Reaction Of Period 3 Chlorides With Water

NaCl

Dissolves in water (not a reaction) to give neutral solution pH=7

NaCl Equation

NaCl (aq) → Na+(aq) + Cl-(aq)

(do not: NaCl + H2O as they DO NOT react)

MgCl2

Hydrolyses slightly in water to give slightly acidic solution (pH=6.5)

MgCl reaction with water

• Dissociate completely in water: MgCl2 (s) + 6H2O (l) → [Mg(H2O)2)]2+(aq) + 2Cl-(aq)

• [Mg(H2O)2)]2+ is a weak acid that slightly hydrolyses in water: [Mg(H2O)2)]2+ + H2O ⇌ [Mg(H2O)5(OH)]+(aq) + H3O+(aq)

AlCl3

hydrolyses in water to give acidic solution (pH=3)

AlCl3 reaction with water

• AlCl3 dissolving in water to form ions: AlCl3(s) + 6H2O(l) → [Al(H2O)6]3+(aq) + 3Cl-(aq)

• [Al(H2O)6]3+ is a weak acid

• Small highly polarising Al3+ cation weakens the O-H bonds in water and breaks the bonds to release the H+ ions in the solution

• [Al(H2O)6]3+ undergoes hydrolysis to give H3O+: [Al(H2O)6]3+(aq) + H2O(l) ⇌ [Al(H2O)5(OH)]2+ + H3O+

AlCl3 in LIMITED water

AlCl3(s) + 3H2O(l) → Al(OH)3(s) + 3HCl(g)

SiCl4 & PCl5

Hydrolysed completely in water to give acidic solution (pH=2)and gaseous HCl gas

PCl5 in LIMITED/COLD water

PCl5 + H2O → POCl3 + 2HCl

(basically just make sure HCl is evolved & use whatever that’s left to write the remaining salt)

PCl5 when MORE water is added

Formation of H3PO4 acid

POCl3 + 3H2O → H3PO4 + 3HCl