Reaction of Period III Chlorides with Water

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Reaction of Period III Chlorides with Water

Before considering the reactions that occur with oxides and chlorides in water, we must first consider the effect of two processes, hydration and hydrolysis.

Hydrated ions are formed when ionic compounds dissolve in water.

Hydration occurs because of the attraction between the ions and the polar water molecules.

For example:

Na+(aq) + 6H2O → [Na(H2O)6]2+(aq)

Diagram showing the shape and bonding of the hydrated Na+ ion:

Reaction of Period III Chlorides with Water

If the radius of the ion is small and ionic charge large, hydrolysis also occurs. Hydrolysis is a reaction in which the O-H bonds in water are broken.

It occurs when the attraction between the ion and water molecules is so strong that some O-H bonds in water are broken.

For example:

[Mg(H2O)6]2+(aq) + H2O (l) → [Mg(OH)(H20)5]+(aq) + H3O+(l)

This explains why aqueous magnesium chloride is slightly acidic.

The chloride ion has a low surface density charge and does not attract water sufficiently strongly for it to hydrolyse. Hence, any reaction of a chloride with water depends upon the element attached to the chloride.

NaCl and MgCl2 are ionic and water soluble. Mg2+ has a higher surface density of charge than Na+ and hydrolysis occurs to a very small extent. Hence, the aqueous solution of MgCl2 is slightly acidic.

AlCl3 shows some covalent character, however it still remains soluble in water. The Al3+ ion produces sufficient surface density of charge for hydrolysis to occur. In the case of AlCl3 the large hydration energy of the Al3+ ion results in a vigorous exothermic reaction with water and the solution formed is quite acidic: pH 3.

Hydration:

AlCl3(s) + 6H2O(l) → [Al(H2O)6]3+(aq) + 3Cl-(aq)

Hydrolysis:

[Al(H2O)6]3+(aq) + H2O(l) → [Al(OH)(H2O)5]2+(aq) + H3O+(l)

The remaining chlorides are covalently bonded. The electronegativity of chlorine induces a slight positive charge on the atom to which it is attached. When added to water these chlorides are hydrolysed. The vigour of the reaction increases across the period as the number of chlorine atoms attached increases.

Summary of period III chlorides with water:

NaCl: Dissolves, negligible heat change, colourless solution produced containing Na+(aq) and Cl-(aq). No hydrolysis; pH 7.

MgCl2: Dissolves, slightly exothermic, colourless solution produced containing Mg2+(aq) and Cl-(aq). Slight hydrolysis; pH 6.5.

AlCl3: Reacts vigorously, heat and fumes produced, colourless solution containing [Al(H2O)5(OH)]2+(aq) and Cl-(aq); pH 3.

SiCl4: Reacts violently, heat evolved, steamy fumes of HCl produced and a white precipitate of SiO2. Complete hydrolysis; pH 0-2.

Mechanism:

Reaction of Period III Chlorides with Water

PCl5: Reacts violently, heat evolved, steamy fumes of HCl, colourless solution of phosphoric acid H3PO4 formed. Complete hydrolysis; pH 0-2

S2Cl2: Reacts violently, heat evolved, steamy fumes of HCl, SO2 produced. Complete hydrolysis; pH 0-2.

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