3. When Will a Chemical Reaction be Product-Favored?
− ΔH system ΔH system
ΔSsurroundings = ΔSsystem >
T T
Enthalpy Entropy Product-Favored?
ΔHsystem < 0 ΔSsystem > 0 Yes
(Exothermic) ΔSuniverse > 0
Only at low T
ΔHsystem < 0 ΔSsystem < 0 ensures positive ΔSsurr
(Exothermic) overcomes negative ΔSsys
Only at high T
ΔHsystem > 0 ΔSsystem > 0 ensures positive ΔSsys
(Endothermic) overcomes negative ΔSsurr
ΔHsystem > 0 ΔSsystem < 0 No
(Endothermic) ΔSuniverse < 0
4. Directionality of Processes
Enthalpy Entropy Product-Favored?
Negative Positive Yes
Negative Negative Only at low T
Positive Positive Only at high T
Positive Negative No
Exothermic reactions tend to be product-favored
Some endothermic processes may also be product-favored
Entropy tends to increase in a product-favored reaction
Some processes involving a decrease in entropy (of the systems)
may also be product-favored
Directionality is dependent on ΔH, ΔS, and T
5. Gibbs Free Energy
Thermodynamic term to relate enthalpy and entropy:
ΔGsystem = -TΔSuniverse
Since ΔSuniverse increases for a product-favored process, ΔGsystem
decreases for a product-favored process (@ constant T and P)
6. Gibbs Free Energy
Thermodynamic term to relate enthalpy and entropy:
ΔGsystem = -TΔSuniverse
Since ΔSuniverse increases for a product-favored process, ΔGsystem
decreases for a product-favored process (@ constant T and P)
- ΔH system
ΔS universe = ΔSsurroundings + ΔSsystem = + ΔSsystem
T
7. Gibbs Free Energy
Thermodynamic term to relate enthalpy and entropy:
ΔGsystem = -TΔSuniverse
Since ΔSuniverse increases for a product-favored process, ΔGsystem
decreases for a product-favored process (@ constant T and P)
- ΔH system
ΔS universe = ΔSsurroundings + ΔSsystem = + ΔSsystem
T
- ΔH system
ΔG system = -T
+ ΔSsystem = ΔH system − T∆S system
T
8. Gibbs Free Energy
Thermodynamic term to relate enthalpy and entropy:
ΔGsystem = -TΔSuniverse
Since ΔSuniverse increases for a product-favored process, ΔGsystem
decreases for a product-favored process (@ constant T and P)
- ΔH system
ΔS universe = ΔSsurroundings + ΔSsystem = + ΔSsystem
T
- ΔH system
ΔG system = -T
+ ΔSsystem = ΔH system − T∆S system
T
ΔG system = ΔH system − T∆Ssystem
** T in Kelvin
9. Gibbs Free Energy
ΔG system = ΔH system − T∆Ssystem
ΔG < 0 product-favored
ΔG > 0 reactant-favored
10. Gibbs Free Energy
ΔG system = ΔH system − T∆Ssystem
ΔG < 0 product-favored
ΔG > 0 reactant-favored
Enthalpy Entropy Gibbs Free Energy Product-Favored?
Negative Positive Always Negative Yes
Negative Negative - at low T; + at high T Only at low T
Positive Positive - at high T; + at low T Only at high T
Positive Negative Always Positive No
11. Calculating Gibbs Free Energy
1. Under standard conditions:
ΔG o = ΔH o − T∆S o ** T in Kelvin
can calculate ΔG at different temperatures
2. From standard Gibbs free energies of formation:
ΔG o = ∑ {(moles of product) × ΔG o (product)} − ∑ {(moles of reactant) × ΔG o (reactant)}
f f
gives ΔG at 25 oC (298 K)
12. Transitions Between Product-Favored
and Rectant-Favored Processes
Enthalpy Entropy Gibbs Free Energy
Negative Negative - at low T; + at high T
Positive Positive - at high T; + at low T
Transition at ΔGo = 0
0 = ΔH o − T∆S o
ΔH o
T=
∆S o
used to calculate the temperature at which the transition occurs
