1. Thermodynamics: Entropy, Free Energy, & Equilibrium
McMurry & Fay ch. 16

2. A quick introduction to Entropy
A measure of disorder possibilities

4. Factors affecting entropy
Mixture vs. Pure Substance
Number of Particles
Volume
Temperature
Physical State

5. Will entropy increase or decrease in each reaction?
H2C=CH2 (g) + Br2 (g)  BrCH2CH2Br (l)
2 C2H6 (g) + 7 O2 (g) 4CO2 (g) + 6 H2O (g)
BaCl2 (s)  Ba2+ (aq) + 2 Cl- (aq)

6. Third law of thermodynamics
The entropy of a perfectly ordered pure crystalline system is zero

7. Entropy increases with temperature
As T increases, movement increases
Crystals become more disordered
Solid crystals can melt or sublime
S = 0 only when T = 0 K!

8. Side effect of 3rd law: Can calculate total amount of entropy a substance has at temperatures above 0 K.

9. Standard entropy, S0: T = 298 K, P = 1 atm

10. Entropy is a State Function

11. DS0 of a reaction can be calculated using standard entropy tables:
DS0rxn = S S0products - S S0reactants

12. Calculate DS0rxn for SO3 (g) + H2O (l) H2SO4 (aq)

13. Calculate DS0rxn for 4 Al (s) + 3 O2 (g)  2Al2O3 (s)

14. 2nd law of thermodynamics
Entropy is increasing in the universe

15. Entropy (S)
DSuniverse > 0 How can DSsystem < 0?

16. Vocabulary: SPONTANEITY (Spontaneous reaction vs
Vocabulary: SPONTANEITY (Spontaneous reaction vs. Non-spontaneous reaction)
A measure of whether a reaction will continue to occur without outside input

17. Spontaneous reaction

18. Non-spontaneous reaction

19. Spontaneous or non-spontaneous?
Ice cream melting at room temperature
Water evaporating at room temperature
Photosynthesis
Propane burning
Egg cooking

20. Will a reaction occur?
Enthalpy: is there enough energy to make the reaction happen?
Entropy: will this increase the entropy of the universe?

21. Gibbs Free Energy: A measure of spontaneity
Combines enthalpic & entropic considerations at constant temperature and pressure:
DG = DH –T DS
Change in Entropy
Change in
Gibbs Free Energy
Change in Enthalpy
Temperature (K)

22. What DG represents
Free energy = maximum work that can be done by a system

23. (some energy is lost as heat)
What DG represents
Free energy
= maximum work that can be done by a system
DE = q + w
(some energy is lost as heat)

24. What DG values tell us
DG < 0 reaction is spontaneous DG > 0 reaction is non- spontaneous DG = 0 system is at equilibrium
(reverse reaction is spontaneous)

25. When is a reaction spontaneous?
Always positive!
DG = DH –T DS
DH < 0, DS >0
T & DS are both positive
-TDS must be negative
Negative
Reaction is always spontaneous

26. When is a reaction spontaneous?
Always positive!
DG = DH –T DS
DH < 0, DS < 0
T is positive but
DS is negative
-TDS must be positive
Negative
Reaction is only spontaneous when
T is small

27. When is a reaction spontaneous?
Always positive!
DG = DH –T DS
DH > 0, DS < 0
T is positive
but DS is negative
-TDS must be positive
Positive
Reaction is only spontaneous
when T is large

28. When is a reaction spontaneous?
Always positive!
DG = DH –T DS
DH > 0, DS < 0
T is positive
but DS is negative
-TDS must be positive
Positive
Reaction is never spontaneous

29. DH DS
Reaction is… - +
Always spontaneous
Spontaneous at low T
Spontaneous at high T
Never spontaneous

30. Two ways to calculate DG0 from thermodynamic tables
Find DH0 & DS0 of reaction, use DG0 = DH0 – TDS0
Use values for DG0

31. Calculating DG0 using DH0 & DS0
DG0 = DH0 – (298.15K) DS0
Use standard
heats of formation
(DHf0)
Use standard
entropy from
table (DS0) T

32. Is the reaction N2 (g) + 2 H2 (g)  N2H4 (g) spontaneous under standard conditions? DHf0 (N2H4) = 95.4 kJ/mol S0 (N2H4) = 23.4 J/mol K S0 (N2 gas) = J/mol K S0 (H2 gas) = J/mol K

33. DGf0rxn = S Gf0products – S Gf0reactants

34. Calculate DG0rxn for CaCO3 (s) + 2 HCl(aq)  CaCl2 (aq) + H2O (l) + CO2 (g)

35. Calculate DG0rxn for NaHCO3 (s) + HCl (aq)  NaCl (aq) + H2O (l) + CO2 (g)

36. DG at temperatures other than 25oC
DGT = DH0 – TDS0

37. What is DG for the reaction CH4 + 3O2  CO2 + 2 H2O at 25 oC? at 250 oC?
DGT ≈ DH0 – TDS0
DH0rxn = kJ/mol
DS0rxn = J/mol K

38. DG values can be used to find phase change temperatures
At a phase change boundary, two phases are in equilibrium

39. DG values can be used to find phase change temperatures
At a phase change boundary, two phases are in equilibrium:
DG0 = DH0 –T DS0 = 0

40. DG values can be used to find phase change temperatures
DH0 –T DS0 = 0 DH0 = T DS0

41. DG values can be used to find phase change temperatures
𝑇= ∆𝐻 0 ∆𝑆 0

42. DH0: Fe (g) 416.3 kJ/mol S0 : Fe (s) 27.3 J/mol K Fe (g) 180.5 J/mol K
At what temperature will solid iron vaporize?
𝑇= ∆𝐻 0 ∆𝑆 0
DH0: Fe (g) kJ/mol S0 : Fe (s) J/mol K Fe (g) J/mol K

43. Free Energy and Equilibrium
Temperature (Kelvin)
DG = DG0 + RT ln Q
Free energy
change
Standard
free energy
change
(from table)
Reaction
quotient
Gas constant

44. Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.

46. At equilibrium
DG = DG0 + RT ln Q
DG0 = - RT ln K