Period 3 Elements, Oxides and Chlorides

1. Reactions of the Period 3 Elements

The spec asks for the reactions of sodium and magnesium with water, and of the elements sodium to sulfur with oxygen and with chlorine.

Sodium and magnesium with water

• Sodium: 2Na + 2H₂O → 2NaOH + H₂. The reaction is vigorous. Sodium floats, melts and fizzes. The solution has a pH of about 14.
• Magnesium with cold water: Mg + 2H₂O → Mg(OH)₂ + H₂. The reaction is very slow. The solution has a pH of about 10, because Mg(OH)₂ is only slightly soluble.
• Magnesium with steam: Mg + H₂O(g) → MgO + H₂.

The elements with oxygen

Sulfur trioxide: sulfur burns to give SO₂, not SO₃. SO₂ is then oxidised further with a vanadium(V) oxide catalyst: 2SO₂ + O₂ ⇌ 2SO₃ (V₂O₅ catalyst). See the Contact process on the catalysts page.

The elements with chlorine

• 2Na + Cl₂ → 2NaCl
• Mg + Cl₂ → MgCl₂
• 2Al + 3Cl₂ → Al₂Cl₆
• Si + 2Cl₂ → SiCl₄
• 2P + 5Cl₂ → 2PCl₅ (with excess chlorine)

Aluminium chloride is often written as AlCl₃. In the vapour it exists as Al₂Cl₆ molecules (see section 3).

2. Period 3 Oxide Formulas, Structures & Bonding

Across Period 3 from left to right, elements increase in electronegativity. Consequently, their bonding with oxygen transitions from ionic with electropositive metals (sodium and magnesium) to macromolecular covalent (silicon) to simple molecular covalent with non-metals (phosphorus and sulfur).

• Na₂O and MgO: giant ionic, high melting point. MgO is higher because it has 2+ and 2- ions, so the attraction between ions is stronger.
• Al₂O₃: giant ionic with some covalent character, high melting point.
• SiO₂: giant covalent, high melting point (1883 K); many strong covalent bonds must be broken. MgO has the highest melting point of these oxides.
• P₄O₁₀, SO₂ and SO₃: simple molecular, low melting points.

3. Structure and Melting Points of the Chlorides

The Al₂Cl₆ dimer

In AlCl₃, the aluminium atom has only six electrons in its outer shell. Two AlCl₃ molecules join together. A lone pair on a chlorine atom of each molecule forms a co-ordinate (dative) bond to the aluminium atom of the other molecule. So Al₂Cl₆ has two Cl→Al dative bonds.

Explaining the trend

• NaCl and MgCl₂ are ionic lattices. There are strong attractions between oppositely charged ions, so a lot of energy is needed to melt them.
• AlCl₃, SiCl₄ and PCl₅ are made of molecules. The molecules are held together only by weak van der Waals forces, so little energy is needed to separate them.
• So across the period, ionic lattices give way to molecules, and the melting point falls sharply.

4. Reactions of Period 3 Oxides with Water & Resulting pH

The behaviour of Period 3 oxides in water mirrors their chemical bonding:

1. Sodium Oxide (Na₂O)

Reacts vigorously and exothermically with water to form a strongly alkaline solution of sodium hydroxide:

Na₂O(s) + H₂O(l) → 2NaOH(aq)   [pH = 14]

Ionic equation: O²⁻(s) + H₂O(l) → 2OH⁻(aq). The oxide ion is a powerful Brønsted-Lowry base that accepts a proton from water.

2. Magnesium Oxide (MgO)

Reacts slowly with water to form sparingly soluble magnesium hydroxide:

MgO(s) + H₂O(l) ⇌ Mg(OH)₂(aq/s)   [pH = 9 to 10]

Because Mg(OH)₂ is only slightly soluble, few OH⁻ ions are released into solution, producing a weakly alkaline pH.

3. Aluminium Oxide (Al₂O₃) and Silicon Dioxide (SiO₂)

Both oxides are completely insoluble in water and produce an unreactive neutral suspension (pH = 7). Al₂O₃ has an extremely high lattice enthalpy that water cannot overcome, while SiO₂ is an immense macromolecular covalent network.

