Chemical periodicity

Periodicity of period 3 elements NaMgAl Si PSCl Ar Sodium, magnesium and aluminium are metals. Silicon has some metalloid traits. Phosphorous, sulphur and chlorine are typical non-metals. Argon is an inert gas

Reaction of the elements with water Sodium rapidly reacts with cold water. Na + H 2 O → NaOH + ½H 2 It floats on the surface, fizzes and may burst into an orange flame.

Magnesium Magnesium reacts very slowly with cold water. Mg + 2H 2 O → Mg(OH) 2 + ½H 2 But magnesium reacts rapidly with steam; Mg + H 2 O (g) → MgO + H 2

Comparison of sodium and magnesium hydroxides. Sodium hydroxide is extremely soluble. It is a strong base with a pH of 12 – 14. Magnesium hydroxide is much less soluble. It is not such a strong base, with a pH of 9 – 11.

Preparation and properties of Period 3 oxides Magnesium burns rapidly with a brilliant white light. Mg + O 2 →2MgO Sodium burns with a yellow flame.

Metal vs Non-metal oxides. Metal oxides are generally basic. They react with water to give alkaline solutions and react with acids. Non-metal oxides are either acidic or neutral. They react with water to give acidic solutions and react with bases.

Properties of magnesium oxide Magnesium oxide is a white, ionic substance, with a high MP and BP.

MgO dissolves slowly in cold water… MgO (s) + H 2 O → Mg(OH) 2(aq) Giving an alkaline solution of magnesium hydroxide, pH 9-11.

Preparation of Aluminium Oxide Aluminium also burns readily in the powdered form. 4Al + 3O 2 →2Al 2 O 3 But solid lumps are not reactive. At room temperature aluminium acquires a thin oxide film which protects the underlying metal from reacting further.

Properties of aluminium oxide Aluminium oxide is a white solid, with a significant degree of covalent character. It is insoluble in water. It is amphoteric, reacting with both acids and alkalis. Al 2 O 3 + 6H + → 2Al H 2 O Al 2 O 3 + 2OH - + 3H 2 O → 2Al(OH) 4

Silicon dioxide Silicon dioxide is also insoluble and is classed as acidic. If heated strongly in air silicon also forms an oxide.

Phosphorous has two allotropes (physical forms) red and white. Red only reacts when heated, white spontaneously ignites in air to give white smoke of phosphorous pentoxide. 4P + 5O 2 → P 4 O 10 In a limited supply of oxygen P 2 O 6 forms instead. Phosphorous

Phosphorous oxides P 4 O 10 reacts vigorously to give phosphoric (v) acid. P 4 O H 2 O → 4H 3 PO 4 P 2 O 6 reacts to give phosphoric (iii) acid; P 2 O 6 + 3H 2 O → 2H 3 PO 3

Preparation of sulphur oxides Sulphur burns with a blue flame to give sulphur dioxide. S + O2→ SO 2

When heated with more oxygen, and passed over a heated vanadium pentoxide catalyst, sulphur dioxide is converted into sulphur trioxide. SO 2 + ½O 2 ⇌ SO 3 Sulphur dioxide is a gas. Sulphur trioxide is a solid, with a MP of 17 o C.

Both oxides react with water to give acids; SO 2 + H 2 O → H 2 SO 3 SO 3 + H 2 O→ H 2 SO 4 Sulphur dioxide reacts with water to give sulphuric (iv) acid; Sulphuric (iv) acid is weak. H 2 SO 3 ⇌ H + + HSO 3 - Sulphur trioxide reacts vigorously to give sulphuric (vi) acid;

Phosphorous oxides P 4 O 10 reacts vigorously to give phosphoric (v) acid. P 4 O H 2 O → 4H 3 PO 4 P 2 O 6 reacts to give phosphoric (iii) acid; P 2 O 6 + 3H 2 O → 2H 3 PO 3

Preparation and properties of period 3 chlorides Sodium burns readily in chlorine. Na + ½Cl 2 → NaCl Sodium chloride is a white, ionic solid.

NaCl dissolves to give hydrated ions. NaCl (s) → Na + (aq) + Cl - (aq) The pH of the resulting solution is neutral.

Preparation and properties of magnesium chloride Magnesium burns in chlorine. Mg + Cl 2 → MgCl 2 Magnesium chloride is a white, ionic solid. It dissolves in water to give hydrated ions. Due to the relatively high charge density of the magnesium ion it can polarise water molecules. The pH of its solution is therefore slightly acidic at 6.5.

Preparation and properties of aluminium chloride Aluminium reacts rapidly when heated with chlorine. 2Al + 3Cl 2 → 2AlCl 3 It has an appreciably degree of covalent character and is usually found as a dimer Al 2 Cl 6.

Aluminium chloride gives fumes of HCl in moist air and dissolves in water to give an acidic solution. AlCl 3 + 6H 2 O →[Al(H 2 O) 6 ] 3+ +3Cl - [Al(H 2 O) 6 ] 3+ → [Al(H 2 O) 5 OH] 2+ + H +

Preparation and properties of silicon chloride Silicon reacts when heated with chlorine. Si + 2Cl 2 → SiCl 4 Silicon tetrachloride is a colourless, volatile liquid. It reacts vigorously with water in a strongly exothermic reaction giving off white fumes of HCl. SiCl 4 + 2H 2 O → SiO 2 + 4HCl The pH of the solution is 2.

Preparation and properties of phosphorous chlorides Phosphorous reacts, without needing to be heated, when chlorine is passed over it. P 4 + 6Cl 2 → 4PCl 3 Phosphorous (iii) chloride is a colourless, volatile liquid. If chlorine is in excess it can react further. PCl 3 + Cl 2 → PCl 5 Phosphorous (v) chloride is a yellow solid. Both chlorides are normally found as dimers.

Both chlorides will react vigorously with water in a strongly exothermic reaction that gives off acid fumes; PCl 3 + 3H 2 O → H 3 PO 3 + 3HCl PCl 5 + 4H 2 O → H 3 PO 4 + 5HCl