Spontaneity

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.