Chapter 3: Periodicity Higher level

Period 3 oxides (continued) You need to be able to talk about the structure of the oxides as well as the bonding. Discuss: State at 25 o C Electrical conductivity in molten state Structure and bonding Melting and boiling points

Phosphorus, Sulfur and Chlorine These three elements have multiple oxides. Work out the oxidation numbers for the elements in the following molecules. P 4 O 6 P = +3 P 4 O 10 P = +5 SO 2 S = +4 SO 3 S = +6 Cl 2 O Cl = +1 Cl 2 O 7 Cl = +7

Acidic or Basic – a bit more General rule Electropositive elements have basic oxides Electronegative elements have acidic oxides What’s the exception? Aluminium oxide = amphoteric

Metal oxides Why is sodium hydroxide more alkaline than magnesium hydroxide? Na 2 O(s) + H 2 O(l)  2Na + (aq) + 2OH - (aq) BUT MgO(s) + H 2 O(l)  Mg(OH) 2 (aq)/(s) Magnesium hydroxide is only sparingly soluble, so it doesn’t release many OH - ions into the solution.

Aluminium Complete these equations: Al 2 O 3 (s) + 6HCl(aq)  3AlCl 3 (aq) + 3H 2 O(l) Al 2 O 3 (s) + 2NaOH(aq)  2NaAl(OH) 4 (aq) 2NaAl(OH) 4 (aq) = sodium aluminate

Period 3 Chlorides Physical properties are linked to type of bonding. Which period 3 elements will form ionic bonds with Cl? Na, Mg and Al. Si, S, P will all form covalent bonds. In fact, when liquid aluminium chloride is covalent as well.

Aluminium Chloride

Physical properties Ionic – high melting point, conductive (solid and molten), soluble in water. Covalent (simple molecular lattice when solid) – low melting point due to van der waals’ forces being weak, doesn’t conduct.

Reactions with water As with the oxides we need to look at whether the period 3 chlorides gives acidic or basic solutions when they react with H 2 O.

Sodium Chloride NaCl(s) + H 2 O(l)  Na + (aq) + Cl - (aq) + H 2 O(l) We can remove H2O, so really the equation is; NaCl(s)  Na + (aq) + Cl - (aq) This is a neutral solution, neither acidic or basic.

Magnesium Chloride Magnesium has a 2+ ion, it is also smaller than the Na + ion. This means that Mg 2+ has a much big charge density than Na +. This charge attracts water molecules and causes some to dissociate a produce acidic H + ions. Therefore it produces a weak acid.

Al, Si, P, S and Cl The rest of the period 3 chlorides all produce acidic solutions after reacting with water. 2AlCl 3 (s) + 3H 2 O(l)  Al 2 O 3 (s) + 6HCl(aq) SiCl 4 + 4H 2 O  Si(OH) 4 + 4HCl PCl 3 + 3H 2 O  H 3 PO 3 + 3HCl PCl 5 + 4H 2 O  H 3 PO 4 + 5HCl