1 / 47

Spontaneity

Spontaneity. I am a sleepless Slowfaring eater Maker of rust and rot In your bastioned fastenings, I am the crumbler: tomorrow Carl Sandburg. Spontaneity, Entropy & Free Energy. First Law of Thermodynamics Basically the law of conservation of energy

stacy-crane
Télécharger la présentation

Spontaneity

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Spontaneity I am a sleepless Slowfaring eater Maker of rust and rot In your bastioned fastenings, I am the crumbler: tomorrow Carl Sandburg

  2. Spontaneity, Entropy & Free Energy • First Law of Thermodynamics • Basically the law of conservation of energy • energy can be neither created nor destroyed • i.e., the energy of the universe is constant • the total energy is constant • energy can be interchanged • e.g. potential energy (stored in chemical bonds) can be converted to thermal energy in a chemical reaction • CH4 + O2 --> CO2 + H2O + energy • Doesn’t tell us why a reaction proceeds in a particular direction

  3. Spontaneity, Entropy & Free Energy • Spontaneous Processes and Entropy • Spontaneous processes occurs without outside intervention • Spontaneous processes can be fast or slow

  4. Spontaneity, Entropy & Free Energy • Thermodynamics • lets us predict whether a process will occur • tells us the direction a reaction will go • only considers the initial and final states • does not require knowledge of the pathway taken for a reaction

  5. Spontaneity, Entropy & Free Energy • Kinetics • depends on the pathway taken • tells us the speed of the process • depends on • activation energy • temperature • concentration • catalysts

  6. Spontaneity, Entropy & Free Energy • Spontaneous Processes • a ball rolls downhill, but the ball never spontaneously rolls uphill • steel rusts, but the rust never spontaneously forms iron and oxygen • a gas fills its container, but a gas will never spontaneously collect in one corner of the container. • Water spontaneously freezes at temperatures below 0o C

  7. Spontaneity, Entropy & Free Energy • What thermodynamic principle explains why these processes occur in one direction? • The driving force for a spontaneous reaction is an increase in the entropy of the universe

  8. Spontaneity, Entropy & Free Energy • Entropy • Symbol: S • A measure of randomness or disorder • The natural progression is from order to disorder • It is natural for disorder to increase • Entropy is a thermodynamic function • Describes the number of arrangements that are available to a system in a given state

  9. Spontaneity, Entropy & Free Energy • Entropy • The greater the number of possible arrangements, the greater the entropy of a system, i.e., there is a large positional probability. • The positional probability or the entropy increases as a solid changes from a liquid or as a liquid changes to a gas

  10. Spontaneity, Entropy & Free Energy • Ssolid < Sliquid < Sgas • Choose the substance with the higher positional entropy: • CO2(s) or CO2(g)? • N2(g) at 1 atm and 25oC or N2(g) at .010 atm and 25oC?

  11. Spontaneity, Entropy & Free Energy • Predict the sign of the entropy change • solid sugar is added to water • iodine vapor condenses onto a cold surface forming crystals

  12. Spontaneity, Entropy & Free Energy • Second Law of Thermodynamics • The entropy of the universe is increasing • The universe is made up of the system and the surroundings • DSuniverse = DSsystem + DSsurroundings

  13. Spontaneity, Entropy & Free Energy • A process is spontaneous if the DSuniverse is positive • If the DSuniverse is zero, there is no tendency for the reaction to occur

  14. Spontaneity, Entropy & Free Energy • The effect of temperature on spontaneity • H2O(l) --> H2O(g) • water is the system, everything else is the surroundings • DSsystem increases, i.e. DSsystem is positive, because there are more positions for the water molecules in the gas state than in the liquid state

  15. Spontaneity, Entropy & Free Energy • What happens to the surrounding? • Heat leaves the surroundings, entering the system to cause the liquid molecules to vaporize • When heat leaves the surroundings, the motion of the molecules of the surroundings decrease, which results in a decrease in the entropy of the surroundings • DSsurroundings is negative

  16. Spontaneity, Entropy & Free Energy • Sign of DS depends on the heat flow • Exothermic Rxn: DSsurr >0 • Endothermic Rxn: DSsurr< 0 • Magnitude of DS is determined by the temperature • DSsurr = - DH T

