Editing Spontaneous process (section)

{{Short description|Thermodynamic operation}}
In [[thermodynamics]], a '''spontaneous process''' is a [[Thermodynamic process|process]] which occurs without any external input to the system. A more technical definition is the time-evolution of a [[Thermodynamic system|system]] in which it releases [[Thermodynamic free energy|free energy]] and it moves to a lower, more thermodynamically stable energy state (closer to [[thermodynamic equilibrium]]).<ref>[http://www.chem.purdue.edu/gchelp/gloss/sponprocess.html Spontaneous process] - Purdue University</ref><ref>[http://chemed.chem.wisc.edu/chempaths/GenChem-Textbook/Entropy-and-Spontaneous-Reactions-623.html Entropy and Spontaneous Reactions] {{webarchive|url=https://web.archive.org/web/20091213190957/http://chemed.chem.wisc.edu/chempaths/GenChem-Textbook/Entropy-and-Spontaneous-Reactions-623.html |date=2009-12-13 }} - ChemEd DL</ref> The sign convention for free energy change follows the general convention for thermodynamic measurements, in which a release of free energy from the system corresponds to a negative change in the free energy of the system and a positive change in the free energy of the [[Environment (systems)|surroundings]].

Depending on the nature of the process, the free energy is determined differently. For example, the [[Gibbs free energy]] change is used when considering processes that occur under constant [[pressure]] and [[temperature]] conditions, whereas the [[Helmholtz free energy]] change is used when considering processes that occur under constant [[volume]] and temperature conditions. The value and even the sign of both free energy changes can depend upon the temperature and pressure or volume.

Because spontaneous processes are characterized by a decrease in the system's free energy, they do not need to be driven by an outside source of energy.

For cases involving an [[isolated system]] where no energy is exchanged with the surroundings, spontaneous processes are characterized by an increase in [[entropy]].

A ''spontaneous reaction'' is a [[chemical reaction]] which is a spontaneous process under the conditions of interest.