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2012 15 3 and 15 4


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2012 15 3 and 15 4

  1. 1. IB Chemistry Power Points Topic 15 LESSON Entropy Spontaneity
  2. 2. ENTROPYEntropy, S is a measure of the DISORDER orrandomness of a system.A single coin can have 2 possible configurations.A system of 4 coins can have 16 (more disorder)
  3. 3. The greater the number of configurations (or microstates) ina particular system, the greater the entropy (disorder) of thesystem
  4. 4. Entropy on the Molecular ScaleImplications to a chemical system (MUST KNOW):• more particles -> more states -> more entropy• higher T -> more energy states -> more entropy• less structure (gas vs solid) -> more states -> more entropy
  5. 5. Entropy changes (dissolving) ΔS = Sfinal – Sinitial
  6. 6. Entropy changes (heating) ΔS = Sfinal – Sinitial
  7. 7. Entropy changes (change of state) ΔS = Sfinal – Sinitial
  8. 8. ѳStandard entropy values: S The standard entropy value for a substance is defined as the entropy increase of the substance when heated from 0 K to 298 K (standard conditions) standard entropy is an absolute value The standard entropy change for a reaction can be determined by: ѳ ѳ ѳ ΔS = ΣS products – ΣS reactants
  9. 9. Consider the following reactionDescribe the change in entropy in this reaction.Calculate the change in entropy in this reaction.
  10. 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, returning to the original state is not spontaneous
  11. 11. Spontaneous Processes Processes that are spontaneous in one direction are non spontaneous in the reverse direction.
  12. 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. 13. Spontaneous reactions produce substantial18.amounts of products at equilibrium and release free4energy. Free energy is energy that is available to do work The Gibbs free energy change, G is the maximum amount of free energy that can be extracted to do work.
  14. 14. Gibbs Free Energy Change (some things to know)1. If the G (for a particular reaction under a specific set of conditions) is negative, the forward reaction is spontaneous. (or vice versa)2. The G for a reaction can be determined from standard free energies of formation, Gf values available on Data Tables. This is analogous to using standard enthalpies of formation, Hf values.
  15. 15. The key equation to predict spontaneity: G H T SThis equation shows how G changes withtemperature. (We assume S & H values are not affected by T.)
  16. 16. 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 spontaneity comes from the entropy term.
  17. 17. By knowing the sign (+ or -) of S and H, we canget the sign of G and determine if a reaction isspontaneous.
  18. 18. endothermic so ΔH must be positive
  19. 19. entropy increasing so ΔS must be positive
  20. 20. G H T S 3176 10 298 284 1 91368 Jmol 1 91.4 kJmol not spontaneous
  21. 21. G H T S 3 176 10 298 284 1 91368 Jmol 1 91.4 kJmolincrease in temperature increases spontaneity
  22. 22. G H T S 30 176 10 T 284T 620 K o 346 C