Chemical stability
Chemical stability means thermodynamic stability of a chemical system, especially of a chemical compound.
States (chemical compounds) that can exist unchanged for "any length of time" are called stable, otherwise metastable states. Unstable (non-stable, unstable) states are those in which a continuous transformation into states with 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 pressure. Quasi-stable states are unstable states in which an outward and a reverse reaction can take place simultaneously, since both states have the same free energy.
It should be noted that chemical reactions in homogeneous phase never proceed completely, but only until a state of equilibrium is reached in which, in addition to the reaction product, the reactants (starting materials) are also partially present. The same final state is reached by starting from the reaction product and subjecting it to the conditions of the formation reaction (same pressure and temperature). Such chemical equilibria play an essential role in all areas of chemistry. If one continuously removes the starting materials in this situation, decomposition thus takes place.
The stability of chemical systems can be influenced, for example, by catalysts and stabilizers. With the latter, one obtains a stabilized (not stable!) system, which breaks down with the removal of the stabilizer. By using stabilizers, chemical changes such as decomposition or oxidation can be prevented in unstable compounds. Catalysts can be used to change a metastable state to an unstable one or to change the reaction rate, but not to shift the position of the chemical equilibrium.
Pseudo-stable states are unstable states that outwardly appear to be in equilibrium with themselves or their environment, but nevertheless undergo very slow, barely noticeable transformations into states with lower free energy content. Examples include lyophobic colloids that coagulate slowly and the extremely slow formation of water in mixtures of hydrogen and oxygen at room temperature. Such states are often thermolabile. Such apparently stable equilibrium states include metastable states.
A chemical system is called thermodynamically stable if 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 called thermodynamically more stable than state B if the free standard 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 chemically stable or inert, but this depends on the conditions and substances present. For example, glass and precious metals are attacked by some acids. For example, explosives and radicals are unstable.
REACH requires safety data sheets to 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. The relevance to safety of any changes in the physical appearance of the substance or mixture must also be stated.
See also
- Reactivity
- Stabiliser (food additive)
- Phlegmatize
- Corrosion
- Weathering
