13.1.2 Describe the reactions of chlorine and the chlorides referred to in 13.1.1 with water.

Simply:

• NaCl/MgCl2 dissolves into its ions when water is added.
• AlCl3undergoes hydrolysis.
• SiCl4 and PCl3/PCl5 form acidic solutions:

In more detail and also extra notes:

CHLORIDES 

Reactions with water:

·     Sodium chloride dissolves readily due to polar water molecules being able to pull oppositely charged ions out of the lattice; NaCl forms a neutral solution;

·     magnesium chloride and aluminium trichloride form acidic solutions as their small highly charged cations (high charge density) attract water molecules (to form complexes)  some of which give up their hydrogen ions to other water molecules surrounding the complex.

               [Mg (H₂O)₆]²⁺+  H₂O    ¬®       [Mg(H₂O)₅ OH] ⁺ + H₃O⁺

               [Al (H₂O)₆]³⁺  + H₂O    ¬®         [Al (H₂O)₅ OH] ²⁺  + H₃O⁺

    

      in addition, AlCl₃ also forms HCl when it reacts with water:

                AlCl₃ (s)   + 3H₂O    ¬®       Al (OH)₃     +  3HCl (=fumes)  (exothermic  reaction)

·     simple molecular chlorides all react rapidly with water to form hydrochloric acid.

                           

                SiCl₄ (l)  +4H₂O (l)   ®    Si(OH)₄ (s)  +   4HCl  (g)                 

                PCl₅  (l)  +  4H₂O    ®    H₃ PO₄ (aq)   + 5HCl  (g)      (= HCl fumes)(exo)

                Cl₂ (g)  +   H₂O    ®    HClO   +   HCl  

OXIDES

Reactions with water

• ionic oxides are soluble and react with water to form alkaline solutions (hydroxides) with a decreasing pH when going to the right of the period (basic oxides);

         Na₂O (s)  +   H₂O (l)     ¾®  2Na⁺   (aq) +2OH^-  (aq)     /2NaOH (aq)

         MgO (s)  +   H₂O (l)     ¾®    Mg²⁺   (aq)  + 2OH^-  (aq)     /Mg(OH)₂ (aq)

•  aluminium oxide is amphoteric:

         reaction with acid:      Al₂ O₃ (s)  + 6HCl (aq)   ¾®  2AlCl₃(aq) + 3H₂O (l)

         reaction with alkali:     Al₂ O₃ (s)  + 2NaOH (aq)  ¾®  2NaAlO₂ (aq) +H₂O (l)

                                                                                                                   (sodium aluminate)

• silicon dioxide does not dissolve in or react with water ( water remains neutral) but it can react with sodium hydroxide which is why it is considered an acidic oxide;

           SiO₂ (s)  +   2NaOH (aq)¾®  Na₂SiO₃(s) +H₂O (l)

                             

• simple molecular oxides are soluble in water and react to form strong acidic solutions like phosphoric and sulphuric acid. HClO, hypochlorous acid, is a weak acid.

            P₄ O₁₀ (s)  + 6H₂O (l)     ¾®  4H₃PO₄   (aq) 

            SO₂ (g)  +H₂O (l)     ¾®  H₂SO₃   (aq) 

            Cl₂O (l)  +    H₂O     ¾®  2HClO  (aq)