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thermodynamics Thermodynamics is a branch of physics that deals with heat, work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed by the four laws ...
, a spontaneous process is a process which occurs without any external input to the system. A more technical definition is the time-evolution of a
system A system is a group of interacting or interrelated elements that act according to a set of rules to form a unified whole. A system, surrounded and influenced by its environment, is described by its boundaries, structure and purpose and express ...
in which it releases free energy and it moves to a lower, more thermodynamically stable energy state (closer to
thermodynamic equilibrium Thermodynamic equilibrium is an axiomatic concept of thermodynamics. It is an internal state of a single thermodynamic system, or a relation between several thermodynamic systems connected by more or less permeable or impermeable walls. In the ...
).Entropy and Spontaneous Reactions
- ChemEd DL 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 surroundings. Depending on the nature of the process, the free energy is determined differently. For example, the
Gibbs free energy In thermodynamics, the Gibbs free energy (or Gibbs energy; symbol G) is a thermodynamic potential that can be used to calculate the maximum amount of work that may be performed by a thermodynamically closed system at constant temperature an ...
change is used when considering processes that occur under constant
pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country a ...
and
temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...
conditions, whereas the
Helmholtz free energy In thermodynamics, the Helmholtz free energy (or Helmholtz energy) is a thermodynamic potential that measures the useful work obtainable from a closed thermodynamic system at a constant temperature (isothermal). The change in the Helmholtz en ...
change is used when considering processes that occur under constant
volume Volume is a measure of occupied three-dimensional space. It is often quantified numerically using SI derived units (such as the cubic metre and litre) or by various imperial or US customary units (such as the gallon, quart, cubic inch). ...
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 In physical science, an isolated system is either of the following: # a physical system so far removed from other systems that it does not interact with them. # a thermodynamic system enclosed by rigid immovable walls through which neither ...
where no energy is exchanged with the surroundings, spontaneous processes are characterized by an increase in
entropy Entropy is a scientific concept, as well as a measurable physical property, that is most commonly associated with a state of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodyna ...
. A ''spontaneous reaction'' is a
chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and breaking ...
which is a spontaneous process under the conditions of interest.


Overview

In general, the spontaneity of a process only determines whether or not a process ''can'' occur and makes no indication as to whether or not the process ''will'' occur. In other words, spontaneity is a necessary, but not sufficient, condition for a process to actually occur. Furthermore, spontaneity makes no implication as to the speed at which as spontaneous may occur. As an example, the conversion of a diamond into graphite is a spontaneous process at room temperature and pressure. Despite being spontaneous, this process does not occur since the energy to break the strong carbon-carbon bonds is larger than the release in free energy.


Using free energy to determine spontaneity

For a process that occurs at constant temperature and pressure, spontaneity can be determined using the change in
Gibbs free energy In thermodynamics, the Gibbs free energy (or Gibbs energy; symbol G) is a thermodynamic potential that can be used to calculate the maximum amount of work that may be performed by a thermodynamically closed system at constant temperature an ...
, which is given by: \Delta G = \Delta H - T \Delta S \,, where the sign of Δ''G'' depends on the signs of the changes in
enthalpy Enthalpy , a property of a thermodynamic system, is the sum of the system's internal energy and the product of its pressure and volume. It is a state function used in many measurements in chemical, biological, and physical systems at a constant ...
(Δ''H'') and
entropy Entropy is a scientific concept, as well as a measurable physical property, that is most commonly associated with a state of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodyna ...
(Δ''S''). If these two signs are the same (both positive or both negative), then the sign of Δ''G'' will change from positive to negative (or vice versa) at the temperature In cases where Δ''G'' is: * negative, the process is spontaneous and may proceed in the forward direction as written. * positive, the process is non-spontaneous as written, but it may proceed spontaneously in the ''reverse direction''. * zero, the process is at equilibrium, with no net change taking place over time. This set of rules can be used to determine four distinct cases by examining the signs of the Δ''S'' and Δ''H''. * When Δ''S'' > 0 and Δ''H'' < 0, the process is always spontaneous as written. * When Δ''S'' < 0 and Δ''H'' > 0, the process is never spontaneous, but the reverse process is always spontaneous. * When Δ''S'' > 0 and Δ''H'' > 0, the process will be spontaneous at high temperatures and non-spontaneous at low temperatures. * When Δ''S'' < 0 and Δ''H'' < 0, the process will be spontaneous at low temperatures and non-spontaneous at high temperatures. For the latter two cases, the temperature at which the spontaneity changes will be determined by the relative magnitudes of Δ''S'' and Δ''H''.


Using entropy to determine spontaneity

When using the entropy change of a process to assess spontaneity, it is important to carefully consider the definition of the system and surroundings. The
second law of thermodynamics The second law of thermodynamics is a physical law based on universal experience concerning heat and energy interconversions. One simple statement of the law is that heat always moves from hotter objects to colder objects (or "downhill"), unle ...
states that a process involving an isolated system will be spontaneous if the entropy of the system increases over time. For open or closed systems, however, the statement must be modified to say that the total entropy of the ''combined'' system and surroundings must increase, or, \Delta S_\text = \Delta S_\text + \Delta S_\text \ge 0 \,. This criterion can then be used to explain how it is possible for the entropy of an open or closed system to decrease during a spontaneous process. A decrease in system entropy can only occur spontaneously if the entropy change of the surroundings is both positive in sign and has a larger magnitude than the entropy change of the system: \Delta S_\text > 0 and \left, \Delta S_\text\ > \left, \Delta S_\text{system}\ In many processes, the increase in entropy of the surroundings is accomplished via heat transfer from the system to the surroundings (i.e. an exothermic process).


See also

*
Endergonic reaction In chemical thermodynamics, an endergonic reaction (; also called a heat absorbing nonspontaneous reaction or an unfavorable reaction) is a chemical reaction in which the standard change in free energy is positive, and an additional driving fo ...
reactions which are not spontaneous at standard temperature, pressure, and concentrations. *
Diffusion Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical ...
spontaneous phenomenon that minimizes Gibbs free energy.


References

Thermodynamics Chemical thermodynamics Chemical processes