5.111 Lecture Summary #16 Wednesday, October 15, 2014
Readings for today: Section 8.1 (Section 7.1 in 4th ed) – Spontaneous Change, Sections 8.2
and 8.8 (Sections 7.2 and 7.8 in 4th ed) - Entropy, Sections 8.12, 8.13, 8.15 (Sections
7.12, 7.13, 7.15 in 4th ed) – Free Energy.
Reading for Lecture #17: Section 8.16 (Section 7.16 in 4th ed) – Free-Energy Changes in
Biological Systems.
Topics: Thermodynamics
I. Spontaneous change and free energy
II. Entropy
III. Free energy of formation
I. SPONTANEOUS CHANGE AND FREE ENERGY
A spontaneous change is a process that, given enough time, occurs without the need for
outside intervention.
For example, the following reactions are spontaneous at constant pressure:
4Fe(s) + 3O2(g) → 2Fe2O3(s) ∆ Hº = kJ/mol
hydrolysis of ATP: NH2
NH2
C
N
C C N
N
C N O O
HC
O O O
HC C CH
+ 2H2O(l) +HPO4–2(aq) + H3O+(aq)
C CH O P O P O CH2 N
O P O P O P O CH2 N N
N O
O
O O HC CH
O O O HC CH
HC CH
HC CH
OH OH
OH OH
(aq) (aq) ∆ Hº = kJ/mol
But so are these …
H2O(s) → H2O(l) ∆H° = +6.95 kJ/mol
NH4NO3(s) → NH4+(aq) + NO3-(aq) ∆H° = +28 kJ/mol
Is ∆ H the key to spontaneity? !
Condition for spontaneity under constant P + T involves GIBBS FREE ENERGY, ∆G.
∆G = ∆H – T∆S
where T = temperature and ∆S = change in entropy, a measure of disorder.
∆G < 0 process
∆G > 0 process
∆G = 0 equilibrium
Under constant pressure and temperature, a process is spontaneous when ∆G < 0, not
necessarily when ∆H < 0.
Figuring this out was one of the towering achievements of thermodynamics!!
1
Readings for today: Section 8.1 (Section 7.1 in 4th ed) – Spontaneous Change, Sections 8.2
and 8.8 (Sections 7.2 and 7.8 in 4th ed) - Entropy, Sections 8.12, 8.13, 8.15 (Sections
7.12, 7.13, 7.15 in 4th ed) – Free Energy.
Reading for Lecture #17: Section 8.16 (Section 7.16 in 4th ed) – Free-Energy Changes in
Biological Systems.
Topics: Thermodynamics
I. Spontaneous change and free energy
II. Entropy
III. Free energy of formation
I. SPONTANEOUS CHANGE AND FREE ENERGY
A spontaneous change is a process that, given enough time, occurs without the need for
outside intervention.
For example, the following reactions are spontaneous at constant pressure:
4Fe(s) + 3O2(g) → 2Fe2O3(s) ∆ Hº = kJ/mol
hydrolysis of ATP: NH2
NH2
C
N
C C N
N
C N O O
HC
O O O
HC C CH
+ 2H2O(l) +HPO4–2(aq) + H3O+(aq)
C CH O P O P O CH2 N
O P O P O P O CH2 N N
N O
O
O O HC CH
O O O HC CH
HC CH
HC CH
OH OH
OH OH
(aq) (aq) ∆ Hº = kJ/mol
But so are these …
H2O(s) → H2O(l) ∆H° = +6.95 kJ/mol
NH4NO3(s) → NH4+(aq) + NO3-(aq) ∆H° = +28 kJ/mol
Is ∆ H the key to spontaneity? !
Condition for spontaneity under constant P + T involves GIBBS FREE ENERGY, ∆G.
∆G = ∆H – T∆S
where T = temperature and ∆S = change in entropy, a measure of disorder.
∆G < 0 process
∆G > 0 process
∆G = 0 equilibrium
Under constant pressure and temperature, a process is spontaneous when ∆G < 0, not
necessarily when ∆H < 0.
Figuring this out was one of the towering achievements of thermodynamics!!
1
