A spontaneous process is one that occurs naturally under certain conditions. A nonspontaneous process, on the other hand, will not take place unless it is “driven” by the continual input of energy from an external source. Processes have a natural tendency to occur in one direction under a given set of conditions. Water will naturally flow downhill (spontaneous process), but uphill flow (nonspontaneous process) requires outside intervention such as the use of a pump. Iron exposed to the earth’s atmosphere will corrode (spontaneous process), but rust is not converted to iron (nonspontaneous process) without intentional chemical treatment. A process that is spontaneous in one direction under a particular set of conditions is nonspontaneous in the reverse direction. At room temperature and typical atmospheric pressure, for example, ice will spontaneously melt, but water will not spontaneously freeze.
Spontaneity is Independent of Rate of Reaction
The spontaneity of a process is not correlated to the speed of the process. While a catalyst may be used to speed up or slow down a process, its presence does not influence spontaneity: nonspontaneous reactions cannot be made spontaneous using a catalyst. A spontaneous change may be so rapid that it is essentially instantaneous or so slow that it cannot be observed over any practical period of time. To illustrate this concept, consider the decay of radioactive isotopes. Radioactive decay is, by definition, a spontaneous process in which the nuclei of unstable isotopes emit radiation as they are converted to more stable nuclei. All the decay processes occur spontaneously, but the rates at which different isotopes decay vary widely. Technetium-99m is a popular radioisotope for medical imaging studies that undergoes relatively rapid decay and exhibits a half-life of about six hours. Uranium-238 is the most abundant isotope of uranium, and its decay occurs much more slowly, exhibiting a half-life of more than four billion years.
Dispersal of Matter and Energy
Consider an isolated system consisting of two flasks connected with a closed valve. Initially, there is an ideal gas in one flask, and the other flask is empty. When the valve is opened, the gas spontaneously expands to fill both flasks equally. Since the system is isolated, no heat has been exchanged with the surroundings. The spontaneity of this process is, therefore, not a consequence of any change in energy that accompanies the process. Instead, the driving force appears to be related to the greater, more uniform dispersal of matter that results when the gas is allowed to expand.
Now consider two objects at different temperatures: object X at temperature TX and object Y at temperature TY, with TX > TY. When these objects come into contact, heat spontaneously flows from the hotter object (X) to the colder one (Y). This corresponds to a loss of thermal energy by X and a gain of thermal energy by Y. From the perspective of this two-object system, there was no net gain or loss of thermal energy; rather the available thermal energy was redistributed among the two objects. This spontaneous process resulted in a more uniform dispersal of energy.
As demonstrated by the two processes, an important factor in determining the spontaneity of a process is the extent to which it changes the dispersal or distribution of matter and/or energy. In each case, a spontaneous process took place that resulted in a more uniform distribution of matter or energy.
This text is adapted from Openstax, Chemistry 2e, Chapter 16.1: Spontaneity.