The tin chloride (II) or stannous chloride, chemical formula SnCl _2, is a white crystalline solid compound, reaction product of tin and concentrated hydrochloric acid solution: Sn (s) + 2HCl (conc) => SnCl ₂ (aq) + H ₂ (g). The process of its synthesis (preparation) consists of adding pieces of tin filed so that they react with the acid.

After adding the pieces of tin, dehydration and crystallization are carried out until the inorganic salt is obtained. In this compound, tin has lost two electrons from its valence shell to form bonds with chlorine atoms.

This can be better understood if the valence configuration of tin is considered (5s ² 5p _x ² p _y ⁰ p _z ⁰), of which the pair of electrons occupying the p _x orbital is transferred to the H ⁺ protons, thus forming a diatomic molecule of hydrogen. That is, this is a redox-type reaction.

Physical and chemical properties

Are the SnCl ₂ bonds ionic or covalent? The physical properties of tin (II) chloride rule out the first option. The melting and boiling points for this compound are 247 ° C and 623 ° C, indicative of weak intermolecular interactions, a common fact for covalent compounds.

Its crystals are white, which translates into zero absorption in the visible spectrum.

Valencia configuration

In the image above, in the upper left corner, an isolated SnCl ₂ molecule is illustrated.

The molecular geometry should be flat because the hybridization of the central atom is sp ² (3 sp ² orbitals and a pure p orbital to form covalent bonds), but the free pair of electrons occupies volume and pushes the chlorine atoms down, giving the molecule an angular geometry.

In the gas phase, this compound is isolated, so it does not interact with other molecules.

As the loss of the pair of electrons in the p _x orbital, tin is transformed into the Sn ²⁺ ion and its resulting electron configuration is 5s ² 5p _x ⁰ p _y ⁰ p _z ⁰, with all its p orbitals available to accept bonds of other species.

The Cl ^- ions coordinate with the Sn ²⁺ ion to give rise to tin chloride. The electron configuration of tin in this salt is 5s ² 5p _x ² p _y ² p _z ⁰, being able to accept another pair of electrons in its free p _z orbital .

For example, it can accept another ion Cl ^-, forming the complex of trigonal plane geometry (a pyramid with a triangular base) and negatively charged ^-.

Reactivity

SnCl ₂ has high reactivity and a tendency to behave like Lewis acid (electron acceptor) to complete its valence octet.

Just as it accepts a Cl ^- ion, the same happens with water, which "hydrates" the tin atom by binding a water molecule directly to the tin, and a second water molecule forms hydrogen bond interactions with the first.

The result of this is that SnCl _{2 is} not pure, but coordinated with water in its dihydrated salt: SnCl ₂ · 2H ₂ O.

SnCl ₂ is very soluble in water and in polar solvents, because it is a polar compound. However, its solubility in water, less than its weight by mass, activates a hydrolysis reaction (breakdown of a water molecule) to generate a basic and insoluble salt:

SnCl ₂ (aq) + H ₂ O (l) <=> Sn (OH) Cl (s) + HCl (aq)

The double arrow indicates that an equilibrium is established, favored to the left (towards the reactants) if the HCl concentrations increase. For this reason, the SnCl ₂ solutions used have an acid pH, to avoid the precipitation of the unwanted salt product of the hydrolysis.

Reducing activity

Reacts with oxygen in the air to form tin (IV) chloride or stannic chloride:

6 SnCl ₂ (aq) + O ₂ (g) + 2H ₂ O (l) => 2SnCl ₄ (aq) + 4Sn (OH) Cl (s)

In this reaction, tin is oxidized, forming a bond with the electronegative oxygen atom and its number of bonds with the chlorine atoms increases.

In general, the electronegative atoms of halogens (F, Cl, Br and I) stabilize the bonds of Sn (IV) compounds and this fact explains why SnCl ₂ is a reducing agent.

When it is oxidized and loses all its valence electrons, the Sn ⁴⁺ ion is left with a 5s ⁰ 5p _x ⁰ p _y ⁰ p _z ⁰ configuration, the pair of electrons in the 5s orbital being the most difficult to be "snatched".

Chemical structure

Riesgos

El SnCl₂ puede dañar las células blancas de la sangre. Es corrosivo, irritante, cancerígeno, y tiene altos impactos negativos en las especies que habitan los ecosistemas marinos.

Puede descomponerse a altas temperaturas, liberando el nocivo gas cloro. En contacto con agentes muy oxidantes desencadena reacciones explosivas.

Referencias

1. Shiver & Atkins. (2008). Química Inorgánica. En Los elementos del grupo 14 (cuarta edición., pág. 329). Mc Graw Hill.
2. ChemicalBook. (2017). Recuperado el 21 de marzo de 2018, de ChemicalBook: chemicalbook.com
3. PubChem. (2018). Tin Chloride. Recuperado el 21 de marzo de 2018, de PubChem: pubchem.ncbi.nlm.nih.gov
4. Wikipedia. (2017). Tin(II) chloride. Recuperado el 21 de marzo de 2018, de Wikipedia: en.wikipedia.org
5. E. G. Rochow, E. W. (1975). The Chemistry of Germanium: Tin and Lead (first ed.). p-82,83. Pergamom Press.
6. F. Hulliger. (1976). Structural Chemistry of Layer-Type Phases. P-120,121. D. Reidel Publishing Company.