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

2. Why does change occur?  H – change in enthalpy (heat transferred btwn system and surroundings)  S – 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  H = heat absorbed by the system – exothermic = -  H – endothermic = +  H 1 st law of thermo = energy is conserved, heat lost by system must equal heat gained by environment & vice versa remember the formula for calculating  H of a rxn:  H o rxn =  H o f products –  H o f reactants

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

6. 3 rd 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 S o = measures order/disorder of pure substances at 1 atm & 298 K units are J/mol K S o is largest for gases and heavier/more complex substances

8. Entropy Changes looks just like the equation for  H o  S o rxn =  S o products –  S o reactants What is the  S o rxn for the following rxn? 2NO(g) + O 2 (g)  2NO 2 (g)

9. Gibbs Free Energy  G = amt of useful work/energy that can be obtained from a process at constant T&P  G =  H – T  S  G o rxn =  G o f products –  G o f reactants G & H are in kJ/mol, S is in J/molK so it must be converted to kJ

10. What does  G tell you? If  G is… The process is… negativespontaneous positivenonspontaneous zeroat equilibrium

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

12. Free Energy & Temperature HH SS-T  S GG The rxn is… -+--spontaneous at all T +-++nonspontaneous at all T --++/-Spontaneous only at low T ++-+/- Spontaneous only at high T KNOW THIS TABLE!!!

13. Is  G for a burning campfire positive or negative? Explain citing the positive or negative signs of  H and  S for the process. Is there a temperature at which  G 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:  G =  G o + RT lnQ have to use R = 8.314 J/molK At equilibrium, Q = K &  G = 0, so the eqn becomes:  G o = -RT lnK

15. Using the std. free energies of formation for the reaction: 2H 2 S(g) + 3O 2 (g)  2H 2 O(g) + 2SO 2 (g) Calculate  G o and  G for a mixture at 25 o C with the following composition: P H2S = 1.00 atm, P O2 = 2.00 atm, P H2O = 0.500 atm, P SO2 = 0.750 atm

16. The standard free energy change for the following reaction at 25 o C is -118.4 kJ/mol: KClO 3 (s)  KCl(s) + 3/2 O 2 (g) Calculate K p for the reaction and the equilibrium pressure of O 2 gas.