  17. Spontaneity, Entropy & Free Energy Signs of Entropy Changes DSsysDSsurr DSunivSpontaneous? + + - - + - - +

  18. Spontaneity, Entropy & Free Energy • Free Energy • aka Gibbs Free Energy • G • another thermodynamic function • related to spontaneity • G = H - TS • for a process that occurs at constant temperature (i.e. for the system): DG = DH - TDS

  19. Spontaneity, Entropy & Free Energy • How does the free energy related to spontaneity? DG = DH - TDS - DG = - DH + DS (remember, - DH = DSsurr ) T T T -DG = DSsurr + DSsys (remember, DSsurr + DSsys = DSuniv) T -DG = DSuniv T

  20. Spontaneity, Entropy & Free Energy • DSuniv > 0 for a spontaneous reaction • DG < 0 for a spontaneous reaction • DG > 0 for a nonspontaneous reaction • Useful to look at DG because many chemical reactions take place under constant pressure and temperature

  21. Spontaneity, Entropy & Free Energy • H2O(s) --> H2O(l) • DHo = 6.03 x 103J/mole • DSo = 22.1 J/K.mole • Calculate DG, DSsurr, and DSuniv at -10oC, 0oC, and 10oC

  22. Spontaneity, Entropy & Free Energy • For the melting of ice • DSsys and DSsurr oppose each other • spontaneity will depend on temperature • DSo is positive because of the increase in positional entropy when the ice melts • DSsurr is negative because the reaction is endothermic

  23. Spontaneity, Entropy & Free Energy • At what temperatures is Br2(l) --> Br2(g) spontaneous? • What is the normal boiling point of Br2? DHo= 31.0 kJ/mol DSo = 93.0 J/K.mol

  24. Spontaneity, Entropy & Free Energy • Entropy Changes in Chemical Reactions • Just like physical changes, entropy changes in the surroundings are determined by heat flow • Entropy changes in the system are determined by positional entropy (the change in the number of possible arrangements)

  25. Spontaneity, Entropy & Free Energy • N2 (g) + 3 H2(g) --> 2 NH3 (g) • The entropy of the this system decreases because • four reactant molecules form two product molecules • there are less independent units in the system • less positional disorder, i.e. fewer possible configurations

  26. Spontaneity, Entropy & Free Energy • When a reaction involves gaseous molecules: • the change in positional entropy is determined by the relative numbers of molecules of gaseous reactants and products • I.e., if you have more product molecules than reactant molecules, DS will be positive

  27. Spontaneity, Entropy & Free Energy • In thermodynamics, the change in a function is usually what is important • usually we can’t assign an absolute value to a function like enthalpy or free energy • we can usually determine the change in enthalpy and free energy

  28. Spontaneity, Entropy & Free Energy • We can assign absolute entropy values, i.e., we can find S • A perfect crystal at 0 K, while unattainable, represents a standard • all molecular motion stops • all particles are in their place • the entropy of a perfect crystal at 0 K is zero = third law of thermodynamics

  29. Spontaneity, Entropy & Free Energy • Increase the temperature of our perfect crystal • molecular motion increases • disorder increases • entropy varies with temperature • See thermodynamic tables for So values (at 298 K and 1 atm)

  30. Spontaneity, Entropy & Free Energy • Entropy is a state function • entropy does not depend on the pathway taken • DSrxn = SnDSoproducts- SnDSoreactant

  31. Spontaneity, Entropy & Free Energy • Calculate DSo at 25oC for • 2NiS(s) + 3 O2(g) --> 2 SO2(g) + 2 NiO(s) Substance So(J/K.mol) SO2 248 NiO 38 O2 205 NiS 53

  32. Spontaneity, Entropy & Free Energy • Calculate DSo for Al2O3(s) + 3 H2(g) --> 2 Al(s) + 3 H2O(g) Substance So (J/K.mol) Al2O3 51 H2 131 Al 28 H2O 189

  33. Spontaneity, Entropy & Free Energy • What did you expect the DSo to be? • Why is it large and positive? • H2O is nonlinear and triatomic • H2O has many rotational and vibrational motions • H2 is linear and diatomic • H2 has less rotational and vibrational motions • The more complex the molecule, the higher the DSo

