Properties of Period 3 Oxides

The oxides of Period 3 elements exhibit a wide range of chemical properties. As we progress across the period, the chemical character transitions from basic metallic oxides to an amphoteric oxide in the middle, and finally to acidic non-metallic oxides. The behaviour of these oxides in water and their reactions with acids and bases are central to this topic.

1. Acidic and Basic Oxides

2. Reactions of Period 3 Oxides with Water

The reaction of each oxide with water reflects its structure and bonding. Soluble ionic oxides react to produce hydroxide ions, whereas soluble covalent oxides react to produce oxoacids.

Sodium Oxide (\(\text{Na}_2\text{O}\))

Sodium oxide is a strongly basic ionic oxide. It dissolves and reacts vigorously with water to form a strongly alkaline solution of sodium hydroxide. The solution is fully dissociated, giving a pH of 13 to 14:

In this reaction, the oxide ion acts as a base by accepting a proton from water: \( \text{O}^{2-}(\text{s}) + \text{H}_2\text{O}(\text{l}) \rightarrow 2\text{OH}^{-}(\text{aq}) \).

Magnesium Oxide (\(\text{MgO}\))

Magnesium oxide is also a basic ionic oxide. However, due to the stronger electrostatic attraction between the \( \text{Mg}^{2+} \) and \( \text{O}^{2-} \) ions in its lattice, it is only sparingly soluble in water. It reacts slowly to form a saturated solution of magnesium hydroxide, which has a pH of approximately 9 to 10:

Aluminium Oxide (\(\text{Al}_2\text{O}_3\)) and Silicon Dioxide (\(\text{SiO}_2\))

Both oxides are completely insoluble in water due to their giant structures: aluminium oxide is ionic with strong covalent character, and silicon dioxide is giant covalent. Neither reacts with water, so when mixed with water the pH remains neutral (pH 7):

• \(\text{Al}_2\text{O}_3\): Insoluble in water. No reaction.
• \(\text{SiO}_2\): Insoluble in water. No reaction.

Phosphorus(V) Oxide (\(\text{P}_4\text{O}_{10}\))

Phosphorus(V) oxide is a simple molecular non-metal oxide. It reacts violently with water to form phosphoric(V) acid, a strong tribasic acid. The reaction is highly exothermic, producing a strongly acidic solution with a pH of 1 to 2:

Sulfur Dioxide (\(\text{SO}_2\)) and Sulfur Trioxide (\(\text{SO}_3\))

Both are molecular acidic oxides that dissolve and react with water:

• Sulfur Dioxide: Dissolves to form weak, unstable sulfurous acid (sulfuric(IV) acid), resulting in an acidic solution with a pH of 2 to 3: \[ \text{SO}_2(\text{g}) + \text{H}_2\text{O}(\text{l}) \rightleftharpoons \text{H}_2\text{SO}_3(\text{aq}) \]
• Sulfur Trioxide: Reacts violently with water to form sulfuric(VI) acid, a strong diprotic acid. This produces a strongly acidic solution with a pH of 0 to 1: \[ \text{SO}_3(\text{g}) + \text{H}_2\text{O}(\text{l}) \rightarrow \text{H}_2\text{SO}_4(\text{aq}) \]

3. Reactions of Period 3 Oxides with Acids and Bases

Because some oxides are basic, some are acidic, and one is amphoteric, they exhibit distinct reactions with acids (like hydrochloric acid, \(\text{HCl}\)) and bases (like sodium hydroxide, \(\text{NaOH}\)).

Reactions of Basic Oxides with Acids

Sodium oxide and magnesium oxide react with dilute acids to undergo neutralisation, forming a salt and water:

• Sodium Oxide with Hydrochloric Acid: \[ \text{Na}_2\text{O}(\text{s}) + 2\text{HCl}(\text{aq}) \rightarrow 2\text{NaCl}(\text{aq}) + \text{H}_2\text{O}(\text{l}) \]
• Magnesium Oxide with Hydrochloric Acid: \[ \text{MgO}(\text{s}) + 2\text{HCl}(\text{aq}) \rightarrow \text{MgCl}_2(\text{aq}) + \text{H}_2\text{O}(\text{l}) \]

Reaction of the Amphoteric Oxide (Aluminium Oxide)

Aluminium oxide does not react with water, but it react as both a base and an acid because it is amphoteric:

• Reacting as a Base (with strong acids): It reacts with hydrochloric acid to form aluminium chloride and water: \[ \text{Al}_2\text{O}_3(\text{s}) + 6\text{HCl}(\text{aq}) \rightarrow 2\text{AlCl}_3(\text{aq}) + 3\text{H}_2\text{O}(\text{l}) \]
• Reacting as an Acid (with strong bases): It reacts with hot, concentrated sodium hydroxide solution to form a colourless solution containing the tetrahydroxoaluminate ion: \[ \text{Al}_2\text{O}_3(\text{s}) + 2\text{NaOH}(\text{aq}) + 3\text{H}_2\text{O}(\text{l}) \rightarrow 2\text{NaAl}(\text{OH})_4(\text{aq}) \] The ionic equation for this reaction is: \[ \text{Al}_2\text{O}_3(\text{s}) + 2\text{OH}^{-}(\text{aq}) + 3\text{H}_2\text{O}(\text{l}) \rightarrow 2[\text{Al}(\text{OH})_4]^{-}(\text{aq}) \]

Reactions of Acidic Oxides with Bases

Silicon, phosphorus, and sulfur oxides react with alkalis to form salts and water:

• Silicon Dioxide with Base: Silicon dioxide is giant covalent and has a high activation energy, but it reacts slowly with hot, concentrated sodium hydroxide to form soluble sodium silicate: \[ \text{SiO}_2(\text{s}) + 2\text{NaOH}(\text{aq}) \rightarrow \text{Na}_2\text{SiO}_3(\text{aq}) + \text{H}_2\text{O}(\text{l}) \]
• Phosphorus(V) Oxide with Base: It reacts with sodium hydroxide to produce sodium phosphate and water: \[ \text{P}_4\text{O}_{10}(\text{s}) + 12\text{NaOH}(\text{aq}) \rightarrow 4\text{Na}_3\text{PO}_4(\text{aq}) + 6\text{H}_2\text{O}(\text{l}) \]
• Sulfur Dioxide with Base: It reacts with sodium hydroxide to form sodium sulfite and water: \[ \text{SO}_2(\text{g}) + 2\text{NaOH}(\text{aq}) \rightarrow \text{Na}_2\text{SO}_3(\text{aq}) + \text{H}_2\text{O}(\text{l}) \]
• Sulfur Trioxide with Base: It reacts with sodium hydroxide to form sodium sulfate and water: \[ \text{SO}_3(\text{g}) + 2\text{NaOH}(\text{aq}) \rightarrow \text{Na}_2\text{SO}_4(\text{aq}) + \text{H}_2\text{O}(\text{l}) \]

4. Summary of Period 3 Oxides Character