CHM112 Home Page Reference Materials Homework Assignments Lectures Exams & Quizzes Grades Study Aids URI Home Page


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

Problem 17.41

Use the data from Appendix C to determine Ho and So, at 298 K, for the following reaction. Then determine Go in two ways, and compare the results.

CS2(l) + 3 O2(g) CO2(g) + 2 SO2(g)






Answer:

The enthalpy of reaction is found from the enthalpies of formation:

Hfo (CS2(l)) = 89.70 kJ/mol

Hfo (O2(g)) = 0 kJ/mol

Hfo (CO2(g)) = –393.5 kJ/mol

Hfo (SO2(g)) = –296.8 kJ/mol

Ho = [(–393.5) + 2(–296.8)] – [(89.70) + 3(0)] = –1076.8 kJ


The entropy of reaction is found from the standard entropies:

So (CS2(l)) = 151.3 J/mol•K

So (O2(g)) = 205.0 J/mol•K

So (CO2(g)) = 213.6 J/mol•K

So (SO2(g)) = 248.1 J/mol•K

So = [(213.6) + 2(248.1)] – [(151.3) + 3(205.0)] = –56.5 J/K


One way to find Go is to use the free energies of formation:

Gfo (CS2(l)) = 65.27 kJ/mol

Gfo (O2(g)) = 0 kJ/mol

Gfo (CO2(g)) = –394.4 kJ/mol

Gfo (SO2(g)) = –300.2 kJ/mol

Go = [(–394.4) + 2(–300.2)] – [(65.27) + 3(0)] = –1060.1 kJ


Another way to find the free energy of reaction is to use the enthalpy and entropy changes:

Go = Ho – TSo = –1076.8 – (298)( –0.0565) = –1060.0 kJ


As required, the Gibb's free energies are the same, within experimental error.