Spontaneous Process in Science: Definition and Examples

Illustration of a red ball rolling down a gray spiral track
A ball rolling down an incline is an example of a spontaneous process.

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In a system, be it chemistry, biology, or physics, there are spontaneous processes and nonspontaneous processes.

Definition of a Spontaneous Process

A spontaneous process 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 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 process 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 iron is exposed to air. A radioactive isotope may decay instantly or after millions or even billions of years; yet, it will decay.

Spontaneous Versus Nonspontaneous

The reverse of a spontaneous process 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.

Gibbs Free Energy and Spontaneity

The change in Gibbs free energy or the Gibbs function may be used to assess the spontaneity of a process. At constant temperature and pressure, the Gibbs equation is ΔG = ΔH - TΔS, in which ΔH is the change in enthalpy, ΔS is the change in entropy, and ΔG is the amount of free or available energy. As for the results:

  • If ΔG is negative, the process is spontaneous;
  • If ΔG is positive, the process is nonspontaneous (but would be spontaneous in the reverse direction);
  • If ΔG is zero, then the process is at equilibrium and no net change is occurring over time.