In some cases the symbol ‘aq’ is used on the left-hand side of an equation to represent a large volume of solvent water.

 

1. Chlorides that dissolve in water without hydrolysis.

NaCl(s)  +  aq

à

Na+ (aq)  +  Cl – (aq)

MgCl2 (aq)  +  aq

à

Mg2+ (aq) +  2Cl – (aq)

 

2. Chlorides that hydrolyse in water.

Aluminium chloride is different from the other chlorides of period 3; it dissolves as ions, the acidity coming not from the presence of HCl but from the interaction of the hexaqua-aluminium ion with water in a deprotonation reaction similar to that which makes solutions of transition metal aqua ions acidic:

AlCl3 (s)  +  6H2O (l)  + aq

à

[Al(H2O)6]3+ (aq)  +  3Cl – (aq)

[Al(H2O)6]3+ (aq) +  H2O (l)

à

[Al(H2O)5 (OH)]2+ (aq)  +  H3O+ (aq)

The remaining chlorides of period 3 hydrolyse to give HCl as one of the products. All react with cold water; the phosphorus chlorides react violently. The HCl is shown in solution in the equations below, but some of it will invariably be evolved as misty acidic fumes, especially if only a small amount of water is used.

SiCl4 (l)  +  2H2O (l)

à

SiO2 (s)  +  4H+ (aq)  +  4Cl – (aq)

 

 

 

PCl3 (l)  +  3H2O (l)

à

H3PO3 (aq)  +  3H+ (aq)  +  3Cl – (aq)

 

 

 

Phosphorus pentachloride (phosphorus(V) chloride) reacts with water in two stages:

PCl5(s)  +  H2O (l)

à

POCl3 (aq)  +  2H+ (aq)  +  2Cl – (aq)

POCl3(aq)  +  3H2O (l)

à

H3PO4 (aq)  +  3H+ (aq)  +  3Cl – (aq)

  The hydrolysis of disulphur dichloride, S2Cl2, is complex. It cannot be represented by a single equation. The acidic mixture from the hydrolysis includes S, SO32 –, S2O3 2 – , H2S, and a range of thionic acids H2SnO6 where n is from 2 to 6 or even more.

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