CIE AS Chemistry (9701) exams from 2022

Revision Notes

2.1.5 Period 3 Chlorides

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

The Periodic Table - Table 1_Reaction of Period 3 Chlorides & Water, downloadable AS & A Level Chemistry revision notes

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 Periodic Table - Hydration of Sodium Chloride, downloadable AS & A Level Chemistry revision notes

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 Periodic Table - Aluminium Chloride, downloadable AS & A Level Chemistry revision notes

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 Periodic Table - Aluminium Hydration, downloadable AS & A Level Chemistry revision notes

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

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