SILVER OXIDE (AG2O): STRUCTURE, PROPERTIES AND USES - CHEMISTRY - 2025

The silver oxide is an inorganic compound whose chemical formula is Ag ₂ O. The force binding the atoms is wholly ionic in nature; therefore, it consists of an ionic solid where there is a proportion of two Ag ⁺ cations interacting electrostatically with an anion O ^2-.

The oxide anion, O ^2-, results from the interaction of the silver atoms on the surface with the oxygen in the environment; in much the same way as iron and many other metals do. Instead of reddening and crumbling into rust, a piece or jewel of silver turns black, characteristic of silver oxide.

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For example, in the image above you can see an oxidized silver cup. Note its blackened surface, although it still retains some ornamental luster; which is why even oxidized silver objects can be considered attractive enough for decorative uses.

The properties of silver oxide are such that they do not, at first glance, eat away at the original metal surface. It is formed at room temperature by simple contact with oxygen in the air; and even more interesting, it can decompose at high temperatures (above 200 ° C).

This means that if the glass in the picture were grasped, and the heat of an intense flame were applied to it, it would regain its silver glow. Therefore, its formation is a thermodynamically reversible process.

Silver oxide also has other properties and, beyond its simple formula Ag ₂ O, encompasses complex structural organizations and a rich variety of solids. However, Ag ₂ O is perhaps, along with Ag ₂ O ₃, the most representative of the oxides of silver.

Silver oxide structure

Source: CCoil, from Wikimedia Commons

How is its structure? As mentioned at the beginning: it is an ionic solid. For this reason, there can be neither Ag-O nor Ag = O covalent bonds in its structure; since, if there were, the properties of this oxide would change drastically. It is then Ag ⁺ and O ^2- ions in a 2: 1 ratio and experiencing electrostatic attraction.

The structure of silver oxide is consequently determined by the way in which ionic forces arrange Ag ⁺ and O ^2- ions in space.

In the image above, for example, there is a unit cell for a cubic crystalline system: the Ag ⁺ cations are the silvery blue spheres, and the O ^{2 -} the reddish spheres.

If the number of spheres is counted, it will be found that there are, to the naked eye, nine silvery blue and four red. However, only the fragments of the spheres contained within the cube are considered; counting these, being fractions of the total spheres, the 2: 1 ratio for Ag ₂ O must be met.

By repeating the structural unit of the AgO ₄ tetrahedron surrounded by four other Ag ⁺, the entire black solid is built (ignoring the holes or irregularities that these crystalline arrangements may have).

Changes with the valence number

Focusing now not on the AgO ₄ tetrahedron but on the AgOAg line (observe the vertices of the upper cube), we will have that the silver oxide solid consists, from another perspective, of multiple layers of ions arranged linearly (although inclined). All this as a result of the "molecular" geometry around Ag ⁺.

This has been corroborated by several studies of its ionic structure.

Silver works predominantly with valence +1, since when losing an electron its resulting electronic configuration is 4d ¹⁰, which is very stable. Other valences, such as Ag ²⁺ and Ag ³⁺ are less stable as they lose electrons from nearly full d orbitals.

The Ag ³⁺ ion, however, is relatively less unstable compared to Ag ²⁺. In fact, it can coexist in the company of Ag ^+, chemically enriching the structure.

Its electronic configuration is 4d ⁸, with unpaired electrons in such a way that it gives it some stability.

Unlike the linear geometries around Ag ⁺ ions, it has been found that that of Ag ³⁺ ions is square plane. Therefore, a silver oxide with Ag ³⁺ ions would consist of layers composed of AgO ₄ squares (not tetrahedra) electrostatically linked by AgOAg lines; such is the case of Ag ₄ O ₄ or Ag ₂ O ∙ Ag ₂ O ₃ with monoclinic structure.

Physical and chemical properties

Source: Benjah-bmm27, from Wikimedia Commons

Scraping the surface of the silver cup in the main image would result in a solid, which is not only black in color, but also has shades of brown or brown (top image). Some of its physical and chemical properties reported at the moment are the following:

Molecular weight

231.735 g / mol

Appearance

Black-brown solid in powder form (note that despite being an ionic solid, it lacks a crystalline appearance). It is odorless and mixed with water gives it a metallic taste

Density

7.14 g / mL.

Melting point

277-300 ° C. Certainly it melts into solid silver; that is, it probably decomposes before forming the liquid oxide.

Kps

1.52 ∙ 10 ^-8 in water at 20 ° C. It is therefore a compound hardly soluble in water.

Solubility

If the image of its structure is carefully observed, it will be found that the Ag ²⁺ and O ^2- spheres do not differ almost in size. This has the consequence that only small molecules can pass through the interior of the crystal lattice, making it insoluble in almost all solvents; except for those where it reacts, such as bases and acids.

