Chapter 17 Spontaneity Entropy and Free Energy Section

Chapter 17 Spontaneity, Entropy, and Free Energy

Section 17. 1 Spontaneous Processes and Entropy Thermodynamics vs. Kinetics § Domain of Kinetics § Rate of a reaction depends on the pathway from reactants to products. § Thermodynamics tells us whether a reaction is spontaneous based only on the properties of reactants and products. Copyright © Cengage Learning. All rights reserved 2

Section 17. 1 Spontaneous Processes and Entropy § Thermodynamics lets us predict the direction in which a process will occur but gives no information about the speed of the process. § A spontaneous process is one that occurs without outside intervention. Copyright © Cengage Learning. All rights reserved 3

Section 17. 1 Spontaneous Processes and Entropy CONCEPT CHECK! Consider 2. 4 moles of a gas contained in a 4. 0 L bulb at a constant temperature of 32°C. This bulb is connected by a valve to an evacuated 20. 0 L bulb. Assume the temperature is constant. a) What should happen to the gas when you open the valve? Copyright © Cengage Learning. All rights reserved 4

Section 17. 1 Spontaneous Processes and Entropy CONCEPT CHECK! Consider 2. 4 moles of a gas contained in a 4. 0 L bulb at a constant temperature of 32°C. This bulb is connected by a valve to an evacuated 20. 0 L bulb. Assume the temperature is constant. b) Calculate ΔH, ΔE, q, and w for the process you described above. All are equal to zero. Copyright © Cengage Learning. All rights reserved 5

Section 17. 1 Spontaneous Processes and Entropy CONCEPT CHECK! Consider 2. 4 moles of a gas contained in a 4. 0 L bulb at a constant temperature of 32°C. This bulb is connected by a valve to an evacuated 20. 0 L bulb. Assume the temperature is constant. c) Given your answer to part b, what is the driving force for the process? Entropy Copyright © Cengage Learning. All rights reserved 6

Section 17. 1 Spontaneous Processes and Entropy The Expansion of An Ideal Gas Into an Evacuated Bulb Copyright © Cengage Learning. All rights reserved 7

Section 17. 1 Spontaneous Processes and Entropy § The driving force for a spontaneous process is an increase in the entropy of the universe. § A measure of molecular randomness or disorder. Copyright © Cengage Learning. All rights reserved 8

Section 17. 1 Spontaneous Processes and Entropy § Thermodynamic function that describes the number of arrangements that are available to a system existing in a given state. § Nature spontaneously proceeds toward the states that have the highest probabilities of existing. Copyright © Cengage Learning. All rights reserved 9

Section 17. 1 Spontaneous Processes and Entropy The Microstates That Give a Particular Arrangement (State)

Section 17. 1 Spontaneous Processes and Entropy Positional Entropy § A gas expands into a vacuum to give a uniform distribution because the expanded state has the highest positional probability of states available to the system. § Therefore: Ssolid < Sliquid << Sgas

Section 17. 1 Spontaneous Processes and Entropy CONCEPT CHECK! Predict the sign of ΔS for each of the following, and explain: + a) The evaporation of alcohol – b) The freezing of water – c) Compressing an ideal gas at constant temperature + d) Heating an ideal gas at constant pressure + e) Dissolving Na. Cl in water Copyright © Cengage Learning. All rights reserved 12

Section 17. 2 Entropy and the Second Law of Thermodynamics § In any spontaneous process there is always an increase in the entropy of the universe. § The entropy of the universe is increasing. § The total energy of the universe is constant, but the entropy is increasing. Suniverse = ΔSsystem + ΔSsurroundings Copyright © Cengage Learning. All rights reserved 13

Section 17. 2 Entropy and the Second Law of Thermodynamics ΔSsurr § ΔSsurr = +; entropy of the universe increases § ΔSsurr = -; process is spontaneous in opposite direction § ΔSsurr = 0; process has no tendency to occur Copyright © Cengage Learning. All rights reserved 14

Section 17. 3 The Effect of Temperature on Spontaneity CONCEPT CHECK! For the process A(l) A(s), which direction involves an increase in energy randomness? Positional randomness? Explain your answer. As temperature increases/decreases (answer for both), which takes precedence? Why? At what temperature is there a balance between energy randomness and positional randomness? Copyright © Cengage Learning. All rights reserved 15

Section 17. 3 The Effect of Temperature on Spontaneity CONCEPT CHECK! Describe the following as spontaneous/non-spontaneous/cannot tell, and explain. A reaction that is: a) Exothermic and becomes more positionally random Spontaneous b) Exothermic and becomes less positionally random Cannot tell a) Endothermic and becomes more positionally random Cannot tell a) Endothermic and becomes less positionally random Not spontaneous Explain how temperature affects your answers.

Section 17. 3 The Effect of Temperature on Spontaneity ΔSsurr § The sign of ΔSsurr depends on the direction of the heat flow. § The magnitude of ΔSsurr depends on the temperature. Copyright © Cengage Learning. All rights reserved 17

Section 17. 3 The Effect of Temperature on Spontaneity ΔSsurr Copyright © Cengage Learning. All rights reserved 18

Section 17. 3 The Effect of Temperature on Spontaneity ΔSsurr Copyright © Cengage Learning. All rights reserved 19

Section 17. 3 The Effect of Temperature on Spontaneity ΔSsurr Heat flow (constant P) = change in enthalpy = ΔH Copyright © Cengage Learning. All rights reserved 20

Section 17. 3 The Effect of Temperature on Spontaneity Copyright © Cengage Learning. All rights reserved 21

