The Third Law of Thermodynamics
The entropy of a pure crystal at Absolute Zero = 0
Why? Because all forms of molecular motion stop at
absolute zero, so there can be no entropy content.
Predicting Reaction Spontaneity:
Gibbs Free Energy
DSuniv = DSsys + DSsurr
Could use this relation to calculate reaction spontaneity, but
not always easy to calculate DSsurr
So the expression is rearranged to only include terms
relating to the system:
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Under constant temperature
and pressure conditions:
Gibbs free energy (G):
the energy available to
perform useful work
DG = DHsys -TDSsys
DG < 0
spontaneous
DG > 0 nonspontaneous as written,
spontaneous in the reverse direction.
DG = 0
The reaction is at equilibrium.
Calculating Free-Energy Changes
aA + bB
cC + dD
0
DGrxn
= [ cDG0f (C) + dDG0f (D) ] - [ aDG0f (A) + bDG0f (B) ]
0
DGrxn
= S nDG0f (products) - S mDG0f (reactants)
Standard free energy of
formation (DG0f ) is the free-energy
change that occurs when 1 mole
of the compound is formed from its
elements in their standard states.
DG0f of any element in its stable
form is zero.
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Practice Exercise, p. 686
Calculate ΔH°, ΔS° and ΔG° for the combustion of methane:
CH4(g) + 2 O2(g)  CO2(g) + 2 H2O(l)
DHof (kJ/mol)
Sof (J/mol K)
DGof (kJ/mol)
-74.8
186.2
-50.8
0
205.0
0
-393.5
213.8
-394.4
-285.8
69.9
-237.2
Temperature and Spontaneity
DG = DH –TDS
DG = –TDS + DH
DG = –DST + DH
y =
rearranging
m x + b
m = –DS
b = DH
At what temperature does reaction spontaneity change?
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For the previous reaction:
CH4(g) + 2 O2(g)  CO2(g) + 2 H2O(l) DH = -890.3 kJ
DS = -242.6 J/K = - 0.2426 kJ/K
m = –DS = +0.2426 kJ/K
DG 
b = DH = -890.3 kJ
DG = 0
T
Another Example: HW #55(a)
N2(g) + O2(g)  2 NO(g)
DHo
f
So
(J/mol K)
f
(kJ/mol)
0
191.5
0
90.3
205.0
210.7
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Effects of ΔH, ΔS, and T on ΔG
(a)
(b)
(c)
(d)
DG = DH –TDS
(a)
lecture example
N2(g) + O2(g)  2 NO(g)
(b)
(c)
(d)
CH4(g) + 2 O2(g)  CO2(g) + 2 H2O(l)
Thermodynamics - Memorize These Equations
DSuniv = DSsys + DSsurr > 0
DSsurr = -DHsys/ T
DS0rxn = S nS0(products) - S mS0(reactants)
DS = DH / T phase transition
DG = DH -TDS and spontaneity changes when T = DH/DS
0
DGrxn
= S nDG0f (products) - S mDG0f (reactants)
DH0rxn = S nDH0f (products) - S mDH0f (reactants)
DH0f and DG0f
for elements = 0
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Driving the Human Engine
 In living systems:
• Breaking down food, generating heat
» ΔG < 0; spontaneous.
• Metabolic processes (building muscle, etc.)
» ΔG > 0, nonspontaneous.
 Process of life increases the entropy of the
universe by converting chemical energy
into heat.
Breakdown of Glucose
 Glucose:
 Used by living systems for
energy.
 Glycolysis:
 Series of reactions to
convert glucose into
pyruvate (major path for
metabolism of glucose).
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Phosphorylation
 Phosphorylation reaction: results in addition
of a phosphate group to an organic molecule.
ΔGrxn° = + 13.8 kJ/mol, nonspontaneous
Adenosine Triphosphate (ATP)
 ATP:
• Produced as a result of the breaking down
(metabolizing) of food.
• Can be used to drive nonspontaneous cellular
reactions.
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Hydrolysis of ATP to ADP
ΔGrxn° = −30.5 kJ/mol,
spontaneous.
Coupled Reactions
• Phosphate group
from ATP used in
phosphorylation
of glucose.
Net process:
ΔGsum° = −16.7 kJ (Spontaneous)
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