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Thermodynamics Chapter 19

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First Law of Thermodynamics You will recall from Chapter 5 that energy cannot be created or destroyed. Therefore, the total energy of the universe is a constant. Energy can, however, be converted from one form to another or transferred from a system to the surroundings or vice versa. © 2012 Pearson Education, Inc.

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Spontaneous Processes and Entropy Thermodynamics lets us predict whether a process will occur but gives no information about the amount of time required for the process. A spontaneous process is one that occurs without outside intervention.

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Spontaneous Processes Spontaneous processes are those that can proceed without any outside intervention. The gas in vessel B will spontaneously effuse into vessel A, but once the gas is in both vessels, it will not spontaneously return to vessel B. © 2012 Pearson Education, Inc.

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Spontaneous Processes Processes that are spontaneous in one direction are nonspontaneous in the reverse direction. © 2012 Pearson Education, Inc.

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Spontaneous Processes Processes that are spontaneous at one temperature may be nonspontaneous at other temperatures. Above 0 C, it is spontaneous for ice to melt. Below 0 C, the reverse process is spontaneous. © 2012 Pearson Education, Inc.

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Reversible v. Irreversible Processes Reversible: The universe is exactly the same as it was before the cyclic process. Irreversible: The universe is different after the cyclic process. All real processes are irreversible -- (some work is changed to heat).

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The Second Law of Thermodynamics...in any spontaneous process there is always an increase in the entropy of the universe. S univ > 0 for a spontaneous process.

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Second Law of Thermodynamics In other words: For reversible processes: S univ = S system + S surroundings = 0 For irreversible processes: S univ = S system + S surroundings > 0 © 2012 Pearson Education, Inc.

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Entropy The driving force for a spontaneous process is an increase in the entropy of the universe. Entropy, S, can be viewed as a measure of randomness, or disorder.

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Entropy Like total energy, E, and enthalpy, H, entropy is a state function. Therefore, S = S final S initial © 2012 Pearson Education, Inc.

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Entropy on the Molecular Scale The number of microstates and, therefore, the entropy, tends to increase with increases in –Temperature –Volume –The number of independently moving molecules. © 2012 Pearson Education, Inc.

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Solutions Generally, when a solid is dissolved in a solvent, entropy increases. © 2012 Pearson Education, Inc.

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Positional Entropy A gas expands into a vacuum because the expanded state has the highest positional probability of states available to the system. Therefore, S solid < S liquid << S gas

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Entropy and Physical States Entropy increases with the freedom of motion of molecules. Therefore, S(g) > S(l) > S(s) S(g) > S(l) > S(s) © 2012 Pearson Education, Inc.

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Third Law of Thermodynamics The entropy of a pure crystalline substance at absolute zero is 0. © 2012 Pearson Education, Inc.

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Entropy

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Standard Entropies These are molar entropy values of substances in their standard states. Standard entropies tend to increase with increasing molar mass. © 2012 Pearson Education, Inc.

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Standard Entropies Larger and more complex molecules have greater entropies. © 2012 Pearson Education, Inc.

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Entropy Changes Entropy changes for a reaction can be estimated in a manner analogous to that by which H is estimated: S = n S (products) — m S (reactants) where n and m are the coefficients in the balanced chemical equation. © 2012 Pearson Education, Inc.

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Entropy Changes in Surroundings Heat that flows into or out of the system changes the entropy of the surroundings. For an isothermal process: S surr = q sys T At constant pressure, q sys is simply H for the system. © 2012 Pearson Education, Inc.

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Entropy Change in the Universe The universe is composed of the system and the surroundings. Therefore, S universe = S system + S surroundings For spontaneous processes S universe > 0 S universe > 0 © 2012 Pearson Education, Inc.

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Free Energy G = H T S (from the standpoint of the system) A process (at constant T, P) is spontaneous in the direction in which free energy decreases: G means + S univ

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Gibbs Free Energy 1.If G is negative, the forward reaction is spontaneous. 2.If G is 0, the system is at equilibrium. 3.If G is positive, the reaction is spontaneous in the reverse direction. © 2012 Pearson Education, Inc.

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Free Energy and Temperature There are two parts to the free energy equation: H — the enthalpy term –T S — the entropy term The temperature dependence of free energy then comes from the entropy term. © 2012 Pearson Education, Inc.

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Effect of H and S on Spontaneity

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Free Energy Change and Chemical Reactions G = standard free energy change that occurs if reactants in their standard state are converted to products in their standard state. G = n p G f (products) n r G f (reactants)

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Free Energy and Equilibrium Under any conditions, standard or nonstandard, the free energy change can be found this way: G = G + RT ln Q (Under standard conditions, all concentrations are 1 M, so Q = 1 and ln Q = 0; the last term drops out.) © 2012 Pearson Education, Inc.

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Free Energy and Pressure G = G + RT ln(Q) Q = reaction quotient from the law of mass action.

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Free Energy and Equilibrium G = RT ln(K) K = equilibrium constant This is so because G = 0 and Q = K at equilibrium.

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Temperature Dependence of K y = mx + b ( H and S independent of temperature over a small temperature range) ln(K) = - H o /R*(1/T) + S o /R

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