Period 3 Chlorides (Cambridge (CIE) AS Chemistry)

Reaction of Period 3 Chlorides & Water

• Chlorides of Period 3 elements show characteristic behaviour when added to water which can be explained by looking at their chemical bonding and structure

Chemical bonding & structure of Period 3 chlorides table

Sodium & magnesium chloride

• NaCl and MgCl₂ do not react with water as the polar water molecules are attracted to the ions dissolving the chlorides and breaking down the giant ionic structures: the metal and chloride ions become hydrated ions

How the giant ionic structure of NaCl and MgCl₂ break down in water

The diagram shows water molecules breaking down the giant ionic structure of NaCl and MgCl₂ to form hydrated ions

Aluminium chloride

• Aluminium chloride exists in two forms:

  • AlCl₃ is a giant lattice with ionic bonds

  • Al₂Cl₆ is a dimer with covalent bonds

  The two forms of aluminium chloride

Aluminium chloride exists as a giant ionic lattice or a covalent dimer

• When water is added to aluminium chloride the dimers are broken down and Al³⁺ and Cl^- ions enter the solution

• The highly charged Al³⁺ ion becomes hydrated and causes a water molecule that is bonded to the Al³⁺ to lose an H⁺ ion which turns the solution acidic

• The H⁺ and the Cl^- form hydrogen chloride gas which is given off as white fumes

How the Al³⁺ makes an acidic solution

The hydrated aluminium causes a water molecule to lose a H⁺ ion turning the solution acidic

Silicon chloride

• SiCl₄ is hydrolysed in water, releasing white fumes of hydrogen chloride gas in a rapid reaction

SiCl₄ (l) + 2H₂O (l) → SiO₂ (s) + 4HCl (g)

• The SiO₂ is seen as a white precipitate and some of the hydrogen chloride gas produced dissolves in water to form an acidic solution

Phosphorus(V) chloride

• PCl₅ also gets hydrolysed in water

PCl₅ (s) + 4H₂O (l) → H₃PO₄ (aq) + 5HCl (g)

• Both H₃PO₄ and dissolved HCl are highly acidic