Ostwald's rule of degrees

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The Ostwald step rule states that a system is not of an energy-rich state directly into the most energetically favorable state transitions, but usually one or more metastable intermediate stages occupies. The rule was established at the end of the 19th century by Wilhelm Ostwald .

The rule can be applied to chemical reactions as well as to physical processes. When crystallizing from the melt , substances do not immediately change into the most stable solid modification, but into one that is closest to the melt in terms of its free energy . The lower-energy modifications can be achieved by monotropic conversion. In addition, the Ostwald-Volmer rule often applies , according to which, in energetically similar systems, the modification with a lower density is preferred. So, for example, separates phosphorus during cooling from the vapor phase first metastable white phosphorus from that slowly transforms into the dense red phosphorus.

Ostwald's rule is not universally applicable. Under certain conditions, the possible intermediate stages are skipped and the most stable phase or the end product of a reaction is formed immediately.

More detailed research shows that i. d. Usually intermediate stages occur that are structurally obvious on the reaction path.

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