Chlorides

Chlorides are compounds formed between chlorine and another element. They are binary compounds meaning chlorine is combined with only one other elements.

Chlorine is a highly reactive element that forms compounds easily with other elements, often by direct combination. Chlorine is an oxidising agent, helping to stabilize the higher oxidation states of the other combining element. For example, chlorine reacts directly with hot iron forming iron(III) chloride, rather than iron(II) chloride.

Chlorides may be ionic or covalent. In ionic chlorides, the chlorine is present as chloride ions. In covalent chlorides, the chlorine atom (or atoms) is covalently bonded by a shared pair of electrons to the other element.

Period 3 chlorides are studied as they exemplify the range of chloride types found throughout the periodic table.

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Sodium chloride - NaCl

Sodium chloride is a white crystalline solid that dissolves in water, dissociating into ions.

NaCl + nH2O Na+(aq) + Cl-(aq)

This is not a chemical reaction as understood by the term, as the sodium chloride can reform into solid crystals by evaporating the water. The sodium chloride solution formed in this process is neutral, pH=7, as sodium chloride is the salt of a strong acid and a strong base.

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Magnesium chloride - MgCl₂

Magnesium chloride is a white crystalline solid that dissolves in water, dissociating into ions.

MgCl2 + nH2O Mg2+(aq) + 2Cl-(aq)

The solution formed is slightly acidic by hydrolysis. The magnesium ion is small and double charged giving it a fairly high charge density. It is able to polarise water molecules releasing free hydrogen ions in solution. It is not possible to recrystallise magnesium chloride due to the high charge density of the magnesium ion. If crystallisation is attempted the final solid contains some basic magnesium chloride Mg(OH)Cl

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Aluminium chloride - Al₂Cl₆

Aluminium chloride is wrtten both as AlCl₃ and the dimeric form Al₂Cl₆. It is a white covalent compound that exists in an ionic form at low temperatures and as a dimeric molecule even in the vapour state. This double identity demonstrates that aluminium lies on the borderline between metallic behaviour and covalent behaviour.

Aluminium chloride is hydrolysed (broken down by water) to give an acidic solution containing the complex species aluminium hexaaqua 3+.

AlCl3 + 6H2O [Al(H2O)6]3+ + 3Cl-

The acidity is caused by the high charge density aluminium 3+ ion polarising the water ligands (attached molecules), releasing free hydrogen ions into solution:

[Al(H2O)6]3+ [Al(H2O)5OH]2+ + H+

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Silicon tetrachloride - SiCl₄

Silicon tetrachloride, unlike carbon tetrachloride, reacts very rapidly with water. It is hydrolysed forming an acidic solution containing silicic acid and hydorchloric acid.

SiCl4 + 3H2O Si(OH)4 + 4HCl

Silicon tetrachloride + water silicic acid + hydrogen chloride

The reaction is usually violent enough to produce fumes of hydrogen chloride, which would dissolve forming hydrochloric acid. Interestingly silicic acid is usually written as though it were a base - Si(OH)₄ instead of H₄SiO₄!

There is no real reason for this, just tradition. However it does give a better idea of the tetrahedral structure of the molecule.

Silicon tetrachloride reacts in this way because of the availability of empty 'd' orbitals (silicon is in the 3rd period), which can accept electron pairs from the incoming water molecules to initiate the reaction. This cannot happen with carbon tetrachloride, as carbon has no available 'd' orbitals.

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Phosphorus chloride - PCl₃ & PCl₅

Both of the phosphorus chlorides react with water giving acidic solutions.

PCl3 + 3H2O H3PO3 + 3HCl

phosphorus(III) chloride + water phosphoric(III) acid + hydrochloric acid

PCl5 + 4H2O H3PO4 + 5HCl

phosphorus(V) chloride + water phosphoric(V) acid + hydrochloric acid

In both cases the reaction is fairly rapid and releases a large amount of energy.

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Sulfur chloride

Sulfur chlorides are not on the IB Syllabus.

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Chlorine - Cl₂

Chlorine undergoes an interesting reaction with water, it disproportionates. This means that it gets simultaneously oxidised and reduced! This is best understood by looking at the equation for the reaction:

Cl2 + H2O ClOH + HCl

The oxidation state of chlorine in ClOH is I, and the oxidation state of chlorine in HCl is -I. One of the atoms from the chlorine molecule is reduced from 0 to -I and the other is oxidised from 0 to +I.

The ClOH is called chloric(I) acid, or hypochlorous acid. It is the active chemical in bleach. For this reason chlorine first turns indicator paper red due to the acidity of the HCl in the solution and then the indicator paper gets bleached (turns white) as the ClOH removes the colour from the vegetable dyes in the indicator.

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Summary of period 3 chlorides

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