3/30/2017. Section 17.1 Spontaneous Processes and Entropy Thermodynamics vs. Kinetics. Chapter 17. Spontaneity, Entropy, and Free Energy

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1 Chapter 17 Spontaneity, Entropy, and 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 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. 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 3 Copyright Cengage Learning. All rights reserved 4 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. 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 5 Copyright Cengage Learning. All rights reserved 6 1

2 The Expansion of An Ideal Gas Into an Evacuated Bulb 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 7 Copyright Cengage Learning. All rights reserved 8 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. The Microstates That Give a Particular Arrangement (State) Copyright Cengage Learning. All rights reserved 9 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: S solid < S liquid << S gas 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 NaCl in water Copyright Cengage Learning. All rights reserved 12 2

3 Section 17.2 Entropy and the Second Law of Thermodynamics Second Law of Thermodynamics 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. = +; entropy of the universe increases = -; process is spontaneous in opposite direction = 0; process has no tendency to occur S universe = ΔS system + oundings Copyright Cengage Learning. All rights reserved 13 Copyright Cengage Learning. All rights reserved 14 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 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. (S universe = ΔS system + Δs surroundings ) The sign of depends on the direction of the heat flow. The magnitude of depends on the temperature. Copyright Cengage Learning. All rights reserved 17 Copyright Cengage Learning. All rights reserved 18 3

4 (S universe = ΔS system + Δs surroundings ) At constant P: Heat flow = change in enthalpy = ΔH = Ssurr H T Copyright Cengage Learning. All rights reserved 19 Copyright Cengage Learning. All rights reserved 20 (G) Suniv = G (at constant T and P) T (G) ΔG = ΔH TΔS (at constant T and P) A process (at constant T and P) is spontaneous in the direction in which the free energy decreases. Negative ΔG means positive ΔS univ. Copyright Cengage Learning. All rights reserved 21 Copyright Cengage Learning. All rights reserved 22 EXERCISE! A liquid is vaporized at its boiling point. Predict the signs of: w q + ΔH + ΔS + ΔG 0 Explain your answers. The value of ΔH vaporization of substance X is 45.7 kj/mol, and its normal boiling point is 72.5 C. Calculate ΔS,, and ΔG for the vaporization of one mole of this substance at 72.5 C and 1 atm. ΔS = 132 J/K mol = -132 J/K mol ΔG = 0 kj/mol Copyright Cengage Learning. All rights reserved 23 Copyright Cengage Learning. All rights reserved 24 4

5 Spontaneous Reactions Effect of ΔH and ΔS on Spontaneity 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 25 Copyright Cengage Learning. All rights reserved 26 Third Law of Thermodynamics 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. The entropy of a perfect crystal at 0 K is zero. The entropy of a substance increases with temperature. Predict the signs of: ΔH ΔS ΔS univ Explain. Copyright Cengage Learning. All rights reserved 27 Copyright Cengage Learning. All rights reserved 28 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 = Σn p S products Σn r S reactants Copyright Cengage Learning. All rights reserved 29 EXERCISE! Calculate ΔS for the following reaction: 2Na(s) + 2H 2 O(l) 2NaOH(aq) + H 2 (g) Given the following information: S (J/K mol) Na(s) 51 H 2 O(l) 70 NaOH(aq) 50 H 2 (g) 131 ΔS = 11 J/K Copyright Cengage Learning. All rights reserved 30 5

6 Section 17.6 and Chemical Reactions Standard 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 = Σn p G products Σn r G reactants Section 17.6 and Chemical Reactions A stable diatomic molecule spontaneously forms from its atoms. Predict the signs of: ΔH ΔS ΔG Explain. Copyright Cengage Learning. All rights reserved 31 Copyright Cengage Learning. All rights reserved 32 Section 17.6 and Chemical Reactions Section 17.7 The Dependence of on Pressure Consider the following system at equilibrium at 25 C. PCl 3 (g) + Cl 2 (g) PCl 5 (g) ΔG = kj 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. and Pressure What happen to entropy as the container becomes smaller? S large volume > S small volume S low pressure > S high pressure Using a detailed argument G = G + RT ln(p) or ΔG = ΔG + RT ln(q) Copyright Cengage Learning. All rights reserved 33 Copyright Cengage Learning. All rights reserved 34 Section 17.7 The Dependence of on Pressure Sketch graphs of: 1. G vs. P 2. H vs. P 3. ln(k) vs. 1/T (for both endothermic and exothermic cases) Section 17.7 The Dependence of 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 35 Copyright Cengage Learning. All rights reserved 36 6

7 Section 17.8 and Equilibrium Section 17.8 and Equilibrium Change in to Reach 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 37 Copyright Cengage Learning. All rights reserved 38 Section 17.8 and Equilibrium Section 17.9 and Work Maximum possible useful work obtainable from a process at constant temperature and pressure is equal to the change in free energy. w max = ΔG Copyright Cengage Learning. All rights reserved 39 Copyright Cengage Learning. All rights reserved 40 Section 17.9 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 41 7