  34. Spontaneity, Entropy & Free Energy • Free Energy and Chemical Reactions • Standard Free Energy Change • DGo • the change in the free energy that occurs if the reactants in their standard states are changed to products in their standard states • can’t be measured directly • calculate from other values • allows us to predict the tendency for a reaction to go

  35. Spontaneity, Entropy & Free Energy • How do we calculate DGo? • DGo = DHo - TDSo (for a reaction carried out at constant temperature) • Use Hess’ Law • Use DGof (standard free energy of formation) • DGo = SnDGof (products) - SnDGof (reactants)

  36. Spontaneity, Entropy & Free Energy • Calculate DGo for the reaction at 25oC 2SO2(g) + O2(g) --> 2 SO3(g) SubstanceDHof(kJ/mol)DSo(J/K.mol) SO2(g) -297 248 SO3 -396 257 O2 0 205

  37. Spontaneity, Entropy & Free Energy • Calculate DGo for the reaction Cdia --> Cgr using the following data: Cdia + O2 --> CO2(g) DGo = -397 kJ Cgr + O2 --> CO2(g) DGo = -394 kJ

  38. Spontaneity, Entropy & Free Energy • Calculate DGo for the reaction 2CH3OH + 3 O2--> 2 CO2 + 4 H2O SubstanceDGof(kJ/mol) CH3OH -163 O2 0 CO2 -394 H2O -229

  39. Spontaneity, Entropy & Free Energy • The dependence of free energy on pressure • How does pressure affect enthalpy and entropy? • Pressure does not affect enthalpy • Pressure does affect entropy because pressure depends on the volume • 1 mole of a gas at 10.0 L has more positions available than 1 mole of a gas at 1.0 L • Slarge volume > Ssmall volume • Slow pressure > Shigh pressure

  40. Spontaneity, Entropy & Free Energy • Given that G = DGo + RTln(P) • where G is the free energy at some P (not necessarily 1 atm) • where DGo is the free energy at 1 atm • Ex: N2(g)+ 3 H2(g) --> 2 NH3(g) (lots of equations…lots of equations…) • DG = DGo + RT ln Q • Q is the reaction quotient (from the law of mass action) • T is the temperature in K • R is the gas constant, 8.3145 J/mol.K

  41. Spontaneity, Entropy & Free Energy • Calculate DG at 25o C for the reaction CO(g) + 2 H2(g) --> CH3OH where carbon monoxide is 5.0 atm and hydrogen gas at 3.0 atm are converted to liquid methanol. • What does the answer tell us about this reaction under these conditions?

  42. Spontaneity, Entropy & Free Energy • Free Energy and Equilibrium • Equilibrium occurs at the lowest value of free energy available to the reaction system, i.e., when DG = 0 • At equilibrium, DG = 0, Q = Keq so DG = 0 = DGo + RT ln Keq DGo = - RT ln Keq • Use this equation to find Keq given DGo, or to find DGogiven Keq

  43. Spontaneity, Entropy & Free Energy • Relationship between DGo and Keq • DGo Keq • = 0 1 • < 0 >1 • > 0 < 1

  44. Spontaneity, Entropy & Free Energy • For N2 + 3 H2 --> 2 NH3, DGo = - 33.3 kJ per mole of N2 consumed at 25oC. Predict the direction in which the reaction will shift to reach equilibrium a. PNH3 = 1.00 atm, PN2 = 1.47 atm, PH2 = 1.00 x 10-2 atm b. PNH3 = 1.00 atm, PN2 = 1.00 atm, PH2 = 1.00 atm

  45. Spontaneity, Entropy & Free Energy • 4Fe + 3 O2 <====> 2Fe2O3Calculate the equilibrium constant using the following information: Substance DHof (kJ/mol) So(J/K.mol) Fe2O3 -826 90 Fe 0 27 O2 0 205

  46. Spontaneity, Entropy & Free Energy • Keq and temperature • We used Le Chatelier’s Principle to determine how Keq would change when temperature changes • Use DG to determine the new Keq at a new temperature • DGo = -RT ln K = DHo - TDSo ln K = - DHo.1 + DSo R T R

  47. Spontaneity Chart ∆S Spontaneous At high temp Nonspontaneous ∆H Spontaneous At all temps Nonspontaneous

More Related