In a system, whether it be in chemistry, biology, or physics there are spontaneous processes and nonspontaneous processes.
Spontaneous Process Definition
A spontaneous process is one that will occur without any energy input from the surroundings. It is a process that will occur on its own. 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 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 when iron is exposed to air, the process will occur. A radioactive isotope may decay instantly or after thousands or millions or even billions of years.
Spontaneous Versus Nonspontaneous
Energy must be added in order for a nonspontaneous process to occur. The reverse of a spontaneous process is a nonspontaneous process. For example, rust doesn't convert back into iron on its own. A daughter isotope won't return to its parent state.
Free Energy and Spontaneity
The change in Gibbs free energy for a process may be used to determine its spontaneity. At constant temperature and pressure, the equation is:
ΔG = ΔH - TΔS
Where ΔH is the change in enthalpy and ΔS is the change in entropy.
- 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 0 then the process is at equilibrium and no net change is occurring over time.