Why are all spontaneous processes not exothermic?
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
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.
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