UNIT4DAY6-LaB Page 1 UNIT4DAY6-LaB Thursday, November 29, 2012 8:13 AM Vanden Bout/LaBrake CH301 The 2 nd Law of Thermodynamics GIBBS FREE ENERGY UNIT 4 Day 6 Important Information HW11 Due T DECEMBER 4 th 9AM End of semester attitude survey closes next Monday UNIVERSITY COURSE INSTRUCTOR SURVEY!! MOST IMPORTANT! PLEASE COMPLETE! IT s LIKE VOTING, IT IS YOUR DUTY TO YOUR UNIVERSITY CH302 Vanden Bout/LaBrake Spring 2012
UNIT4DAY6-LaB Page 2 What are we going to learn today? Second & Third Law of Thermodynamics Quantify change in Gibbs Free Energy CH302 Vanden Bout/LaBrake Spring 2012 Quiz: CLICKER QUESTION 1 Which system would you predict would have a negative change in entropy, ΔS? a) H 2 O(s) H 2 O(l) b) H 2 O(s) H 2 O(g) c) Na(s) Na(l) d) I 2 (g) I 2 (s) How could you verify you answer is correct?
From the activity last class: Use the following tabulated data: S (H2)g = 131 J/K mol S (O2)g = 205 J/ K mol S (H2O)l = 70 J/K mol to determine the change in entropy for the combustion of 1 mole of hydrogen. Use the following tabulated data: ΔHf (H2O) = -286 kj/mol to determine the change in entropy for the surroundings for this change under standard conditions Is this change spontaneous? Free Energy, G predict spontaneity based only on system! DERIVE NEW EQUATION!! UNIT4DAY6-LaB Page 3
UNIT4DAY6-LaB Page 4 Poll: CLICKER QUESTION 2 Given the following data, calculate the ΔG rxn for the combustion of 1 mole of hydrogen at 298 K. ΔH f (H 2 O)l = -286 kj/mol S (H 2 )g = 131 J/K mol S (O 2 )g = 205 J/ K mol S (H 2 O)l = 70 J/K mol
S (Br )l = + 152.23 J/K mol UNIT4DAY6-LaB Page 5 Reaction Gibbs Free Energy ΔG r = ΣnΔG f(products) ΣnΔG f(reactants) Standard Gibbs Free Energy of formation, ΔG fº, of a substance is the standard reaction Gibbs Free Energy per mole for the formation of a compound from its elements in their most stable from. Poll: CLICKER QUESTION 3 Calculate the standard reaction free energy of 2Fe 2 O 3 (s) + C(s) 3CO 2 (g) + 4Fe(s) from standard free energy of formation data. ΔG f (Fe 2 O 3 )s = -740 kj/mol ΔG f (CO 2 )g = -394 kj/mol Poll: CLICKER QUESTION 4 This process will be spontaneous at temperatures for which ΔG is negative. At what temperature is this process first spontaneous at 1 atm? Br 2 (l) Br 2 (g) ΔH fus (Br 2 ) = +29.96 kj/mol
At what temperature is this process first spontaneous at 1 atm? Br2 (l) Br2 (g) ΔHfus (Br2) = +29.96 kj/mol S (Br2)l = + 152.23 J/K mol S (Br2)g = + 245.46 J/ K mol The Effect of Temperature ΔG = ΔH - TΔS UNIT4DAY6-LaB Page 6
UNIT4DAY6-LaB Page 7 Poll: CLICKER QUESTION 5 If the ΔH is positive and the ΔS is positive, but the reaction is nonspontaneous at a given temperature you can: a) Not force it to change, once a reaction is nonspontaneous it is always nonspontaneous b) Increase the temperature and move to a spontaneous change c) Decrease the temperature and move to a spontaneous change d) This makes absolutely no sense, please explain again DOES THE MAGNITUDE OF ΔG HAVE SIGNIFICANCE?
UNIT4DAY6-LaB Page 8 TWO IMPORTANT IDEAS FROM THERMODYNAMICS: HOW MUCH HEAT ENERGY IS GIVEN OFF or TAKEN UP with CHANGE? CHANGE IN ENTHALPY WILL THE CHANGE HAPPEN (in isolation)? CHANGE IN GIBBS FREE ENERGY.. A system at equilibrium has no tendency to change in either direction. Dynamic equilibrium, forward and reverse processes still continue but at matching rates. Thermal Equilibrium Mechanical Equilibrium Chemical Equilibrium At Equilibrium, ΔS univ = 0 ΔG = 0
UNIT4DAY6-LaB Page 9 Δ r G is a comparison between ALL Products and ALL Reactants Therefore the change in free energy for reactions that go to completion (all reactants make product). However some reactions stop in the middle (come to an equilibrium). As a result Δ r G < 0 is spontaneous and favors the products Δ r G > 0 is non-spontaneous and favors the reactants Learning Outcomes Calculate change in free energy for a chemical change from change in free energy of formation of the products and the reactants Understand the concept of the change in free energy Calculate change in free energy for a chemical change from change in enthalpy and change in entropy values Recognize based on the sign and magnitude of change in enthalpy and change in entropy how changing the temperature affects the spontaneity of a physical or chemical change Calculate the temperature at which a particular change (physical or chemical) will be spontaneous
UNIT4DAY6-LaB Page 10 The Second Law of Thermodynamics 2nd Law of Thermodynamics states that any process that happens spontaneously will lead to an increase in the entropy of the universe Dealing with the universe is sometimes difficult. Let s consider only the system.