Covalent character

Although silver oxide has been repeatedly said to be an ionic compound, certain properties, such as its low melting point, contradict this statement.

Certainly, the consideration of the covalent character does not destroy what has been explained for its structure, since it would be enough to add a model of spheres and bars to the Ag ₂ O structure to indicate the covalent bonds.

Likewise, the tetrahedra and square AgO ₄ planes, as well as the AgOAg lines, would be linked by covalent bonds (or ionic covalent).

With this in mind, Ag ₂ O would actually be a polymer. However, it is recommended to consider it as an ionic solid with a covalent character (whose nature of the bond remains a challenge today).

Decomposition

At first it was mentioned that its formation is thermodynamically reversible, so it absorbs heat to return to its metallic state. All of this can be expressed by two chemical equations for such reactions:

4Ag (s) + O ₂ (g) => 2Ag ₂ O (s) + Q

2Ag ₂ O (s) + Q => 4Ag (s) + O ₂ (g)

Where Q represents heat in the equation. This explains why the fire burning the surface of the oxidized silver cup returns it to its silvery glow.

Therefore, it is difficult to assume that there is Ag ₂ O (l) since it would decompose instantly from heat; unless the pressure is raised too high to obtain said brown black liquid.

Nomenclature

When the possibility of Ag ²⁺ and Ag ³⁺ ions was introduced in addition to the common and predominant Ag ⁺, the term 'silver oxide' began to seem insufficient to refer to Ag ₂ O.

This is because the Ag ⁺ ion is more abundant than the others, so Ag ₂ O is taken as the only oxide; which is not quite correct.

If Ag ²⁺ is considered as practically non-existent given its instability, then only the ions with valences +1 and +3 will be had; that is, Ag (I) and Ag (III).

Valencias I and III

Since Ag (I) is the one with the lowest valence, it is named by adding the suffix –oso to its argentum name. Thus, Ag ₂ O is: silver oxide or, according to the systematic nomenclature, diplate monoxide.

If Ag (III) is completely ignored, then its traditional nomenclature should be: silver oxide instead of silver oxide.

On the other hand, Ag (III) being the highest valence, the suffix –ico is added to its name. Thus, Ag ₂ O ₃ is: silver oxide (2 Ag ³⁺ ions with three O ^2-). Also, its name according to the systematic nomenclature would be: diplata trioxide.

If the structure of Ag ₂ O ₃ is observed, it can be presumed that it is the product of oxidation by ozone, O ₃, instead of oxygen. Therefore, its covalent character must be greater as it is a covalent compound with Ag-OOO-Ag or Ag-O ₃ -Ag bonds.

Systematic nomenclature for complex silver oxides

AgO, also written as Ag ₄ O ₄ or Ag ₂ O ∙ Ag ₂ O ₃, is an oxide of silver (I, III), since it has both +1 and +3 valences. Its name according to the systematic nomenclature would be: tetraoxide of tetraplata.

This nomenclature is of great help when it comes to other stoichiometrically complex oxides of silver. For example, suppose the two solids 2Ag ₂ O ∙ Ag ₂ O ₃ and Ag ₂ O ∙ 3Ag ₂ O ₃.

Writing the first in a more appropriate way would be: Ag ₆ O ₅ (counting and adding the atoms of Ag and O). Its name would then be hexaplate pentoxide. Note that this oxide has a less rich silver composition than Ag ₂ O (6: 5 <2: 1).

While writing the second solid in another way, it would be: Ag ₈ O ₁₀. Its name would be octa silver decaoxide (with an 8:10 or 4: 5 ratio). This hypothetical silver oxide would be "very oxidized".

Applications

Studies in search of new and sophisticated uses for silver oxide continue to this day. Some of its uses are listed below:

-It dissolves in ammonia, ammonium nitrate and water to form the Tollens reagent. This reagent is a useful tool in qualitative analysis within organic chemistry laboratories. It allows determining the presence of aldehydes in a sample, with the formation of a "silver mirror" in the test tube as a positive response.

-Together with metallic zinc, it forms the primary zinc-silver oxide batteries. This is perhaps one of its most common and home uses.

-It serves as a gas purifier, absorbing for example CO ₂. When heated, it releases trapped gases and can be reused multiple times.

-Due to the antimicrobial properties of silver, its oxide is useful in bioanalysis and soil purification studies.

-It is a mild oxidizing agent capable of oxidizing aldehydes to carboxylic acids. Likewise, it is used in the Hofmann reaction (of tertiary amines) and participates in other organic reactions, either as a reagent or a catalyst.

References

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