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Chemical Thermodynamics: Entropy, Free Energy and Equilibrium

Chemical Thermodynamics: Entropy, Free Energy and Equilibrium. Chapter 18 18.1-18.6. Chemical Thermodynamics. Science of interconversion of energy Heat into other forms of energy Amount of heat gained/released from a system Spontaneity of a reaction Gibbs free energy function

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Chemical Thermodynamics: Entropy, Free Energy and Equilibrium

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  1. Chemical Thermodynamics: Entropy, Free Energy and Equilibrium Chapter 18 18.1-18.6

  2. Chemical Thermodynamics • Science of interconversion of energy • Heat into other forms of energy • Amount of heat gained/released from a system • Spontaneity of a reaction • Gibbs free energy function • Relationship between Gibbs Free Energy and chemical equilibrium

  3. Spontaneous Processes • Main objective • Spontaneous Reaction- a reaction does occur under specific conditions • Non-spontaneous Reaction- a reaction does not occur under specific conditions

  4. Spontaneous Processes • A waterfall runs downhill • A lump of sugar dissolves in a cup of coffee • At 1 atm, water freezes below 0ºC and ice melts above 0ºC • Heat flows from a hotter object to a colder object • Iron exposed to oxygen and water forms rust

  5. Spontaneous Processes

  6. CH4(g) + 2O2(g) CO2(g) + 2H2O (l)DH0 = -890.4 kJ H+(aq) + OH-(aq) H2O (l)DH0 = -56.2 kJ H2O (s) H2O (l)DH0 = 6.01 kJ H2O NH4NO3(s) NH4+(aq) + NO3-(aq)DH0 = 25 kJ Spontaneous Processes Does a decrease in enthalpy mean a reaction proceeds spontaneously? Spontaneous reactions

  7. Entropy • To predict spontaneity we need: • Change in enthalpy • Entropy • Entropy- a measure of the randomness or disorder of a system. • ↑ Disorder = ↑ Entropy

  8. Entropy • New Deck Order • Shuffled Deck Order • Probability • Ordered state • Disordered State

  9. Microstates and Entropy

  10. Microstates and Entropy • Boltzmann, 1868 • S = k ln W • k = 1.38 x 10-23 J/K • ↑ W = ↑ Entropy • ∆ S = Sf – Si • ∆ S = k ln Wf Wi Wf> Wi thenDS > 0 Wf< Wi thenDS < 0

  11. Entropy and Disorder If the change from initial to final results in an increase in randomness Sf > Si DS > 0 For any substance, the solid state is more ordered than the liquid state and the liquid state is more ordered than gas state Ssolid < Sliquid << Sgas

  12. Entropy and Disorder

  13. Entropy and Disorder How does the entropy of a system change for each of the following processes? (a) Forming sucrose crystals from a supersaturated solution Randomness decreases Entropy decreases (DS < 0) (b) Heating hydrogen gas from 600C to 800C Randomness increases Entropy increases (DS > 0)

  14. Entropy and Disorder

  15. Standard Entropy The standard entropy of reaction (DS0 ) is the entropy change for a reaction carried out at 1 atm and 250C. rxn

  16. The Second Law of Thermodynamics • The entropy of the universe increases in a spontaneous process and remains unchanged in an equilibrium process. • Importance? DSuniv = DSsys + DSsurr > 0 Spontaneous process: Equilibrium process: DSuniv = DSsys + DSsurr = 0

  17. aA + bB cC + dD - S mS0(reactants) S nS0(products) = DS0 rxn Entropy Changes in the System • To calculate ΔSuniv, we need both ΔSsysΔSsurr • ΔSsys

  18. Entropy Changes in the System

  19. Entropy Changes in the System When gases are produced (or consumed) • If a reaction produces more gas molecules than it consumes, DS0 > 0. • If the total number of gas molecules diminishes, DS0 < 0. • If there is no net change in the total number of gas molecules, then DS0 may be positive or negative BUT DS0 will be a small number.

  20. Entropy Changes in the System

  21. Entropy Changes in the Surroundings

  22. Entropy Changes in the Surroundings ΔSsurr = -ΔHsys T Using the information from Example 18.2, determine whether or not the reaction is spontaneous. N2(g) + 3H2(g) → 2 NH3(g) ΔHºrxn = -92.6 kJ/mol ΔSsys = -199 J/K ∙ mol ΔSsurr = -(-92.6 x 1000) J/mol 298 K ΔSsurr = 311 J/mol ΔSuniv = ΔSsys + ΔSsurr ΔSuniv = -199 J/K ∙ mol + 311 J/mol ΔSUNIV = 112 J/K ∙ mol

  23. The Third Law of Thermodynamics and Absolute Entropy • Third Law of Thermodynamics-the entropy of a perfect crystalline substance is zero at the absolute zero of temperature.

  24. Gibbs Free Energy • Predicts the direction of a spontaneous reaction. • Uses properties of the system to calculate. • For a constant pressure-temperature process: DG = DHsys -TDSsys DG < 0 The reaction is spontaneous in the forward direction. DG > 0 The reaction is nonspontaneous as written. The reaction is spontaneous in the reverse direction. DG = 0 The reaction is at equilibrium.

  25. Standard Free-Energy Changes The standard free-energy of reaction (DG0 ) is the free-energy change for a reaction when it occurs under standard-state conditions. rxn - mDG0 (reactants) S S = f Standard free energy of formation (DG0) is the free-energy change that occurs when 1 mole of the compound is formed from its elements in their standard states. DG0 rxn f nDG0 (products) f

  26. Standard Free-Energy Changes

  27. Factors Affecting ΔG

  28. Free Energy and Chemical Equilibrium DG = DG0 + RT lnQ R is the gas constant (8.314 J/K•mol) T is the absolute temperature (K) Q is the reaction quotient At Equilibrium DG = 0 Q = K 0 = DG0 + RT lnK DG0 = -RT lnK

  29. Free Energy and Chemical Equilibrium

  30. Free Energy and Chemical Equilibrium

  31. Free Energy and Chemical Equilibrium

  32. Thermodynamics of a Rubber Band

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