Section 17. 4 Free Energy (G) § A process (at constant T and P) is spontaneous in the direction in which the free energy decreases. § Negative ΔG means positive ΔSuniv. Copyright © Cengage Learning. All rights reserved 22

Section 17. 4 Free Energy (G) § ΔG = ΔH – TΔS (at constant T and P) Copyright © Cengage Learning. All rights reserved 23

Section 17. 4 Free Energy CONCEPT CHECK! A liquid is vaporized at its boiling point. Predict the signs of: – w + q + ΔH + ΔS – ΔSsurr 0 ΔG Explain your answers. Copyright © Cengage Learning. All rights reserved 24

Section 17. 4 Free Energy EXERCISE! The value of ΔHvaporization of substance X is 45. 7 k. J/mol, and its normal boiling point is 72. 5°C. Calculate ΔS, ΔSsurr, and ΔG for the vaporization of one mole of this substance at 72. 5°C and 1 atm. ΔS = 132 J/K·mol ΔSsurr = -132 J/K·mol ΔG = 0 k. J/mol Copyright © Cengage Learning. All rights reserved 25

Section 17. 4 Free Energy Spontaneous Reactions To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE Copyright © Cengage Learning. All rights reserved 26

Section 17. 4 Free Energy Effect of ΔH and ΔS on Spontaneity Copyright © Cengage Learning. All rights reserved 27

Section 17. 5 Entropy Changes in Chemical Reactions CONCEPT CHECK! Gas A 2 reacts with gas B 2 to form gas AB at constant temperature and pressure. The bond energy of AB is much greater than that of either reactant. Predict the signs of: ΔH ΔSsurr – + ΔS 0 ΔSuniv + Explain. Copyright © Cengage Learning. All rights reserved 28

Section 17. 5 Entropy Changes in Chemical Reactions Third Law of Thermodynamics § The entropy of a perfect crystal at 0 K is zero. § The entropy of a substance increases with temperature. Copyright © Cengage Learning. All rights reserved 29

Section 17. 5 Entropy Changes in Chemical Reactions Standard Entropy Values (S°) § Represent the increase in entropy that occurs when a substance is heated from 0 K to 298 K at 1 atm pressure. ΔS°reaction = Σnp. S°products – Σnr. S°reactants Copyright © Cengage Learning. All rights reserved 30

Section 17. 5 Entropy Changes in Chemical Reactions EXERCISE! Calculate ΔS° for the following reaction: 2 Na(s) + 2 H 2 O(l) → 2 Na. OH(aq) + H 2(g) Given the following information: S° (J/K·mol) Na(s) 51 H 2 O(l) 70 Na. OH(aq) 50 H 2(g) 131 ΔS°= – 11 J/K Copyright © Cengage Learning. All rights reserved 31

Section 17. 6 Free Energy and Chemical Reactions Standard Free Energy Change (ΔG°) § The change in free energy that will occur if the reactants in their standard states are converted to the products in their standard states. ΔG° = ΔH° – TΔS° ΔG°reaction = Σnp. G°products – Σnr. G°reactants Copyright © Cengage Learning. All rights reserved 32

Section 17. 6 Free Energy and Chemical Reactions CONCEPT CHECK! A stable diatomic molecule spontaneously forms from its atoms. Predict the signs of: ΔH° ΔS° – – ΔG° – Explain. Copyright © Cengage Learning. All rights reserved 33

Section 17. 6 Free Energy and Chemical Reactions CONCEPT CHECK! Consider the following system at equilibrium at 25°C. PCl 3(g) + Cl 2(g) PCl 5(g) ΔG° = − 92. 50 k. J What will happen to the ratio of partial pressure of PCl 5 to partial pressure of PCl 3 if the temperature is raised? Explain. The ratio will decrease. Copyright © Cengage Learning. All rights reserved 34

Section 17. 7 The Dependence of Free Energy on Pressure Free Energy and Pressure G = G° + RT ln(P) or ΔG = ΔG° + RT ln(Q) Copyright © Cengage Learning. All rights reserved 35

Section 17. 7 The Dependence of Free Energy on Pressure CONCEPT CHECK! Sketch graphs of: 1. G vs. P 2. H vs. P 3. ln(K) vs. 1/T (for both endothermic and exothermic cases) Copyright © Cengage Learning. All rights reserved 36

Section 17. 7 The Dependence of Free Energy on Pressure The Meaning of ΔG for a Chemical Reaction § A system can achieve the lowest possible free energy by going to equilibrium, not by going to completion. Copyright © Cengage Learning. All rights reserved 37

Section 17. 8 Free Energy and Equilibrium § The equilibrium point occurs at the lowest value of free energy available to the reaction system. ΔG = 0 = ΔG° + RT ln(K) ΔG° = –RT ln(K) Copyright © Cengage Learning. All rights reserved 38

Section 17. 8 Free Energy and Equilibrium Change in Free Energy to Reach Equilibrium Copyright © Cengage Learning. All rights reserved 39

Section 17. 8 Free Energy and Equilibrium Copyright © Cengage Learning. All rights reserved 40

Section 17. 9 Free Energy and Work § Maximum possible useful work obtainable from a process at constant temperature and pressure is equal to the change in free energy. wmax = ΔG Copyright © Cengage Learning. All rights reserved 41

Section 17. 9 Free Energy and Work § Achieving the maximum work available from a spontaneous process can occur only via a hypothetical pathway. Any real pathway wastes energy. § All real processes are irreversible. § First law: You can’t win, you can only break even. § Second law: You can’t break even. § As we use energy, we degrade its usefulness. Copyright © Cengage Learning. All rights reserved 42