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Silver chloride (AgCl): structure, properties, uses

Chlorine is a slightly irritating greenish-yellow gas with an unpleasant odor. Its chemical symbol is Cl. To form chemical compounds with metals, chlorine is reduced (it gains one electron to make eight electrons at its last energy level) to its negatively charged chloride anion.

Being in ionic form, both elements can form the compound silver chloride, either naturally —as it can be found in some deposits— or by chemical synthesis, which is less expensive to obtain.

Silver chloride occurs natively as chlorargyrite (“chlorine” for chlorine, “argyr” for argentum). The ending “ite” indicates a mineral name.

The synthetically obtained silver chloride appears as white crystals very similar to the cubic form of sodium chloride, although as a whole it would appear as a white powder.

How to obtain silver chloride?

Silver chloride
Chemical structure of silver chloride

In the laboratory it can be easily obtained as follows:

Silver nitrate is reacted with sodium chloride and silver chloride is produced, which precipitates as indicated by the arrow, downward, and the sodium nitrate dissolves in water.

AgNO 3 (aq) + NaCl (aq)   ————> AgCl (s) + NaNO 3 (aq)

Dissociation

Dissociation in chemistry refers to the possibility that an ionic substance can separate into its components or ions when it encounters a substance that allows such separation.

Silver chloride is called the haloid salt, because it is formed with the element chlorine that corresponds to the VIIA family of the periodic table, called halogens. Haloid salts are ionic compounds, mostly poorly soluble in water.

Low dissociation in water

AgCl, which belongs to this type of compound, has a very low dissociation in water. This behavior can occur for the following reasons:

– When the AgCl is formed, it is in a colloidal state that allows that, when the molecule dissociates into its silver (+) and chlorine (-) ions, the original molecule of silver chloride AgCl is immediately re-formed, establishing a dynamic equilibrium between these (dissociated product and neutral molecule).

– Due to the molecular stability of AgCl, when the bond is formed, its strength tends to be more covalent than ionic, creating resistance to dissociation.

– The density of silver, which is much higher than that of chlorine, and it is silver that makes the dissociation less and increases the precipitation of AgCl in solution.

One of the factors that affects the solubility of a substance is temperature. By heating a substance dissolved in water, the solubility increases and, therefore, the dissociation of its components is easier. However, in the face of heat, AgCl undergoes decomposition into Ag and Cl gas.

Silver Chloride Properties

Silver chloride appearance. Source: Vano3333, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons

Physical properties

They are the characteristics that a substance has and that allow it to be identified and differentiated from the others. These properties do not alter the internal structure of the substance; that is, they do not alter the arrangement of the atoms in the formula.

Silver chloride is a solid, odorless, white crystalline color and in its purest form it has an octahedron-shaped geometry. The main physical properties are described below:

– Melting point: 455 ° C

– Boiling point : 1547 ° C

– Density: 5.56 g / mL

– Molar mass: 143.32 g / mol.

When it is found as chlorargyrite (mineral) it has a solid appearance and can be colorless, green-yellow, green-gray, or white, depending on the place and the substances that are around it. It has a hardness on the Mohs scale of 1.5 to 2.5.

It is also considered luster, adamantine (diamond), resinous and silky. This refers to a somewhat shiny appearance.

Chemical properties

It is the reactivity of a chemical substance when it is in contact with another. In this case, its internal structure is not preserved, so the atomic arrangement within the formula changes.

Decomposition with heat or light

Silver chloride breaks down into its elements.

(Light) 2 AgCl (s) ——————->     2 Ag (s) + Cl 2 (g) (Heat)

Silver Precipitation

Silver precipitation is the best way to extract this element from photographic and radiographic films.

AgCl (aq) + NaClO (aq)  ————–> Ag (s) + NaCl ( aq) + CL 2 O (g)

Solubility

Paw chloride is highly insoluble in water, but it is soluble in low molecular weight alcohols (methanol and ethanol), in ammonia, and in concentrated sulfuric acid.

Uses / applications

Photography

Silver chloride is used due to its high sensitivity to light. This process was discovered by William Henry Fox Talbot in 1834.

Gravimetry

Gravimetric analysis consists of finding the amount of an element, radical or compound, that is contained in a sample. For this, it is necessary to remove all substances that may present interference and convert the substance under study into a substance of defined composition that can be weighed.

This is obtained with the help of substances that can easily precipitate in aqueous medium, such as AgCl.

Water analysis

This process is carried out through a titration that is done, using AgNO3 as a titrant and an indicator that determines the end of the reaction (color change); that is, when there are no more chlorides in the water.

This reaction leads to the precipitation of AgCl, due to the affinity that the chloride ion has for the silver cation.

Volumetry

It is the assessment of a sample of unknown concentration (chlorides or bromides). To find the concentration of the sample, it is reacted with a substance; the end point of the reaction is recognized by the formation of a precipitate. In the case of chlorides, it would be silver chloride.

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