Spontaneous Process

The step which has a natural tendency to take place on its own without the help of any other external agency is called a spontaneous process. It is one that occurs on its own, without any energy input from the outside. For example, a ball will roll down an incline; water will flow downhill; ice will melt into the water; radioisotopes will decay, and iron will rust. No intervention is required because these processes are thermodynamically favorable. In other words, the initial energy is higher than the final energy.

Note that how quickly a step occurs has no bearing on whether or not it is spontaneous: It may take a long time for rust to become obvious, yet it will develop when the iron is exposed to air. A radioactive isotope may decay instantly or after millions or even billions of years; yet, it will decay.

Whereas The reverse is a nonspontaneous process: Energy must be added in order for one to occur. For example, rust doesn’t convert back into iron on its own; a daughter isotope won’t return to its parent state.

Some other examples of it are given below;

1. Water always flows from a higher level to a lower level.
2. Heat always flows from the body at high temperatures to the body at low temperatures.
3. Zinc dissolves in an aqueous solution of copper sulfate to form zinc sulfate and copper is deposited.
   CuSO4 + Zn  →  ZnSO4 + Cu↓

Characteristics

• All spontaneous processes have neutral tendencies to take place on their own such process can be reversed only with the help of an external agency.
• A spontaneous process only tells the tendency of the process in a preferred direction, but the process may be slow or fast.
• A spontaneous process continues until a state of equilibrium is reached.
• A spontaneous process may or may not be instantaneous.