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Entropy and Free Energy

Entropy and Free Energy. Chapter 19. First Law Energy is conserved in chemical processes neither created nor destroyed converted from one form into another Second Law For any spontaneous process, the entropy of the universe increases the real criterion for spontaneity

raymond-anderson
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Entropy and Free Energy

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  1. Entropy and Free Energy Chapter 19

  2. First Law • Energy is conserved in chemical processes • neither created nor destroyed • converted from one form into another • Second Law • For any spontaneous process, the entropy of the universe increases • the real criterion for spontaneity • changes in randomness of the universe is + Laws of Thermodynamics

  3. Free energy- the energy that is available to do work. • Entropy- a measure of the disorder of a system. • Enthalpy-at constant pressure, it is the heat evolved or absorbed in the reaction. • Spontaneous Reaction • Non-spontaneous Reaction • Law of Disorder-processes move in the direction of maximum disorder or randomness Some definitions…

  4. Entropy of a gas is greater than that of a liquid or a solid. • Entropy increases when a substance is divided into parts. • Entropy tends to increase in chemical reactions in which the total number of product molecules is greater than the total number of reactant molecules. • Entropy tends to increase when temperature increases. Entropy “Rules” (see p 729 for examples)

  5. The size and direction of heat (enthalpy) changes and entropy changes together determine whether a reaction is spontaneous. Reaction Spontaneity

  6. ΔHΔS Spontaneous? How ΔH and ΔS Affect Reaction Spontaneity

  7. Units for S: J/K • Usually given as J/K x mol because we are interested in a specific substance. • S° signifies entropy at standard conditions (101.3 kPa and 25°C). • Theoretical entropy of a perfect crystal at 0 K is zero. Some background on the standard conditions for entropy…

  8. Standard Entropy change (ΔS°) can be calculated using: • ΔS°(reaction) = ΔS°(products)- ΔS° (reactants) Entropy Calculations

  9. Calculate the standard entropy change (ΔS°) that occurs when 1 mol H2O(g) at 25°C and 101.3 kPa condenses to 1 mol H2O(l) at the same temperature. • H2O(g) S°= 188.7 J/Kxmol • H2O(l) S°= 69.94 J/Kxmol • ΔS°=69.94 – 188.7 = -118.8 J/Kxmol • The negative sign indicates that entropy decreases. Practice Problem:

  10. Josiah Gibbs formulated the Gibbs free Energy change (ΔG) equation. • It is the maximum amount of energy that can be coupled to another process to do useful work. The change in Gibbs free energy is related to the change in entropy (ΔS) and the change in enthalpy (ΔH) of the system by the following equation: ΔG = ΔH – TΔS (T=temperature in Kelvin) Free Energy Calculations

  11. If –ΔG, reaction is spontaneous in forward direction • If +ΔG, reaction is non-spontaneous in forward direction but spontaneous in reverse direction. Work must be supplied from surroundings to make it occur. • If ΔG=0, reaction is at equilibrium • All spontaneous processes release free energy. • In a spontaneous reaction ΔG is negative because the system loses free energy. ΔG

  12. C2H5OH(l) + 3O2(g) 2CO2(g) + 3H2O(g) + 1235 kJ • What is the sign of H? • What is the sign of S? • Plug signs into: ΔG = ΔH – TΔS • Prediction??? • spontaneous Qualitative Prediction of Spontaneity

  13. ΔG = ΔG° + RT ln Q(where R=8.314 J/mol K) • At equilibrium ΔG=0 and Q=K, therefore: • ΔG°=-RT ln K and K= e-ΔG°/RT (find these formulas on your AP cheat sheet) • Turn to page 740 and try practice exercise 19.12 Free Energy and Keq

  14. G negative  reaction proceeds right to equilibrium • G positive  reaction proceedsleft to equilibrium • G = 0  at equilibrium G is the energy change by a system going from initial conditions to equilibrium

  15. Infinite number of combinations of variables • conc, T, P, etc. • Reference values based on standard conditions • gases at 1 atm • solids and liquids – most stable form at 1 atm and 298 K • solutions at 1 M But…

  16. Horxn standard Tabulated Hof formation Horxn = npHof (products) - nrHof (reactants) Standard Enthalpy

  17. S of a pure crystal at 0 K = 0 Third Law of Thermodynamics Sorxn standard Tabulated So Sorxn = npSo (products) - nrSo(reactants) Standard Entropy

  18. Gorxn Tabulated Gof formation Gorxn = npGof (products) - nrGof (reactants) Standard Free Energyone method

  19. SUM SUM Gorxn A reaction A Gorxn B reaction B Gorxn C reaction C Standard Free Energyanother method Gorxn reaction of interest

  20. Gorxn Horxn - T Sorxn = Standard Free Energyyet another method

  21. Gorxn Standard states of all reactants and products 1 M ; 1 atm Equilibrium Standard Free EnergyALL THREE METHODS

  22. Grxn = Gorxn + RT lnQ Free Energy related to Standard Free Energy

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