Period 3 Chlorides (Cambridge International 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

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

Aluminium chloride

• Aluminium chloride exists in two different forms depending on conditions: #### The two forms of aluminium chloride

  • Anhydrous AlCl₃ is covalent and exists as a dimer, Al₂Cl₆
  • In aqueous solution, AlCl₃ dissociates into Al³⁺ and Cl^- ions and behaves ionically

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 hydrogen 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