Silicon tetrachloride, SiCl4 reactions, Phosphorus(V) chloride (phosphorus pentachloride), PCl5,Phosphorus(III) chloride (phosphorus trichloride), PCl3

Silicon tetrachloride, SiCl₄

Silicon tetrachloride is a simple no-messing-about covalent chloride. There isn't enough electronegativity difference between the silicon and the chlorine for the two to form ionic bonds.

 

Silicon tetrachloride is a colourless liquid at room temperature which fumes in moist air. The only attractions between the molecules are van der Waals dispersion forces.

It doesn't conduct electricity because of the lack of ions or mobile electrons.

It fumes in moist air because it reacts with water in the air to produce hydrogen chloride. If you add water to silicon tetrachloride, there is a violent reaction to produce silicon dioxide and fumes of hydrogen chloride. In a large excess of water, the hydrogen chloride will, of course, dissolve to give a strongly acidic solution containing hydrochloric acid.

The chlorides

There are two phosphorus chlorides - phosphorus(III) chloride, PCl₃, and phosphorus(V) chloride, PCl₅.

Phosphorus(III) chloride (phosphorus trichloride), PCl₃

This is another simple covalent chloride - again a fuming liquid at room temperature.

It is a liquid because there are only van der Waals dispersion forces and dipole-dipole attractions between the molecules.

doesn't conduct electricity because of the lack of ions or mobile electrons.

Phosphorus(III) chloride reacts violently with water. You get phosphorous acid, H₃PO₃, and fumes of hydrogen chloride (or a solution containing hydrochloric acid if lots of water is used).

Phosphorus(V) chloride (phosphorus pentachloride), PCl₅

Unfortunately, phosphorus(V) chloride is structurally more complicated.

Phosphorus(V) chloride is a white solid which sublimes at 163°C. The higher the temperature goes above that, the more the phosphorus(V) chloride dissociates (splits up reversibly) to give phosphorus(III) chloride and chlorine.

Solid phosphorus(V) chloride contains ions - which is why it is a solid at room temperature. The formation of the ions involves two molecules of PCl₅.

A chloride ion transfers from one of the original molecules to the other, leaving a positive ion, [PCl₄]⁺, and a negative ion, [PCl₆]^-.

At 163°C, the phosphorus(V) chloride converts to a simple molecular form containing PCl₅ molecules. Because there are only van der Waals dispersion forces between these, it then vaporises.

Solid phosphorus(V) chloride doesn't conduct electricity because the ions aren't free to move.

Phosphorus(V) chloride has a violent reaction with water producing fumes of hydrogen chloride. As with the other covalent chlorides, if there is enough water present, these will dissolve to give a solution containing hydrochloric acid.

The reaction happens in two stages. In the first, with cold water, phosphorus oxychloride, POCl₃, is produced along with HCl.

If the water is boiling, the phosphorus(V) chloride reacts further to give phosphoric(V) acid and more HCl. Phosphoric(V) acid is also known just as phosphoric acid or as orthophosphoric acid.

The overall equation in boiling water is just a combination of these: