PRINCIPLES OF CHEMISTRY I
PRINCIPLES OF CHEMISTRY I CH 301
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This 5 page Class Notes was uploaded by Brady Spinka on Monday September 7, 2015. The Class Notes belongs to CH 301 at University of Texas at Austin taught by Fatima Fakhreddine in Fall. Since its upload, it has received 13 views. For similar materials see /class/181868/ch-301-university-of-texas-at-austin in Chemistry at University of Texas at Austin.
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Date Created: 09/07/15
SPONTANEITY ENTROPY AND FREE ENERGY o A spontaneous process is one that can happen without outside intervention It is a process in which products are favored over reactants at specified conditions Examples sublimation of C02s at room temperature rusting of iron Many products favored spontaneous reactions are exothermic However not all exothermic processes are spontaneous Also not all spontaneous processes are exothermic The dissolution of NaCls in water is an endothermic process yet it is spontaneous The melting of ice is an endothermic yet spontaneous process at a temperature above 0 C What are the factors that make a process spontaneous 0 Two factors determine the spontaneity of a process under specified conditions the enthalpy change and the entropy change An exothermic process favors but does not imply spontaneity and an increase in disorder favors by does not imply spontaneity either Therefore processes are more likely to be spontaneous if the products have lower heat content and greater disorder ENTROPY o The thermodynamic state function entropy S is a measure of the disorder of a system It is one of two factors that affect spontaneity o Entropy is associated with probability The more ways a certain state can be reached the higher the probability for that state to occur 0 The entropy change depends on both heat q and temperature T Therefore the quantitative entropy change for a reversible process at constant temperature is AS qT The more heat we add to a system the more the disorder However the heat added to the system causes more disorder at lower temperature than it does at higher temperature think about how adding the same amount of heat to an ice cube causes more disorder than adding the same amount of heat to steam The effect of entropy change on spontaneity is summarized by the Second Law of Thermodynamics In spontaneous changes the universe tends toward a state of greater disorder ASuniverse ASsystem ASsurrounding If a system undergoes a decrease in disorder ASsystem lt0 but ASsumunding is more positive then ASuniverse gt0 and the process is spontaneous Example solidification of a liquid below its freezing temperature Also if ASsystem gt0 but ASsumunding is more negative then AS universe is still negative and the process is nonspontaneous Example melting of a solid below its melting point Since it is not possible to make direct measurements of ASunivem entropy changes are reported in terms of ASsystem as AS The Third law of Thermodynamics states that the entrog of a pure perfect cgstalline substance at 0 K is zero As temperature increases disorder increases The absolute entropies of various substances under standard conditions are tabulated as S0298 in the appendix The standard entropy change ASO of a reaction can be determined from the absolute entropies of reactants and products ASOrxn 2nS0products 39 Ensoreactants Gases have higher entropy than liquids Liquids have high entropy than solids Increasing the temperature of a system increases its entropy Increasing the volume of gases increases the entropy Mixing of substances increases the entropy Increasing the number of moles of gases increases the entropy Example Using S0 values determine the standard entropy change for the reaction described below at 25 C NH3g HNO31 N20g 2H201 FREE ENERGY CHANGE AND SPONTANEITY When deciding whether a process is spontaneous or not we need to account for the effects of both the enthalpy and the entropy The contribution of these factors was formulated by J Willard Gibbs in terms of another state function the Gibbs free energy change AG AG AH TAS at constant temperature and pressure The value of AG is an indicator of the spontaneity of a process AG is positive process nonspontaneous or reactant process AG is zero system at equilibrium AG is negative process is spontaneous or products favored process The dependence of the AG on the temperature implies that some processes might be either spontaneous or nonspontaneous depending on the temperature The standard state for Gibbs free energy change is 1 atm and 298 K Values of standard molar free energy of formation AGOf for many substances are tabulated in the appendix Elements in their standard state have AGOf 0 o The values of AGOf could be used to calculate the standard free energy change of a reaction at 298 K with the following equation AGorxn 2nAG fpmducts2nAG freactant at 1 arm and 298 K only The value of AGorxn allows us to predict the spontaneity of a hypothetical reaction called the standard reaction In the standard reaction the number of moles shown in the chemical equation all in their standard state are all converted into the number of moles of products shown in the balanced equation and in their standard state as well The value of AG0 can also be calculated by the equation AGO AHO T A80 at constant temperature and pressure This equation applies to standard conditions but can be used to estimate free energy changes at other temperature Example a Use tabulated AGO to calculate the standard free energy change for the reaction below at 298 K Is the reaction spontaneous at 298 K 2NH3g 3CuOs 3H201 N2g 3Cus b Evaluate AGorxn using AHOf and S0 values EQUILIBRIUM For any system at equilibrium AG 0 So AH TAS and therefore T AHAS We can assume AH AHO ana S S0 and use the tabulatea AHo ana S0 values to estimate the equilibrium temperature Example Use thermodynamic data in the appendix to estimate the boiling point of mercury TEMPERATURE DEPENDENCE OF SPONTANEITY Since the free energy change and spontaneity of a reaction depend on both enthalpy and entropy we can group reactions on four classes With respect to spontaneity AH AS TAS AG T range of spontaneity Example Estimate the temperature range over which the reaction of NH3 and CuO in the example above is spontaneous