•ionic solids > as ionic attractions weaken
•dissolved substances > pure substances
•evolved gas > dissolved gas
small entropy
large entropy
DS = qrev / T where q is heat transferred in a
reversible reaction to the system at constant T
DE = q + w
q = heat absorbed by system
w = work done on the system
(review Ch. 5, 1st Law & Enthalpy)
Problem: Calculate the DS for the vaporization of
benzene. DHvap = 30.9 kJ/mol at 80.1C
DS = q / T
= 30.9 x 103 J/mol
353.1 K
= 87.5 J/mol K
liquid to vapor
+ 87.5 J/molK
vapor to liquid
- 87.5 J/molK
Problem: Predict if entropy will increase or decrease
with each process
CO2(g) 
CO2(s)
decrease
KCl(s)  KCl(aq)
increase
MgCO3(s)  MgO(s) + CO2(g)
increase
Second Law of Thermodynamics:
The total entropy of the universe continually
increases for an irreversible process
•For any product-favored reaction, DSuniverse is +

D Suniverse = D S system + D S surroundings

D S surroundings = qsurr. / T = -DHsystem / T
For an individual system

DSsystem = S S products - S Sreactants
Problem:
Calculate DS system , DSsurroundings and
DSuniverse to determine if
H2(g) + Cl2(g)  2HCl(g) is
product-favored.
DS  system = +20.05 J/mol K
= S S
products
DS  surroundings =619.5 J/mol K
= -DHsystem / T
DS universe = +639.6 J/mol K
= D S
system
- S Sreactants
+ D S surroundings
Since D Suniverse is positive, product-favored
Standard Molar Entropy
• Denoted as S
• Defined at 1 atm and 298 K
• SME’s of elements not zero
• S S products - S Sreactants
• Calculate the SME of
N2(g) + 3H2(g) 3(g)
Gibbs Free Energy:
D Suniverse = DSsurroundings + DSsystem
D Suniverse = -DHsystem / T + DSsystem
-T DSuniverse = DHsystem - T DSsystem
D G = -T D Suniverse = Gibbs Free Energy
D G system = DHsystem - T DSsystem
Prediction of Spontaneous Rxn:
D G system = DHsystem - T DSsystem
DHsystem
DSsystem
Spontaneous
-
+
yes, always
-
-
at low temperature
+
+
at high temperature
+
-
no, never
Standard Free Energy of Formation
D Grxn = S DGfprod - S DGfreact
Standard Enthalpy of Formation
D Hrxn = S DHfprod - S DHfreact
Standard Molar Entropy of Rxn
DSrxn = S Sprod - S Sreact
Free Energy & Eq. Constant
• Free energy is related to equil. constant
 DG = DG  + RT ln Q for rxn not at equil.
 0 = DG  + RT ln K for a rxn at equil.
• DG free energy, non-standard conditions
• DG  free energy, standard conditions
• DG  negative
DG  zero
DG  positive
K>1
K=1
K<1