10.4    Period 3 Chlorides

Summary of  the properties of  Period 3 chlorides

Reaction with water, H2O

• Sodium chloride dissolves in water to form a neutral solution of sodium chloride.

NaCl(s) + aq → Na⁺(aq) + Cl⁻(aq)      ; pH = 7

• Magnesium chloride dissolves in water with slight hydrolysis to form a solution of magnesium chloride

MgCl2(s) + 6H2O → [Mg(H2O)6]²⁺(aq) + 2Cl⁻(aq)

                                                                                                                                     hexaaquamagnesium ions

[Mg(H2O)6]²⁺(aq) +H2O(l) ⇌ [Mg(H2O)5(OH)]⁺(aq) + H3O⁺(aq)   ; pH = 6.5

• Aluminium chloride, AlCl3 hydrolyses in water to give an acidic solution, white fumes of hydrogen chloride gas are formed

AlCl3(s) + 6H2O(l) →  [Al(H2O)6]³⁺(aq) + 3Cl⁻(aq) [Al(H2O)6]³⁺(aq) +H2O(l) ⇌ [Al(H2O)5(OH)]²⁺(aq) + H3O⁺(aq)                                           ; pH = 3

• Silicon tetrachloride undergoes complete hydrolysis in water to form a strongly acidic solution, white fumes of hydrogen chloride gas are formed

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

• Phosphorus(III) chloride reacts violently with water in a hydrolysis reaction to give a strongly acidic solution and fumes of hydrogen chloride gas

PCl3(l) + 3H2O(l) → H3PO3(aq) + 3HCl(g)    ; pH = 2

Phosphorus(V) chloride is an off-white ionic solid at room temperature and sublimes at 163 °C to give phosphorus(III) chloride and chlorine gas.

PCl5(s) ⇌ PCl3(l) + Cl2(g)

It reacts violently with water in a hydrolysis reaction to give a strongly acidic solution and fumes of  hydrogen chloride gas.

PCl5(s) + 4H2O(l) → H3PO4(aq) + 5HCl(g)     ; pH = 2

• The acidity of the chlorides increases across the Period as the nature of the chlorides changes from ionic to covalent