Thermodynamics2009

1. Thermodynamics2009

2. Part IIChemical ThermodynamicsChemical thermodynamics is concerned with energy relationships and with equilibrium positions in chemical reactions.

3. Key Concepts of Lecture • Zeroth and First Laws of thermodynamics • • Identifying Spontaneous Processes. • • Identifying reversible and irreversible processes. • • Entropy and its relation to randomness. • • Second Law of Thermodynamics. • • Predicting Entropy Changes of a Process. • • Third Law of Thermodynamics. • • Relate temperature change to entropy change. • • Calculating change in standard entropy.

4. • Free energy in terms of enthalpy and entropy. • Relating free energy change to spontaneity. • Calculating standard free energy change. • Relationship between free energy change and equilibrium constant

5. The Zeroth Law of Thermodynamics is a generalization about the thermal equilibrium among bodies, or thermodynamic systems, in contact. It results from the definition and properties of temperature. It can be stated as:"If A and C are each in thermal equilibrium with B, A is also in thermal equilibrium with C."

6. First Law of Thermodynamics • You will recall from Part I 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.

7. The first law of thermodynamics: the internal energy of a system can be changed by doing work on it or by heating/cooling it. U = Q + W This statement is equivalent to the statement of conservation of energy. For a cyclic process (Ui = Uf) Q= - W. If, in addition, Q = 0 thenW = 0 P V T An equivalent formulation: Perpetual motion machines of the first type do not exist. Perpetual motion machines come in two types: type 1 violates the 1st Law (energy would be created from nothing), type 2 violates the 2nd Law (the energy is extracted from a reservoir in a way that causes the net entropy of the machine+reservoir to decrease).

8. W. Escher

9. The second law of thermodynamics: Spontaneous processes are those that can proceed without any outside intervention The spontanaeity of a reaction is defined by a state function called entropy The entropy of the universe does not change for reversible processes andincreases for spontaneous processes.

10. 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

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

12. 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.

13. Spontaneous Processes

14. 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. Changes are infinitesimally small in a reversible process.

15. Irreversible Processes • Irreversible processes cannot be undone by exactly reversing the change to the system. • All Spontaneous processes are irreversible. • All Real processes are irreversible.

16. Irreversibility due tofriction • Work is done to raise the block and overcome friction. • Block is getting hotter due to friction. • In the reverse process, the block is getting even hotter due to friction. • Heat should be rejected to the surrounding to bring it back to its initial position. • Hence, irreversible process.

17. Reversible Processes • A reversible process is defined as a process that can be reversed without leaving any trace on either system or surroundings. • This is possible if the net of heat and net work exchange between the system and the surrounding is zero for the combined process (original and reverse). Quasi-equilibrium expansion or compression of a gas

18. Reversible processes actually do not occur in nature. • They are simply idealization of actual processes. • Reversible processes can never be achieved. • You may be wondering, then, why we are bothering with such fictitious processes: • Easy to analyze • Serve as idealized model

19. Engineers are interested in reversible processes because: when Reversible processes are approximated instead of the Actual ones • Work-producing devices such as car engine and gas or steam turbine deliver the most work, and • Work-consuming devices such as compressors, fan, and pumps consume the least work.

20. Reversible processes can be viewed as theoretical limits for the corresponding not reversible ones. • We may never be able to have a reversible process, but we may certainly approach it. • The more closely we approximate a reversible process, the more work delivered by a work-producing device or the less work required by a work-consuming device. • Processes that are not reversible are called Irreversible processes.

21. Entropy • For a reversible process occurring at constant temperature (an isothermal process): For an irreversible process ΔS≥ qrev ⁄ T qrev = the heat that is transferred when the process is carried out reversibly at a constant temperature. T = temperature in Kelvin.

22. 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.

23. Entropy on the Molecular Scale • Ludwig Boltzmann described the concept of entropy on the molecular level. • Temperature is a measure of the average kinetic energy of the molecules in a sample.

24. 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.

25. Entropy on the Molecular Scale • Boltzmann envisioned the motions of a sample of molecules at a particular instant in time. • This would be akin to taking a snapshot of all the molecules. • He referred to this sampling as a microstate of the thermodynamic system.

26. Entropy on the Molecular Scale • Each thermodynamic state has a specific number of microstates, W, associated with it. • Entropy is S = k lnW where k is the Boltzmann constant, 1.38  1023 J/K.

27. Entropy • Like total energy, E, and enthalpy, H, entropy is a state function. • Therefore, S = SfinalSinitial

28. Second Law of Thermodynamics The entropy of the universe does not change for reversible processes and increases for spontaneous processes. Reversible (ideal): Irreversible (real, spontaneous):

29. Second Law of Thermodynamics The entropy of the universe increases (real, spontaneous processes). But, entropy can decrease for individual systems. Reversible (ideal): Irreversible (real, spontaneous):

30. Entropy on the Molecular Scale • The number of microstates and, therefore, the entropy tends to increase with increases in • Temperature. • Volume (gases). • The number of independently moving molecules.

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

32. Solutions Dissolution of a solid: Ions have more entropy (more states) But, Some water molecules have less entropy (they are grouped around ions). Usually, there is an overall increase in S. (The exception is very highly charged ions that make a lot of water molecules align around them.)

33. 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.

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

35. Third Law of Thermodynamics The entropy of a pure crystalline substance at absolute zero is 0. http://www.garageband.com/artist/entropy_1

36. Perfect structure at 0 K

37. Structure at 298 K

38. Structure at 310 K

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

40. Standard Entropies Larger and more complex molecules have greater entropies.

41. Entropy Changes Entropy changes for a reaction can be calculated the same way we used for H: S° for each component is found in a table. Note for pure elements:

42. Practical uses: surroundings & system Entropy Changes in Surroundings • Heat that flows into or out of the system also changes the entropy of the surroundings. • For an isothermal process:

43. Practical uses: surroundings & system Entropy Changes in Surroundings • Heat that flows into or out of the system also changes the entropy of the surroundings. • For an isothermal process: • At constant pressure, qsys is simply H for the system.

44. For water: Hfusion = 6 kJ/mol Hvap = 41 kJ/mol If we do this reversibly: Ssurr = –Ssys Change of S during phase changes A phase change is isothermal (no change in T). Entropysystem

45. Entropy of surroundings & system. The introduction of Gibbs free energy. Entropy Change in the Universe • The universe is composed of the system and the surroundings. Therefore, Suniverse = Ssystem + Ssurroundings • For spontaneous processes Suniverse > 0

46. = – Gibbs Free Energy

47. = – Gibbs Free Energy Make this equation nicer:

48. Gibbs free energy function TDSuniverse is defined as the Gibbs free energy, G. For spontaneous processes: Suniverse > 0 And therefore: G < 0 G is easier to determine than Suniverse. So: Use G to decide if a process is spontaneous.

49. 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.

50. Standard Free Energy Changes Standard free energies of formation, Gf are analogous to standard enthalpies of formation, Hf. G can be looked up in tables, or calculated from S° and H.