Tang 01b enthalpy, entropy, and gibb's free energy

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Tang 01b enthalpy, entropy, and gibb's free energy

  1. 1. ENTHALPY, ENTROPY, & GIBB’S FREE ENERGY
  2. 2. Chemical energy is stored in: – moving electrons – vibration of chemical bonds – rotation and translation of molecules – stored nuclear energy of protons & neutrons – energy stored in chemical bonds ENTHALPY
  3. 3. Enthalpy enthalpy (H) – total kinetic and potential energy of a system at a constant pressure change in enthalpy ( H) – change in heat of a system H = Hfinal - Hinitial H = Hproducts – Hreactants The units for enthalpy are in J or kJ per mol (i.e. kJ/mol) ENTHALPY
  4. 4. Enthalpy For an exothermic reaction: Hproducts < Hreactants H is negative Describe what has happened. H reaction progress reactants products ENTHALPY
  5. 5. Enthalpy For an endothermic reaction: Hproducts > Hreactants H is positive Describe what has happened. H reaction progress reactants products ENTHALPY
  6. 6. ENTHALPY N2O4 (g)  2 NO2 (g) H° = +57.93 kJ 2 NO2 (g)  N2O4 (g) H° = -57.93 kJ Reversing a chemical reaction causes a sign change in front of the H° value. Enthalpy
  7. 7. ENTROPY
  8. 8. entropy ( S)- a measure of disorder or randomness entropy =  disorder The units for entropy are J/mol·K or J/K ENTROPY
  9. 9. ENTROPY SECOND LAW OF THERMODYNAMICS the entropy of the universe is constantly increasing
  10. 10. S solid < S liquid < S gas ENTROPY AND GIBBS FREE ENERGY
  11. 11. ΔS = Sproducts – Sreactants When Sproducts>Sreactants, ΔS>0 ENTROPY AND GIBBS FREE ENERGY
  12. 12. ΔS = Sproducts – Sreactants When Sproducts<Sreactants, ΔS<0 ENTROPY AND GIBBS FREE ENERGY
  13. 13. ENTROPY Predict the sign of S for the following reactions: H2(g)  2 H(g) S 2 H2(g) + O2(g)  2 H2O(l) - S 2 NO2(g)  N2O4(g) - S C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(g) + S condensation of steam to liquid - S
  14. 14. ENTROPY Predict the sign of S for the following reactions: sublimation of dry ice + S 2 NH3(g)  N2(g) + 3 H2(g) + S C6H12O6(s) + 6O2(g)  6CO2(g) + 6H2O(g) + S 2 H2O(l)  2 H2(g) + O2(g) + S NaCl(s)  NaCl(aq) + S
  15. 15. ENTROPY Example 1: Calculate the standard entropy of the following reaction: C2H4(g) + H2(g)  C2H6(g) Do you think this reaction will spontaneously occur at room temperature? Since molecules tend to become more and more disordered, and this reaction results in more organization (fewer moles of gas), then this reaction is most likely not spontaneous at room temperature
  16. 16. GIBBS FREE ENERGY How are entropy and enthalpy related? ΔGº = ΔHº - TΔSº Gibbs free energy (J or kJ) enthalpy Temperature (K) entropy Gibbs free energy is the energy that is available to do useful work. A reaction will spontaneously occur if ΔG<0 (exergonic reaction) A reaction will NOT spontaneously occur if ΔG>0 (endergonic reaction)
  17. 17. GIBBS FREE ENERGY A reaction will spontaneously occur if ΔG<0 (exergonic reaction) A reaction will NOT spontaneously occur if ΔG>0 (endergonic reaction)
  18. 18. GIBBS FREE ENERGY Reactions with a negative ΔH and positive ΔS all have a negative ΔG ΔGº = ΔHº - TΔSº (-) - T(+) Exergonic & spontaneous at all temperatures Reactions with a positive ΔH and negative ΔS all have a positive ΔG ΔGº = ΔHº - TΔSº (+) - T(-) Endergonic & nonspontaneous at all temperatures and will only occur with the continuous input of energy
  19. 19. GIBBS FREE ENERGY Reactions with a negative ΔH and negative ΔS all have a negative ΔG ΔGº = ΔHº - TΔSº (-) - T(-) ΔG will be negative in temperature conditions where the value of TΔS is lower than the value of ΔH. Thus the reaction is spontaneous only at lower temperatures. Example: 2SO2(g) + O2(g)  2SO3(g) This reaction is spontaneous at temperatures below 786ºC (1059K) and nonspontaneous above this temperature
  20. 20. GIBBS FREE ENERGY Reactions with a positive ΔH and positive ΔS all have a negative ΔG ΔGº = ΔHº - TΔSº (+) - T(+) ΔG will be negative in temperature conditions where the value of TΔS is higher than the value of ΔH. Thus the reaction is spontaneous only at higher temperatures. Example: H2O(s)  H2O(l) This reaction is spontaneous at temperatures above 0ºC (273K) and nonspontaneous below this temperature
  21. 21. GIBBS FREE ENERGY ΔH>0 ΔH<0 ΔS>0 Spontaneity depends on T (spontaneous at higher temperatures) Spontaneous at all temperatures ΔS<0 Nonspontaneous (proceeds only with a continuous input of energy) Spontaneity depends on T (spontaneous at lower temperatures) Summary: Spontaneous and Non-Spontaneous Reactions
  22. 22. GIBBS FREE ENERGY Example 2: Is the following reaction spontaneous at SATP? CO(NH2)2(aq) + H2O(l)  CO2(g) + 2NH3(g) ΔHº =119kJ ΔSº = 354.8J/K = 0.3548KJ/K T = 25ºC = 298K ΔGº = ΔHº - TΔSº = (119kJ) – (298K)(0.3548kJ/K) = (119kJ) – (105.7kJ) = 13kJ ΔGº>0, so this reaction is not spontaneous Therefore this reaction is not spontaneous at SATP
  23. 23. GIBBS FREE ENERGY

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