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 17.72

On several occasions, we have made the assumption that Ho and So undergo little change with temperature. Why can we not make the same assumption about Go? If we assume that Ho298 and So298 do not change with temperature, is it possible for a reaction that is nonspontaneous under standard-state conditions to become spontaneous both at some higher temperature and some lower temperature? Explain.






Answer:

Since Go = Ho – TSo, the Gibb's energy changes linearly with temperature. If So is a large value, then Go can change significantly with temperature.

If the enthalpy change and the entropy change are both constant with respect to temperature and a reaction is nonspontaneous at standard-state conditions, the reaction can not become spontaneous both at higher and lower temperature. For example, if both the enthalpy change and the entropy change are negative, the reaction could be nonspontaneous at standard-state conditions and could become spontaneous at lower temperature; however, this scenario does not allow spontaneity at higher temperature (the Gibb's energy change is always positive after a certain temperature is attained).