5. E.D. Eastman and W.C. McGavock studied the transition of rhombic sulfur to mo

Question

5. E.D. Eastman and W.C. McGavock studied the transition of rhombic sulfur to monoclinic ulfur J. Am. Chem. Soc.. 59, 145 (1937). At one atmosphere pressure the following phase transition takes place at 368.5 K: At the transition tempcrature they obtained a value of +96 cal mol for the enthalpy of transi tion. They also determined that, over the temperature range of 298 K to 369 K, for this Copyrighted m TI 11 RMODYNAMICS (V 11 ) 12-17 reaction dCP =0.085 + 0.66 x 10-3T where ACp is in cal mol-1 K and Tis in kelvin. At 25°C and one bar pressure, rhombic sulfur is the more stable form; therefore its standard molar free energy of formation is taken as zero at this temperature. What is the standard molar free cnergy of formation of monoclinic sulfur at 25°C?

Explanation / Answer

25 C =298K

Heat of reaction to convert rhombic sulphur to monoclinic sulphur at 298K = Heat required to raise rhombic sulphur temperature from 298K to 368.5 K +Heat of reaction at 368.5 K +Heat release to reduce monoclinic sulphur temperature from 368.5K to 298 K

Heat required to raise temperature from 298K to 368.5 K= (.085+.66*10^-3*T)*dT = (.085T+.33*10^-3*T^2)298368.5

Heat of reaction to convert rhombic sulphur to monoclinic sulphur at 298K =(.085T+.33*10^-3*T^2)298368.5+96+(.085T+.33*10^-3*T^2)298368.5

Heat of reaction to convert rhombic sulphur to monoclinic sulphur at 298K=96 cal/mol

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