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Thermodynamics (the other part)

Thermodynamics (the other part). Topic 19 in Red Book Chapter 16 in textbook. Why does change occur?. D H – change in enthalpy (heat transferred btwn system and surroundings) D S – change in entropy (randomness or disorder of the system)

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Thermodynamics (the other part)

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  1. Thermodynamics(the other part) Topic 19 in Red Book Chapter 16 in textbook

  2. Why does change occur? • DH – change in enthalpy (heat transferred btwn system and surroundings) • DS – change in entropy (randomness or disorder of the system) • Systems tend to move towards lower enthalpy (releasing heat) and higher entropy (more disorder)

  3. Spontaneous Processes • reaction that proceeds on its own with no outside help • if a reaction is spontaneous in one direction, it is nonspontaneous in the other direction • does not necessarily mean that the reaction happens quickly, just that it happens

  4. Enthalpy • DH = heat absorbed by the system • exothermic = -DH • endothermic = +DH • 1st law of thermo = energy is conserved, heat lost by system must equal heat gained by environment & vice versa • remember the formula for calculating DH of a rxn: DHorxn = SDHof products – SDHof reactants

  5. Entropy & 2nd Law • measure of randomness (gases are more random than liquids, so gases have higher S) • DS is positive for solid  liquid  gas changes and negative for the reverse • DS is higher for more complex molecules (propane has higher S than methane) • 2nd law of thermo states that any spontaneous change is always associated with an overall +DS

  6. 3rd Law • states that S of a pure crystalline solid at 0K is always zero. • As the temperature is warmed above 0K, S increases gradually • At phase changes, S increases a lot

  7. Standard Molar Entropies • So = measures order/disorder of pure substances at 1 atm & 298 K • units are J/mol K • So is largest for gases and heavier/more complex substances

  8. Entropy Changes • looks just like the equation for DHo DSorxn = SSo products – SSo reactants • What is the DSorxn for the following rxn? 2NO(g) + O2(g)  2NO2(g)

  9. Gibbs Free Energy • DG = amt of useful work/energy that can be obtained from a process at constant T&P DG = DH – TDS DGorxn = SGof products – SGof reactants • G & H are in kJ/mol, S is in J/molK so it must be converted to kJ

  10. What does DG tell you?

  11. What is the standard free energy for the reaction: 2NO(g) + O2(g)  2NO2(g)

  12. Free Energy & Temperature KNOW THIS TABLE!!!

  13. Is DG for a burning campfire positive or negative? Explain citing the positive or negative signs of DH and DS for the process. Is there a temperature at which DG for a burning campfire will change signs? Explain.

  14. Free Energy & Equilibrium • you can calculate free energy at non-standard conditions using the following formula: DG = DGo + RT lnQ • have to use R = 8.314 J/molK • At equilibrium, Q = K & DG = 0, so the eqn becomes: DGo = -RT lnK

  15. Using the std. free energies of formation for the reaction: 2H2S(g) + 3O2(g)  2H2O(g) + 2SO2(g) Calculate DGo and DG for a mixture at 25oC with the following composition: PH2S = 1.00 atm, PO2 = 2.00 atm, PH2O = 0.500 atm, PSO2 = 0.750 atm

  16. The standard free energy change for the following reaction at 25oC is -118.4 kJ/mol: KClO3(s)  KCl(s) + 3/2 O2(g) Calculate Kp for the reaction and the equilibrium pressure of O2 gas.

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