Overview

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 T_X and object Y at temperature T_Y, with T_X > T_Y. 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.

Procedure

A spontaneous reaction is one that occurs without any external intervention under a particular set of conditions. A nonspontaneous process is one that requires external forces to proceed.

Spontaneity is affected by conditions such as temperature and pressure.

For example, the melting of ice at a temperature of 0 °C or below is not spontaneous; however, the melting of ice at room temperature is spontaneous because it does not need any external intervention.

A spontaneous reaction is usually unidirectional, and its reversal requires an external force to make it proceed as a nonspontaneous reaction. For example, ice that melts at room temperature cannot spontaneously freeze back to its original form.

Spontaneous processes occur due to the dispersal of energy and matter. If a flask containing a gas is connected to another empty flask through a valve, opening the valve will lead to the spontaneous transfer of the gas to the empty flask, which will continue until the gas is equally distributed in both flasks.

Similarly, if a hot object is placed in cold water, the heat from the object will be spontaneously transferred to the water until the temperatures of the object and the water reach equilibrium.

Spontaneity in thermodynamics does not indicate the speed of a process. The reaction could be extremely fast, like an acid-base neutralization, or extremely slow, like the conversion of diamonds to graphite. 

Consider the decomposition of hydrogen peroxide into water and oxygen gas. The reaction, although spontaneous, proceeds very slowly at room temperature. Adding a catalyst, such as manganese oxide, speeds up the reaction and therefore increases the decomposition rate of hydrogen peroxide.

Although the catalyst enhances the rate of the spontaneous reaction, it cannot make the nonspontaneous reverse reaction be spontaneous.