1 (391)

Prévia do material em texto

Chapter 18 Free Energy and Thermodynamics 391 more ordered. For to be positive, must be positive and greater in absolute value (or magnitude) than We learned in Chapter 6 that freezing is an exothermic process: it gives off heat to the surroundings. If we think of entropy as the dispersal or randomization of energy, the release of heat energy by the system disperses that energy into the surroundings, increasing the entropy of the surroundings. The freezing of water below increases the entropy of the universe because the heat given off to the surroundings increases the entropy of the surroundings to a sufficient degree to overcome the entropy decrease in the water. The freezing of water becomes nonspontaneous above because the magnitude of the increase in the entropy of the surroundings due to the dispersal of energy into the surroundings is temperature-dependent. The higher the temperature, the smaller the percent increase in entropy for a given rise in temperature. Therefore, the impact of the heat released to the surroundings by the freezing of water depends on the temperature of the surroundings-the higher the temperature, the smaller the impact. 18.15 The change in Gibbs free energy for a process is proportional to the negative of Because is a criterion for spontaneity, AG is also a criterion for spontaneity (though opposite in sign). 18.17 The third law of thermodynamics states that the entropy of a perfect crystal at absolute zero (0K) is zero. For enthalpy, we defined a standard state so that we could define a "zero" for the scale. This is not necessary for entropy because there is an absolute zero. 18.19 The larger the molar mass, the greater the entropy at 25 °C. For a given state of matter, entropy generally increases with increasing molecular complexity. 18.21 The three ways of calculating the are: Use tabulated values of standard enthalpies of formation to calculate and use tabulated values of stan- dard entropies to calculate then use the values of and calculated in these ways to calculate the standard free energy change for a reaction by using the equation = Use tabulated values of the standard free energies of formation to calculate using an equation similar to that used for standard enthalpy of a reaction = products) Use a reaction pathway or stepwise reaction to sum the changes in free energy for each of the steps in a manner similar to that used in Chapter 6 for enthalpy of stepwise reactions. The method to calculate the free energy of a reaction at temperatures other than at 25 °C is the first method. The second method is only applicable at 25 °C. The third method is only applicable at the temperature of the individual reactions, generally 25 °C. 18.23 The standard free energy change for a reaction applies only to standard conditions. For a gas, standard condi- tions are those in which the pure gas is present at a partial pressure of 1 atm. For nonstandard conditions, we need to calculate (not to predict spontaneity. 18.25 The free energy of reaction under nonstandard conditions can be calculated from using the relationship = + RT InQ, where Q is the reaction quotient (defined in Section 14.7), T is the temperature in K, and R is the gas constant in the appropriate units (8.314 J/mol K). Problems by Topic Entropy, the Second Law of Thermodynamics, and the Direction of Spontaneous Change 18.27 (a) and (c) are spontaneous processes. 18.29 System B has the greatest entropy. There is only one energetically equivalent arrangement for System A. However, the particles of System B may exchange positions for a second energetically equivalent arrangement. 18.31 Given: 1.00 mol C₃H₈O, fus = 5.37 kJ/mol, -89.5°C Find: S Conceptual plan: °C K then fus then kJ J then T K=273.15+°C = 1kJ Copyright © 2017 Pearson Education, Inc.