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- 1. IB Chemistry Power Points Topic 15 Energeticswww.pedagogics.ca LESSON Entropy Spontaneity
- 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. The greater the number of configurations (or microstates) ina particular system, the greater the entropy (disorder) of thesystem
- 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. Entropy changes (dissolving) ΔS = Sfinal – Sinitial
- 6. Entropy changes (heating) ΔS = Sfinal – Sinitial
- 7. Entropy changes (change of state) ΔS = Sfinal – Sinitial
- 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. Consider the following reactionDescribe the change in entropy in this reaction.Calculate the change in entropy in this reaction.
- 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. Spontaneous Processes Processes that are spontaneous in one direction are non spontaneous 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 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. 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. 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. 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. By knowing the sign (+ or -) of S and H, we canget the sign of G and determine if a reaction isspontaneous.
- 18. endothermic so ΔH must be positive
- 19. entropy increasing so ΔS must be positive
- 20. G H T S 3176 10 298 284 1 91368 Jmol 1 91.4 kJmol not spontaneous
- 21. G H T S 3 176 10 298 284 1 91368 Jmol 1 91.4 kJmolincrease in temperature increases spontaneity
- 22. G H T S 30 176 10 T 284T 620 K o 346 C

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