1. Chapter 19 Chemical Thermodynamics
Chemistry, The Central Science, 10th edition
Theodore L. Brown; H. Eugene LeMay, Jr.; and Bruce E. Bursten
Chapter 19 Chemical Thermodynamics
John D. Bookstaver
St. Charles Community College
St. Peters, MO
 2006, Prentice Hall, Inc.

2. First Law of Thermodynamics
You will recall from Chapter 5 that energy cannot be created nor destroyed.
Therefore, the total energy of the universe is a constant.
Energy can, however, be converted from one form to another or transferred from a system to the surroundings or vice versa.

3. Spontaneous Processes
Spontaneous processes are those that can proceed without any outside intervention.
The gas in vessel B will spontaneously effuse into vessel A, but once the gas is in both vessels, it will not spontaneously

4. Spontaneous Processes
Processes that are spontaneous in one direction are nonspontaneous in the reverse direction.

5. Spontaneous Processes
Processes that are spontaneous at one temperature may be nonspontaneous at other temperatures.
Above 0C it is spontaneous for ice to melt.
Below 0C the reverse process is spontaneous.

6. Time to think… Draw a picture/diagram of a spontaneous process/event.
If a process is nonspontaneous, does that mean that it cannot occur under any circumstances? Justify your answer.

7. Reversible Processes
In a reversible process the system changes in such a way that the system and surroundings can be put back in their original states by exactly reversing the process.

8. Irreversible Processes
Irreversible processes cannot be undone by exactly reversing the change to the system.
Spontaneous processes are irreversible. Why?

9. Time to think…
If you evaporate water and then condense it, have you necessarily performed a reversible process? Explain.

10. Entropy
Entropy (S) is a term coined by Rudolph Clausius in the 19th century.
Clausius was convinced of the significance of the ratio of heat delivered and the temperature at which it is delivered, q T

11. Entropy
Entropy can be thought of as a measure of the randomness of a system.
It is related to the various modes of motion in molecules.

12. Entropy
Like total energy, E, and enthalpy, H, entropy is a state function.
Therefore,
S = Sfinal  Sinitial
REVIEW: What is a state function?

13. Entropy
For a process occurring at constant temperature (an isothermal process), the change in entropy is equal to the heat that would be transferred if the process were reversible divided by the temperature:
S =
qrev T

14. Example 19.2
The element mercury, Hg, is a silvery liquid at room temperature. The normal freezing point of mercury is –38.9ºC, and its molar enthalpy of fusion is fusion = 2.29 kJ/mol. What is the entropy change of the system when 50.0 g of Hg(l) freezes at the normal freezing point?

15. Your turn…
The normal boiling point of ethanol, C2H5OH, is 78.3o C, and its molar enthalpy of vaporization is kJ/mol. What is the change in entropy in the system when 68.3 g of C2H5OH(g) at 1 atm condenses to liquid at the normal boiling point?
-163J/K

16. Second Law of Thermodynamics
The second law of thermodynamics states that the entropy of the universe increases for spontaneous processes, and the entropy of the universe does not change for reversible processes.

17. Second Law of Thermodynamics
In other words:
For reversible processes:
Suniv = Ssystem + Ssurroundings = 0
For irreversible processes:
Suniv = Ssystem + Ssurroundings > 0

18. Second Law of Thermodynamics
These last truths mean that as a result of all spontaneous processes the entropy of the universe increases.

19. Time to think…
The rusting of iron is accompanied by a decrease in the entropy of the system (the iron & oxygen). What can we conclude about the entropy of the suroundings?
HINT: Suniv = Ssystem + Ssurroundings

20. Entropy on the Molecular Scale
Molecules exhibit several types of motion:
Translational: Movement of the entire molecule from one place to another.
Vibrational: Periodic motion of atoms within a molecule.
Rotational: Rotation of the molecule on about an axis or rotation about  bonds. 

21. Entropy on the Molecular Scale
The entropy tends to increase with increases in
Temperature.
Volume.
The number of independently moving molecules.

