por What is chemical inertness?
Chemical inertness is the property that a substance or material has to resist degradation caused by external agents. In this sense, its physical properties, and especially the chemical ones, remain unchanged. There are no bond breaks or new ones formed.
Now, chemical inertness is relative. Some substances or materials are more inert than others, which is due to the nature and strength of their interactions. Such a quality could, in principle, be opposed to the phenomena of change, essential for matter to evolve into various products.
That is why, no matter how inert a substance or material is, there will always be a condition under which it becomes reactive. For example, gold is the noblest of metals and is considered inert. However, it is attacked and dissolved by aqua regia, a solution towards which it is highly reactive.
Perhaps, and to date, the only chemical element that has shown absolute chemical inertness is neon. No compound is known of it, not even under ultra-pressure conditions, such as those found in the nuclei of planets or moons.
Characteristics of chemical inertness
lack of oxidation
For a material or substance to be inert, in principle, it must not react with the air that surrounds it. This means that it does not tend to form bonds with the oxygen or nitrogen molecules that surround its surface. In other words: it does not oxidize on exposure to air.
Food and all organic substances in question tend to oxidize. Therefore, it is said that they are not inert.
The lack of oxidation in chemical inertness must be maintained at temperatures higher than 100 ºC. The higher the temperature, the substances or materials will begin to oxidize more quickly, reacting with oxygen or nitrogen in the air to form oxides or nitrides, respectively.
Resistance to acids or alkalis
Another feature present in chemical inertness is resistance to acids or bases. This means that an inert substance or material should resist attack by acids, with no tendency to degrade due to the acceptance of H+ ions or very strong electrophiles; or the attack of the bases, without degrading due to the OH– ions.
Again, this is relative, as there are different types of acids and bases. Some inert substances can be very resistant to, say, sulfuric acid, but degrade at the slightest drop of hydrofluoric acid instead. Such is the case of glass bottles when they react with HF.
electronic stability
The previous characteristics have to do with the thermodynamic stability derived from the nature of intermolecular interactions, in addition to other factors. Instead, chemical inertness is also glimpsed in the electronic characteristics of the same atoms.
The more stable the electronic configuration of an atom, the less its tendency to gain or lose electrons. Therefore, it will exhibit greater chemical inertness. This is the case of noble gases, which will be seen in the next section.
bioinactivity
In medicine, a substance or material is inert if it lacks bioactivity. That is, it can be placed inside an organism without being assimilated during its metabolism. This feature is highly desirable in bone prostheses, or in tissue reconstruction.
radiation resistance
Finally, inert substances or materials must also be resistant to radiation, be it ultraviolet or nuclear.
Examples of chemical inertness
Glass
Examples of materials that exhibit chemical inertness include glass. If they were not inert, they would not be used to make containers or containers, as they would react with their content. Depending on their composition, like those of borosilicates, they can become very resistant to corrosion and temperature.
However, as mentioned at the beginning, glass is not immune to all substances: it reacts with HF, even dilute, hot alkalis, such as NaOH, and hot and highly concentrated H3PO4.
plastics
Plastics also perform similar functions to glass, but are much more versatile (they don’t break as easily). Some plastics, such as Teflon (polytetrafluoroethylene), Kynar (polyvinylidene fluoride), and Telene (polydicyclopentadiene), are extremely resistant to acid attack and corrosion.
ceramics
Inert ceramics go one step further than plastics. They are intended for applications where high temperatures predominate, quite common in the automotive and aerospace industries; or in biochemical systems, as occurs in the pharmaceutical industry and in the implementation of bone prostheses.
Among some of these ceramics with great chemical inertness we have: alumina (Al2O3, present in corundum and sapphire), silicates (specialized glasses), silicon carbide (SiC, hard and tenacious), and zirconia (ZiO2).
inert gases
Leaving inert materials aside, we now have inert substances. Inert gases are not very reactive, so their presence in the air does not pose any risk of reaction under normal conditions.
Among these gases we have CO2, CO and N2. Nitrogen is the most inert of all these gases; and yet, it is capable of reacting hot with some metals to form nitrides, M3Nn, being no the valence or oxidation state of the metal.
CO2 is relatively inert; except when it meets alkaline solutions, where it is transformed into carbonates, or in the presence of the enzymes carbonic anhydrases.
For its part, CO remains inert at room temperature; but at high temperatures it reacts with carbon, water vapor, metal oxides, olefins, among other compounds.
Such reactions can proceed in the presence of metal catalysts. Likewise, CO, even without breaking its covalent bonds, is capable of coordinating to neutral metal atoms.
noble metals
Noble metals are the most resistant metals to corrosion and attack by acids and alkalis. Each, at high temperatures, or in powder form, will react with oxygen or fluorine. Therefore, the chemical inertness of these elements is quite relative.
Among the noble metals we have: gold (Au), ruthenium (Ru), platinum (Pt), palladium (Pd), osmium (Os) and iridium (Ir). Of all of them, gold is the most noble, being found even in a metallic state in the earth’s crust.
Noble gases
And finally, at the highest level of chemical inertness, we have the noble gases: helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe) and radon (Rn). All of them are extremely inert. However, many xenon compounds have been synthesized, including salts known as perxenates, with the anion XeO64-.
Its chemical inertness is due to the fact that its atoms contain their energy shells and orbitals completely filled with electrons. From argon, it is possible that under ultrapressures they agree to receive electrons using empty orbitals of more energetic layers (3d and 4s, for example); something that is impossible for helium or neon.
Of the noble gases, precisely helium and neon are the most inert. Helium is capable of forming compounds with sodium at very high pressures (HeNa).
Meanwhile, neon is not known to have any compound at all, being even more inert than helium itself because of its greater effective nuclear charge, which strongly repels any atom that tries to approach neon atoms.
References
Whitten, Davis, Peck & Stanley. (2008). Chemistry. (8th ed.). CENGAGE Learning.
Shiver & Atkins. (2008). Inorganic Chemistryica. (Fourth edition). Mc Graw Hill.
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Clara Moskowitz. (March 20, 2018). A Noble Gas Surprise: Helium Can Form Weird Compounds. Retrieved from: scientificamerican.com
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