1. Chapter 19 Chemical Thermodynamics Lecture Presentation John D. Bookstaver St. Charles Community College Cottleville, MO © 2012 Pearson Education, Inc.

2. What does the term spontaneous mean? Provide an example of a spontaneous process. Can the process be reversed without adding additional energy from the surroundings? Take Five!

3. First Law of Thermodynamics You will recall from Chapter 5 that energy cannot be created or 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. © 2012 Pearson Education, Inc.

4. 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 return to vessel B. © 2012 Pearson Education, Inc.

5. Spontaneous Processes Processes that are spontaneous in one direction are nonspontaneous in the reverse direction. © 2012 Pearson Education, Inc.

6. 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. © 2012 Pearson Education, Inc.

7. Reversible Processes © 2012 Pearson Education, Inc. 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 © 2012 Pearson Education, Inc. Irreversible processes cannot be undone by exactly reversing the change to the system. Spontaneous processes are irreversible.

9. Entropy Entropy (S) is a term coined by Rudolph Clausius in the nineteenth century. Clausius was convinced of the significance of the ratio of heat delivered and the temperature at which it is delivered, qTqT © 2012 Pearson Education, Inc.

10. 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. The change in entropy of a system may predict if a reaction will be spontaneous. © 2012 Pearson Education, Inc.

11. Entropy: microstates

12. Predict which substance in each pair has the higher entropy and justify your answer. 1 mol of NH 3 (g) or 1 mol of He(g), both at 25°C 1 mol of Pb(s) at 25°C or 1 mol of Pb(l) at 800°C

13. Entropy What increases the entropy of a system? What increases the entropy of the surroundings?

14. Entropy Like total energy, E, and enthalpy, H, entropy is a state function. Therefore,  S = S final  S initial © 2012 Pearson Education, Inc.

15. Entropy When would entropy be positive for a system? When would entropy be negative for a system?

16. Entropy Driving force provided by the energy flow (heat) = Magnitude of the entropy change of the surroundings =

17. Remember quantity of heat ΔH? Rules? Exothermic Process ΔS surr = Endothermic Process ΔS surr =

18. Determining ΔS surr

19. 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 = q rev T © 2012 Pearson Education, Inc.

20. 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. © 2012 Pearson Education, Inc.

21. Second Law of Thermodynamics In other words: For reversible processes:  S univ =  S system +  S surroundings = 0 For irreversible processes:  S univ =  S system +  S surroundings > 0 © 2012 Pearson Education, Inc.

22. Second Law of Thermodynamics These last truths mean that as a result of all spontaneous processes, the entropy of the universe increases. © 2012 Pearson Education, Inc.

23. A.Yes. Nonspontaneous processes can never occur under any circumstances. B.No. Nonspontaneous processes can occur with some continuous external assistance.

24. A.Yes, because the system is restored to its original state. B.Yes, because the surroundings are also restored to their original state. C.No, because there is no evidence that the surroundings are also restored to their original state. D.No, because energy had to be used to restore the system.

25. A.The value of  S is a state function because q is constant for a specified T irrespective of the path chosen. B.  S depends not merely on q but on q rev. There is only one reversible isothermal path between two states regardless of the number of possible paths. C.  S has negligible dependence on q and thus q does not affect the state function properties of S or  S. D.  H = q p.  H is directly related to q.  H is a state function; thus,  S is also a state function.

26. A.We need to know if the change involves a closed or open system to make a conclusion. B. The entropy of the surroundings must increase by the same amount as the entropy decrease of the system. C.The entropy of the surroundings must increase by a greater amount than the entropy decrease of the system. D.The entropy of the surroundings must decrease by a smaller amount than the entropy decrease of the system.

28. Does the potential energy of the eggs change during this process? A.Yes, as the eggs moves downward. B.Yes, because the eggs undergo a change in velocity upon release. C.No, because the egg breaks upon hitting the surface. D.No, the yellow yolk is a liquid and changes locations within the egg.

30. If flask B were smaller than flask A, would the final pressure after the stopcock is opened be greater than, equal to, or less than 0.5 atm? A.Greater than 0.5 atm B.Equal to 0.5 atm C.Less than 0.5 atm

32. In which direction is this process exothermic? A.Freezing of liquid water to ice B.Melting of ice to liquid water

34. If the flow of heat into or out of the system is to be reversible, what must be true of δT? A.δT must be very large. B.δT must be close to zero. C.δT must be zero. D.δT must be infinitesimally small.