22. Entropy and Physical States
Entropy increases with the freedom of motion of molecules.
Therefore,
S(g) > S(l) > S(s)

23. Solutions
Generally, when a solid is dissolved in a solvent, entropy increases.

24. Entropy Changes In general, entropy increases when
Gases are formed from liquids and solids.
Liquids or solutions are formed from solids.
The number of gas molecules increases.
The number of moles increases.

25. Time to think…
Indicate whether each of the following processes produces an increase or decrease in the entropy of the system and briefly support your answer.

26. Time to think…
Choose the substance with the greater entropy in each case:
1 mol of H2(g) at STP or 1 mol of H2(g) at 100°C and 0.5 atm,
1 mol of H2O(s) at 0°C or 1 mol of H2O(l) at 25°C,
1 mol of H2(g) at STP or 1 mol of SO2(g) at STP,
1 mol of N2O4(g) at STP or 2 mol of NO2(g) at STP.

27. Third Law of Thermodynamics
The entropy of a pure crystalline substance at absolute zero is 0.

28. Standard Entropies
These are molar entropy values of substances in their standard states.
Standard entropies tend to increase with increasing molar mass.

29. Standard Entropies
Larger and more complex molecules have greater entropies.
Draw a diagram to illustrate this.

30. S° = nS°(products) - mS°(reactants)
Entropy Changes
Entropy changes for a reaction can be estimated in a manner analogous to that by which H is estimated:
S° = nS°(products) - mS°(reactants)
where n and m are the coefficients in the balanced chemical equation.

31. Time to think…
Calculate S° for the synthesis of ammonia from N2(g) and H2(g) at 298 K:
Ans: J/K

32. Entropy Changes in Surroundings
Heat that flows into or out of the system changes the entropy of the surroundings.
For an isothermal process:
Ssurr =
qsys T
At constant pressure, qsys is simply H for the system.

33. Entropy Change in the Universe
The universe is composed of the system and the surroundings.
Therefore,
Suniverse = Ssystem + Ssurroundings
For spontaneous processes
Suniverse > 0

34. Entropy Change in the Universe
This becomes:
Suniverse = Ssystem +
Multiplying both sides by T,
TSuniverse = Hsystem  TSsystem
Hsystem T

35. Gibbs Free Energy
TDSuniverse is defined as the Gibbs free energy, G.
When Suniverse is positive, G is negative.
Therefore, when G is negative, a process is spontaneous.

36. Gibbs Free Energy
If DG is negative, the forward reaction is spontaneous.
If DG is 0, the system is at equilibrium.
If G is positive, the reaction is spontaneous in the reverse direction.

37. Standard Free Energy Changes
Analogous to standard enthalpies of formation are standard free energies of formation, G. f
DG = SnDG(products)  SmG(reactants) f
where n and m are the stoichiometric coefficients.

38. Time to think…
By using data from Appendix C, calculate G° at 298 K for the combustion of methane:
Ans: kJ

39. Free Energy Changes At temperatures other than 25°C, DG° = DH  TS
How does G change with temperature?

40. Free Energy and Temperature
There are two parts to the free energy equation:
H— the enthalpy term
TS — the entropy term
The temperature dependence of free energy, then comes from the entropy term.

41. Free Energy and Temperature

42. Free Energy and Equilibrium
Under any conditions, standard or nonstandard, the free energy change can be found this way:
G = G + RT lnQ
(Under standard conditions, all concentrations are 1 M, so Q = 1 and lnQ = 0; the last term drops out.)

43. Free Energy and Equilibrium
At equilibrium, Q = K, and G = 0.
The equation becomes
0 = G + RT lnK
Rearranging, this becomes
G = RT lnK
or,
K = eG/RT

44. Time to think…
As we saw in Section 11.5, the normal boiling point is the temperature at which a pure liquid is in equilibrium with its vapor at a pressure of 1 atm.
Write the chemical equation that defines the normal boiling point of liquid carbon tetrachloride, CCl4(l).
What is the value of G° for the equilibrium in part (a)?
Use thermodynamic data in Appendix C and Equation to estimate the normal boiling point of CCl4.

45. Time to think…
Use data in Appendix C to estimate the normal boiling point, in K, for elemental bromine, Br2(l).
330K