Period 3 Chlorides (CIE A Level Chemistry)

• 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

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 as a giant lattice and with ionic bonds
  • Al₂Cl₆ as a dimer with covalent bonds

The two forms in which aluminium chloride exists

• 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

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