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 spontaneous process ： ウィキペディア英語版
spontaneous process
A spontaneous process is the time-evolution of a system in which it releases free energy (usually as heat) and moves to a lower, more thermodynamically stable energy state.〔(Spontaneous process ) - Purdue University〕〔(Entropy and Spontaneous Reactions ) - ChemEd DL〕 The sign convention of changes in free energy follows the general convention for thermodynamic measurements, in which a release of free energy from the system corresponds to a negative change in free energy, but a positive change for the surroundings.
Depending on the nature of the process, the free energy is determined differently. For example, the Gibbs free energy is used when considering processes that occur under constant pressure and temperature conditions whereas the Helmholtz free energy is used when considering processes that occur under constant volume and temperature conditions.
A spontaneous process is capable of proceeding in a given direction, as written or described, without needing to be driven by an outside source of energy. The term is used to refer to macro processes in which entropy increases; such as a smell diffusing in a room, ice melting in lukewarm water, salt dissolving in water, and iron rusting.
The laws of thermodynamics govern the direction of a spontaneous process, ensuring that if a sufficiently large number of individual interactions (like atoms colliding) are involved then the direction will always be in the direction of increased entropy (since entropy increase is a statistical phenomenon).
==Overview==
For a reaction at standard temperature and pressure, Δ''G'' in the Gibbs free energy is:
:$\Delta G = \Delta H - T \Delta S \,$
The sign of Δ''G'' depends on the signs of the changes in Enthalpy (Δ''H'') and entropy (Δ''S''), as well as on the absolute temperature (''T'', in kelvin). Δ''G'' changes from positive to negative (or vice versa) where
''T'' = Δ''H''/Δ''S''.
For heterogeneous systems where all of the species of the reaction are in different phases and can be mechanically separated, the following is true.
When Δ''G'' is negative, a process or chemical proceeds spontaneously in the forward direction.
When Δ''G'' is positive, the process proceeds spontaneously in reverse.
When Δ''G'' is zero, the process is already in equilibrium, with no net change taking place over time.
We can further distinguish four cases within the above rule just by examining the signs of the two terms on the right side of the equation.
When Δ''S'' is positive and Δ''H'' is negative, a process is always spontaneous.
When Δ''S'' is positive and Δ''H'' is positive, the relative magnitudes of Δ''S'' and Δ''H'' determine if the reaction is spontaneous. High temperatures make the reaction more favorable.
When Δ''S'' is negative and Δ''H'' is negative, the relative magnitudes of Δ''S'' and Δ''H'' determine if the reaction is spontaneous. Low temperatures make the reaction more favorable.
When Δ''S'' is negative and Δ''H'' is positive, a process is not spontaneous at any temperature, but the reverse process is spontaneous.
For homogeneous systems where all of the species of the reaction are in the same phase, Δ''G'' cannot accurately predict reaction spontaneity.
The second law of thermodynamics states that for any spontaneous process the overall Δ''S'' must be greater than or equal to zero, yet a spontaneous chemical reaction can result in a negative change in entropy. This does not contradict the second law, however, since such a reaction must have a sufficiently large negative change in enthalpy (heat energy) that the increase in temperature of the reaction surroundings (considered to be part of the system in thermodynamic terms) results in a sufficiently large increase in entropy that overall the change in entropy is positive. That is, the Δ''S'' of the ''surroundings'' increases enough because of the exothermicity of the reaction that it overcompensates for the negative Δ''S'' of the system, and since the overall Δ''S'' = Δ''S''surroundings + Δ''S''system, the overall change in entropy is still positive.
Another way to view the fact that some spontaneous chemical reactions can lead to products with lower entropy is to realize that the second law states that entropy of an isolated system must increase (or remain constant). Since a negative enthalpy change in a reaction means that energy is being released to the surroundings, then the 'isolated' system includes the chemical reaction plus its surroundings. This means that the heat release of the chemical reaction sufficiently increases the entropy of the surroundings such that the overall entropy of the isolated system increases in accordance with the second law of thermodynamics.for a irreversible process change in entropy always >0
Spontaneity does not imply that the reaction proceeds with great speed. For example, the decay of diamonds into graphite is a spontaneous process that occurs very slowly, taking millions of years. The ''rate'' of a reaction is independent of its spontaneity, and instead depends on the chemical kinetics of the reaction. Every reactant in a spontaneous process has a tendency to form the corresponding product. This tendency is related to stability. Stability is gained by a substance if it is in a minimum energy state or is in maximum randomness. Only one of these can be applied at a time. e.g. Water converting to ice is a spontaneous process because ice is more stable since it is of lower energy. However, the formation of water is also a spontaneous process as water is the more random state.

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