Reaction of chlorides with water

Question 1

NaCl

Answer 1

• dissolves in water to form colourless solution of neutral pH

NaCl (s) + aq —> Na+ (aq) + Cl- (aq)

Na+ has Low charge density and does not hydrolysis

Question 2

MgCl2

Answer 2

• dissolves in water to form colourless solution of slightly acidic pH 6.5 (hydrated mg ion has higher charge density and undergoes slight hydrolysis)

MgCl2 (s) + 6H2O (l) —> [Mg(H2O)6]2+ (aq) + 2Cl- (aq)
[Mg(H2O)6]2+ (aq) + H2O (l) —> [Mg(H2O)5(OH)]+ (aq) + H3O+

Question 3

AlCl3 (limited amount of water)

Answer 3

• steamy white fume of HCl and white solid remains (insoluble in water)

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

Question 4

AlCl3 (large amount of water ie excess)

Answer 4

• dissolves to form colourless solution of pH3

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)

• Due to very high charge density of Al3+, the O-H bonds in the H2O ligands are sufficiently polarised and weakened & break readily and donate a proton
• hydrated aluminium complex can undergo hydrolysis to a further extent than Mg2+

Question 5

SiCl4/ PCl5: general observation

Answer 5

• violent reaction water producing fumes of HCl gas

- if water is in excess, HCl fumes dissolve to form a strongly acidic solution of pH1

Question 6

SiCl4

Answer 6

SiCl4 (s) + 4H2O (l) —> SiO2 . 2H2O (s) + 4HCl (aq)

Complete hydrolysis

Question 7

PCl5

Answer 7

PCl5 (s) + 4H2O (l) —> H3PO4 (aq) + 5HCl (aq)

Complete hydrolysis

Question 8

Dropwise addition of water (or exposed to moist air) for PCl5

Answer 8

PCl5 (s) + H2O (l) —> POCl3 (l) + 2HCl (g)