One of the major concerns of thermodynamics is whether a particular process will occur under specified conditions to give predominantly products.

A change for which the collection of products is thermodynamically more stable than the collection of reactants under the given conditions is said to be product-favored or spontaneous under the conditions.

A change for which the products are thermodynamically less stable than the reactants under the given conditions is described as reactant-favored or nonspontaneous.

Remember, in this description, spontaneous does not necessarily refer to the speed of the reaction. A spontaneous reaction can be a very slow reaction, as is the case in rusting of iron. Also, we can cause some nonspontaneous reactions to occur by applying additional forces to the system (electrical energy or the like).

Two Aspects of Spontaneity
The two factors that affect the spontaneity of any physical or chemical change are: