Thermodynamics is the only science about which I am firmly convinced that, within the framework of the applicability of its basic principles, it will

Transcription

1 Thermodynamics is the only science about which I am firmly convinced that, within the framework of the applicability of its basic principles, it will never be overthrown - Albert Einstein OFP Chapter 11 1

2 Chapter 11 Spontaneous Change and Equilibrium 11-1 Enthalpy and Spontaneous Change 11-2 Entropy 11-3 Absolute Entropies and Chemical Reactions 11-4 The Second Law of Thermodynamics 11-5 The Gibbs Function 11-6 The Gibbs Function and Chemical Reactions 11-7 The Gibbs Function and the Equilibrium Constant 11-8 The Temperature Dependence of Equilibrium Constants OFP Chapter 11 2

3 This entire Chapter deals with a concept called Entropy and the Gibbs function Both are new state properties Entropy = S which has the units JK -1 mol -1 OFP Chapter 11 3

4 Consider Energy, it is perhaps obvious that processes occur spontaneously to produce a state of lower energy Gas E Liquid Solid But, a chunk of ice at Room Temperature, spontaneously melts, forming a state of higher Energy Apparently more than energy is involved in determining the direction of spontaneous change OFP Chapter 11 4

5 This additional factor is the tendency of a system to assume the most random molecular arrangement possible Systems become disordered, more random Entropy S H O 2 ( l) = > S Disordered State S H O ( s) Natural processes are favored which result in Decreased Energy (favored) Increased Entropy (favored) These two factors can oppose each other. Which one wins out? 2 > S Ordered State OFP Chapter 11 5

6 Recall from Chapter 10 lecture notes Third Law of Thermodynamics S = 0 at 0 K At absolute zero the Entropy term contributes nothing to the direction of spontaneous change The most stable state has the lowest energy A temperature increases, molecular motion increases and the tendency to disorder becomes more important At sufficiently high temperatures the Entropy factor becomes large enough to overcome even an unfavorable energy change OFP Chapter 11 6

7 For H 2 O (s) H 2 O (l) Above T m, the Entropy is dominant so spontaneous melting takes place Below T m, the energy decrease is dominant so spontaneous freezing takes place Temperature is a critical factor >T m S H2O is dominant <T m E H2O is dominant OFP Chapter 11 7

8 Disorder and Entropy Entropy is a quantitative measure of the number of microstates available to the molecules in a system. It is a measure of the number of ways in which Energy can be arranged. Entropy is the degree of randomness or disorder in a system The Entropy of all substances is positive S solid < S liquid < S gas S is the Entropy Change in a system Entropy = S which has the units JK -1 mol -1 OFP Chapter 11 8

9 For phase transitions, at temperature = T, under equilibrium conditions Melting (solid to liquid) Fusion (liquid to solid) Vaporization (liquid to gas) Condensation (gas to liquid) S = H/T units are JK -1 mol -1 E.g., S vap = H/T b at boiling point Define S as the absolute molar Entropy, which is the absolute entropy of 1 mol of a substance in standard state Appendix D gives standard molar entropy values, S in units JK -1 mol -1 OFP Chapter 11 9

10 OFP Chapter 11 10

11 Entropies of Reaction S rxn = ΣS products ΣS reactants S rxn is the sum of products minus the sum of the reactants For a general reaction a A + b B c C + d D S o = c S o (C) + d S o (D) a S o (A) b S o (B) Appendix D gives standard molar entropy values, S in units JK -1 mol -1 OFP Chapter 11 11

12 Exercise 11-3 (a) Use Data from Appendix D to calculate S r at K for the reaction 2H 2 S(g) + 3O 2 (g) 2SO 2 (g) + 2H 2 O(g) (b) Calculate S of the system when g of H 2 S(g) reacts with excess O 2 (g) to give SO 2 (g) and H 2 O(g) and no other products at K OFP Chapter 11 12

13 Exercise 11-3 (a) Use Data from Appendix D to calculate S r at K for the reaction 2H 2 S(g) + 3O 2 (g) 2SO 2 (g) + 2H 2 O(g) Solution (a) S r = 2S (SO 2 (g) ) + 2S (H 2 O(g)) -2S (H 2 S(g) ) - 3S (O 2 (g)) S r = 2(248.11)+2(188.72) -2(205.68)-3(205.03) = JK -1 mol -1 Notice that this is minus, which is consistent with 5 to 4 decrease in the amount of gas OFP Chapter 11 13

14 Exercise H 2 S(g) + 3O 2 (g) 2SO 2 (g) +2H 2 O(g) (a) S r = JK -1 mol -1 (b) Calculate S of the system when g of H 2 S(g) reacts with excess O 2 (g) to give SO 2 (g) and H 2 O(g) and no other products at K Solution: S = S 26.71g H2S = 60.0JK 1 1mol H2S 34 g H2S 1 1mol reaction ( 152.8JK ) 2 mol H2S 1mol reaction OFP Chapter 11 14

15 PRS Quiz Entropy is the thermodynamic property which constitutes a quantitative measure of the 1. heat content of a system. 2. degree of disorder of a system. 3. internal energy of system. 4. chaos on Halloween night OFP Chapter 11 15

16 PRS Quiz For a given substance, the most disordered physical state is 1. gas 2. liquid 3. solid 4. californium OFP Chapter 11 16

17 PRS Quiz Which of the following processes should show the greatest increase in entropy? 1. C 6 H 6 (g) + 15/2 O 2 (g) 6CO 2 (g) + 3H 2 O (g) 2. 2NO 2 (g) N 2 O 4 (g) 3. C 2 H 4 (g) + H 2 (g) C 2 H 6 (g) 4. BaS (s) + 2 NaNO 3 (s) Ba(NO 3 ) 2 (s) + Na 2 S (s) OFP Chapter 11 17

18 Chapter 11 Spontaneous Change and Equilibrium Second Law of Thermodynamics In a real spontaneous process the Entropy of the universe (meaning the system plus its surroundings) must increase. S universe > 0 (spontaneous process) S universe = S system + S surroundings > 0 if S universe = 0, then everything is in equilibrium The 2 nd Law of Thermodynamics profoundly affects how we look at nature and processes OFP Chapter 11 18