4. Phosphorus(V) Oxide (P₄O₁₀)

Reacts violently and exothermically with water to form an acidic solution of phosphoric(V) acid:

P₄O₁₀(s) + 6H₂O(l) → 4H₃PO₄(aq)   [pH = 0 to 1]

5. Sulfur Dioxide (SO₂) and Sulfur Trioxide (SO₃)

Sulfur dioxide dissolves readily to form weak sulfurous / sulfuric(IV) acid:

SO₂(g) + H₂O(l) ⇌ H₂SO₃(aq)   [pH = 2 to 3]

Sulfur trioxide reacts violently with water in a dense mist of sulfuric(VI) acid droplets:

SO₃(l) + H₂O(l) → H₂SO₄(aq)   [pH = 0 to 1]

5. Reactions of the Chlorides with Water & Resulting pH

Explaining the trend in terms of bonding

• Ionic chlorides just dissolve. NaCl and MgCl₂ separate into hydrated ions, so the solution is neutral or almost neutral.
• Covalent chlorides are hydrolysed. Water attacks the central atom. Si and P can accept a lone pair from a water molecule. A Cl atom is replaced by an OH group, and HCl forms. So the solutions are strongly acidic.
• AlCl₃ is in between. In water it forms [Al(H₂O)₆]³⁺. The small, highly charged Al³⁺ ion polarises a water ligand, which releases H⁺ (as H₃O⁺). So the solution is acidic.

6. Structures of the Acids and Anions

The acidic oxides and the covalent chlorides form acids in water. You need to draw the displayed formulae of these acids and of the anions they form. Show the P=O and S=O double bonds, as in the diagrams below.

• H₃PO₄ and PO₄³⁻: from P₄O₁₀ (with water) and from PCl₅ (hydrolysis).
• H₂SO₃ and SO₃²⁻: from SO₂.
• H₂SO₄ and SO₄²⁻: from SO₃.
• HCl: from the hydrolysis of SiCl₄ and PCl₅.

Each acid loses its H⁺ ions to a base to form the anion. The negative charges sit on the oxygen atoms that have lost a hydrogen.

7. Period 3 Oxide Structure & pH Continuum Diagram

8. Acid-Base Reactions of Period 3 Oxides and Chlorides

Basic Oxides (Na₂O and MgO)

React with acids to form a simple salt and water:

• Na₂O(s) + 2HCl(aq) → 2NaCl(aq) + H₂O(l)   [Ionic: O²⁻ + 2H⁺ → H₂O]
• MgO(s) + 2HCl(aq) → MgCl₂(aq) + H₂O(l)
• MgO(s) + H₂SO₄(aq) → MgSO₄(aq) + H₂O(l)

Acidic Oxides (SiO₂, P₄O₁₀, SO₂, SO₃)

React with aqueous alkalis / bases to form salts and water:

• Silicon dioxide: Reacts with hot concentrated sodium hydroxide:
  SiO₂(s) + 2NaOH(aq) → Na₂SiO₃(aq) + H₂O(l)   (sodium silicate)
• Phosphorus(V) oxide: Reacts with sodium hydroxide:
  P₄O₁₀(s) + 12NaOH(aq) → 4Na₃PO₄(aq) + 6H₂O(l)   (sodium phosphate)
• Sulfur dioxide: Reacts with sodium hydroxide:
  SO₂(g) + 2NaOH(aq) → Na₂SO₃(aq) + H₂O(l)   (sodium sulfite)
• Sulfur trioxide: Reacts with sodium hydroxide:
  SO₃(l) + 2NaOH(aq) → Na₂SO₄(aq) + H₂O(l)   (sodium sulfate)

Amphoteric Oxide (Al₂O₃)

Aluminium oxide reacts with both acids and bases. Its equations are in section 9.

The Chlorides with Bases

When SiCl₄ and PCl₅ are hydrolysed, HCl forms. A base neutralises this HCl in the same way as any acid:

• HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)

The other acid formed (H₃PO₄ from PCl₅) is neutralised too, so the overall products are salts and water.

9. Amphoteric Behaviour of Aluminium Oxide (Al₂O₃)

1. Aluminium Oxide Acting as a Base (Reaction with Acids)

Al₂O₃ reacts with warm dilute hydrochloric acid or sulfuric acid to form aluminium salts:

Al₂O₃(s) + 6HCl(aq) → 2AlCl₃(aq) + 3H₂O(l)

Ionic: Al₂O₃(s) + 6H⁺(aq) → 2Al³⁺(aq) + 3H₂O(l)

2. Aluminium Oxide Acting as an Acid (Reaction with Alkalis)

Al₂O₃ dissolves in hot concentrated aqueous sodium hydroxide to form sodium tetrahydroxoaluminate:

Al₂O₃(s) + 2NaOH(aq) + 3H₂O(l) → 2Na[Al(OH)₄](aq)

Ionic: Al₂O₃(s) + 2OH⁻(aq) + 3H₂O(l) → 2[Al(OH)₄]⁻(aq)

10. Worked Examples & Equations

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