Chemical stability

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Chemical stability means thermodynamic stability of a chemical system, especially a chemical compound .

States (chemical compounds) that can remain unchanged for "any length of time" are referred to as stable, otherwise as metastable states . Unstable (unstable, unstable) states are those in which a continuous transformation into states with a lower free energy content (or higher entropy) takes place. Examples are substances or mixtures of substances in which measurable reactions take place at constant temperature and constant pressure. Quasi-stable states are understood to be unstable states in which a forward and backward reaction can take place at the same time, since both states have the same free energy.

It should be noted that chemical reactions never run completely in a homogeneous phase, but only until a state of equilibrium is reached, in which, in addition to the reaction product, some of the reactants (starting materials) are also present. The same final state is reached when starting from the reaction product and subjecting it to the conditions of the formation reaction (same pressure and same temperature). Such chemical equilibria play an essential role in all areas of chemistry. If the starting materials are removed continuously in this situation, decomposition takes place.

The stability of chemical systems can be influenced , for example, by catalysts and stabilizers . With the latter you get a stabilized (not a stable!) System that collapses when the stabilizer is no longer available. By using stabilizers, chemical changes such as decomposition or oxidation can be prevented in the case of unstable compounds. Catalysts can be used to convert a metastable state into an unstable one or to change the reaction rate, but not to shift the position of the chemical equilibrium.

Pseudostable states are unstable states, which outwardly appear to be in equilibrium with themselves or their surroundings, but in which very slow, barely noticeable transformations into states with a lower content of free energy take place. Examples are lyophobic colloids , which coagulate slowly, and the extremely slow formation of water in mixtures of hydrogen and oxygen at room temperature. Such states are often thermally unstable . The metastable states are counted among such apparently stable equilibrium states.

A chemical system is called thermodynamically stable when it is in a state of lowest energy or in chemical equilibrium with its environment. This can also be a steady state . A state A is said to be thermodynamically more stable than state B if the standard free enthalpy is positive when changing from A to B.

For drugs , pesticides and environmental toxins, the half-life and metabolism (i.e. the breakdown of the compound) are of crucial importance.

Compounds such as glass , noble gases and noble metals are considered to be chemically stable or inert , but this depends on the existing conditions and substances. Glass and precious metals are attacked by some acids . Explosives and radicals , for example, are unstable .

According to REACH , safety data sheets must state whether the substance or mixture is stable or unstable under normal ambient conditions and under the temperature and pressure conditions expected during storage and handling. It must also be stated what significance any changes in the physical appearance of the substance or mixture have for safety.

See also

Individual evidence

  1. a b entry on stability. In: Römpp Online . Georg Thieme Verlag, accessed on May 25, 2014.
  2. a b c Gabriele Janssen: The safety data sheet according to REACH: Requirements for specialist knowledge ... Hüthig Jehle Rehm, 2011, ISBN 3-609-65125-3 , p. 38, 143 ( limited preview in Google Book search).
  3. a b Entry on chemical equilibria. In: Römpp Online . Georg Thieme Verlag, accessed on May 25, 2014.