3/1/2016
Entropy changes
Standard molar entropy
S°[O2(g)] = 205.0 J/mol K
S°[H2O(ℓ)] = 69.91 J/ mol K
S°[Na(s)] = 51.45 J/ mol K
S°[CH4(g)] = 186.3 J/ mol K
S°[H2O(g)] = 188.83 J/ mol K
S°[K(s)] = 64.47 J/ mol K
S°[Rb(s)] = 76.78 J/ mol K
S°[C2H6(g)] = 229.5 J/ mol K
S°[C3H8(g)] = 269.0 J/ mol K
1
3/1/2016
The change in entropy during a reaction can be calculated from tabulated standard
molar entropies because entropy is a state function.
For an isothermal process:
Ssurr 
qsys
T
at constant P


Ssurr 
H sys
T
How do we put the two ΔS calculations together? See in a moment….
Overall prediction of spontaneity depends on:
2
3/1/2016
Gibbs free energy
For a spontaneous process, the second law states
> 0 Now, multiply both sides by -T
= ΔHsys –T ΔSsys
3
3/1/2016
Gibbs free energy
Josiah Willard Gibbs proposed a new state function,
G, now called Gibbs Free Energy or just free energy.
when ΔSuniv is positive
4
3/1/2016
In any spontaneous process at constant T and P,
If ∆G < 0
If ∆G > 0
If ∆G = 0
5
3/1/2016
Standard free energy of formation
Standard free energy, ∆G°, is
∆G° can be calculated from the standard enthalpy change and standard
entropy change.
ΔHf⁰ (kJ/mol)
S⁰ (J/mol K)
Na2CO3(s)
-1130.9
135.0
CO2(g)
-393.5
213.6
H2O(g)
-241.8
188.7
6
3/1/2016
Thermodynamic standard state:
Standard free energy of formation, ΔG°f
For elements in their most stable form at standard conditions
Free energy is a state function:
7
3/1/2016
Free energy and temperature
ΔG at non-standard conditions depends on temperature.
For a reaction to be spontaneous,
Which is more important ΔHsys or -TΔSsys for determining spontaneity?
When ΔH and ΔS have the same sign,
8
3/1/2016
Free energy and the equilibrium constant
Most chemical reactions do not occur under standard conditions, hence we need to obtain
ΔG from ΔG°.
It can be shown that: under any nonstandard conditions
Q depends on actual, nonstandard conditions and is useful in predicting the direction of a
reaction.
9
3/1/2016
Free energy and the equilibrium constant
Q is useful in predicting the direction of a reaction.
Q<K
Q>K
Q =K
The direction of spontaneity can be changed
10
3/1/2016
Relationship between ΔG° and K
At equilibrium, ΔG = 0 and Q = K.
Importance of the magnitude of ΔG°
11
3/1/2016
Driving nonspontaneous reactions
Many desirable reactions are nonspontaneous as written.
Often a reaction can be driven forward by coupling it to another reaction that
is spontaneous.
e.g. Extraction of Cu metal from chalcolite containing Cu2S:
Cu2S(s) → 2Cu(s) + S(s)
∆G° = +86.2 kJ
S(s) + O2(g) → SO2(g)
∆G° = -300.4 kJ
12
3/1/2016
Driving nonspontaneous reactions
In biochemical processes the necessary reactions are coupled to spontaneous reactions
that release energy.
C6H12O6(s) + 6 O2(g)  6CO2(g) + 6 H2O(ℓ)
ΔG° = -2880 kJ
13