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Chapter 16

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  1. Chapter 16 Spontaneity, entropy and free energy

  2. Spontaneous • A reaction that will occur without outside intervention. • We can’t determine how fast. • We need both thermodynamics and kinetics to describe a reaction completely. • Thermodynamics compares initial and final states. • Kinetics describes pathway between.

  3. Thermodynamics • 1st Law- the energy of the universe is constant. • Keeps track of thermodynamics doesn’t correctly predict spontaneity. • Entropy (S) is disorder or randomness • 2nd Law the entropy of the universe increases.

  4. Entropy • Defined in terms of probability. • Substances take the arrangement that is most likely. • The most likely is the most random. • Calculate the number of arrangements for a system.

  5. 2 possible arrangements • 50 % chance of finding the left empty

  6. 4 possible arrangements • 25% chance of finding the left empty • 50 % chance of them being evenly dispersed

  7. 4 possible arrangements • 8% chance of finding the left empty • 50 % chance of them being evenly dispersed

  8. Gases • Gases completely fill their chamber because there are many more ways to do that than to leave half empty. • Ssolid <Sliquid <<Sgas • there are many more ways for the molecules to be arranged as a liquid than a solid. • Gases have a huge number of positions possible.

  9. Entropy • Solutions form because there are many more possible arrangements of dissolved pieces than if they stay separate. • 2nd Law • DSuniv = DSsys + DSsurr • If DSuniv is positive the process is spontaneous. • If DSuniv is negative the process is spontaneous in the opposite direction.

  10. For exothermic processes DSsurr is positive. • For endothermic processes DSsurr is negative. • Consider this process H2O(l)® H2O(g) • DSsys is positive • DSsurr is negative • DSuniv depends on temperature.

  11. Temperature and Spontaneity • Entropy changes in the surroundings are determined by the heat flow. • An exothermic process is favored because by giving up heat the entropy of the surroundings increases. • The size of DSsurr depends on temperature • DSsurr = -DH/T

  12. - + ? DSsurr DSuniv Spontaneous? DSsys - - - No, Reverse + + + Yes + - ? At High temp. At Low temp.

  13. Gibb's Free Energy • G=H-TS • Never used this way. • DG=DH-TDS at constant temperature • Divide by -T • -DG/T = -DH/T-DS • -DG/T = DSsurr + DS • -DG/T = DSuniv • If DG is negative at constant T and P, the Process is spontaneous.

  14. Let’s Check • For the reaction H2O(s) ® H2O(l) • DSº = 22.1 J/K mol DHº =6030 J/mol • Calculate DG at 10ºC and -10ºC • Look at the equation DG=DH-TDS • Spontaneity can be predicted from the sign of DH and DS.

  15. Spontaneous? DS DH DG=DH-TDS + - At all Temperatures At high temperatures, “entropy driven” + + At low temperatures, “enthalpy driven” - - Not at any temperature, Reverse is spontaneous - +

  16. Third Law of Thermo • The entropy of a pure crystal at 0 K is 0. • Gives us a starting point. • All others must be>0. • Standard Entropies Sº ( at 298 K and 1 atm) of substances are listed. • Products - reactants to find DSº (a state function). • More complex molecules higher Sº.

  17. Free Energy in Reactions • DGº = standard free energy change. • Free energy change that will occur if reactants in their standard state turn to products in their standard state. • Can’t be measured directly, can be calculated from other measurements. • DGº=DHº-TDSº • Use Hess’s Law with known reactions.

  18. Free Energy in Reactions • There are tables of DGºf . • Products-reactants because it is a state function. • The standard free energy of formation for any element in its standard state is 0. • Remember- Spontaneity tells us nothing about rate.

  19. Spontaneous? DS DH DG=DH-TDS + - At all Temperatures At high temperatures, “entropy driven” + + At low temperatures, “enthalpy driven” - - Not at any temperature, Reverse is spontaneous - +

  20. Third Law of Thermo • The entropy of a pure crystal at 0 K is 0. • Gives us a starting point. • All others must be>0. • Standard Entropies Sº ( at 298 K and 1 atm) of substances are listed. • Products - reactants to find DSº (a state function) • More complex molecules higher Sº.

  21. Free Energy in Reactions • DGº = standard free energy change. • Free energy change that will occur if reactants in their standard state turn to products in their standard state. • Can’t be measured directly, can be calculated from other measurements. • DGº=DHº-TDSº • Use Hess’s Law with known reactions.

  22. Free Energy in Reactions • There are tables of DGºf • Products-reactants because it is a state function. • The standard free energy of formation for any element in its standard state is 0. • Remember- Spontaneity tells us nothing about rate.

  23. Free energy and Pressure • DG = DGº +RTln(Q) where Q is the reaction quotients (P of the products /P of the reactants). • CO(g) + 2H2(g) ® CH3OH(l) • Would the reaction be spontaneous at 25ºC with the H2 pressure of 5.0 atm and the CO pressure of 3.0 atm? • DGºf CH3OH(l) = -166 kJ • DGºf CO(g) = -137 kJ DGºf H2(g) = 0 kJ

  24. How far? • DG tells us spontaneity at current conditions. When will it stop? • It will go to the lowest possible free energy which may be an equilibrium. • At equilibrium DG = 0, Q = K • DGº = -RTlnK

  25. DGº K =0 =1 <0 >0 >0 <0

  26. Temperature dependence of K • DGº= -RTlnK = DHº - TDSº • ln(K) = DHº/R(1/T)+ DSº/R • A straight line of lnK vs 1/T

  27. Free energy And Work • Free energy is that energy free to do work. • The maximum amount of work possible at a given temperature and pressure. • Never really achieved because some of the free energy is changed to heat during a change, so it can’t be used to do work.