Why are all spontaneous processes not exothermic?

1 Answer
Apr 18, 2014

All spontaneous processes are not exothermic, because it is the Gibbs Free energy that determines spontaneity, not the enthalpy.

A process is spontaneous if the Gibbs free energy is negative. An important expression for the Gibbs free energy is given by
#DeltaG = DeltaH - T DeltaS#
Where #Delta S# is the change in entropy and T is the absolute temperature in K.

You will notice that this expression may be positive even with a negative enthalpy change (exothermic process) if the entropy change is negative and the temperature is high enough.

A practical example is the condensation of steam. This is a very exothermic process. But it also has a negative change in entropy, because a liquid is more orderly than a gas. Because this is the case, the process is only spontaneous at low temperatures at 1 atm ( <373 K). Otherwise, at temperatures above 373 K, water will remain vapor even though condensation is exothermic.