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

Period 3 chloride	NaCl	MgCl2	Al2Cl6	SiCl4	PCl5	SCl2
Chemical bonding	Ionic	Ionic	Covalent	Covalent	Covalent	Covalent
Structure	Giant ionic	Giant ionic	Simple molecular	Simple molecular	Simple molecular	Simple molecular
Observations	White solids dissolve to form colourless solutions		Chlorides react with water giving off white fumes of hydrogen chloride gas
pH of solution formed	7.0	6.5	3.0	2.0	2.0	2.0

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