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J. W. Hill, R. H. Petrucci, T. W. McCreary, & S. S. Perry General Chemistry 4th edition

Problem 17.61

The thermodynamic data listed below are for 298 K. Use these data and data from Appendix C to determine Keq at 45 oC for the reaction

CO2(g) + SF4(g)CF4(g) + SO2(g)


 

Hfo, kJ/mol

So, J mol–1 K–1

SF4(g)

–763

299.6

CF4(g)

–925

261.6






Answer:

Find the enthalpy of reaction at 298 K:

Hfo(CF4(g)) = –925 kJ/mol

Hfo(SO2(g)) = –296.8 kJ/mol

Hfo(SF4(g)) = –763 kJ/mol

Hfo(CO2(g)) = –393.5 kJ/mol

Ho = [(–925) + (–296.8)] – [(–393.5) + (–763)] = –65 kJ

Find the entropy of reaction at 298 K:

So(CF4(g)) = 261.6 J/mol•K

So(SO2(g)) = 248.1 J/mol•K

So(SF4(g)) = 299.6 J/mol•K

So(CO2(g)) = 213.6 J/mol•K

So = [(261.6) + (248.1)] – [(213.6) + (299.6)] = –3.5 J/K

Assuming that Ho and So do not change significantly between 298 K and 318 K (45 oC):

Go = Ho – TSo = –65 – (318)(–0.0035) = –64 kJ = –64000 J

Go = –RTlnKeq

–64000 = –(8.314)(318)lnKeq

Keq = e24.2 = 3×1010