The Acidic/Basic Nature of the Period 3 Oxides

This lesson covers: 

1. The transition from alkaline to acidic oxides across period 3
2. The reactions of period 3 oxides with acids and alkalis

Trend from alkaline to acidic oxides

As you move left to right across period 3, the oxides transition from alkaline to acidic:

• Alkaline oxides - Sodium oxide, magnesium oxide
• Acidic oxides - Silicon dioxide, phosphorus pentoxide, sulfur dioxide, sulfur trioxide

So the alkaline oxides on the left will react with acids, while the acidic oxides on the right will react with bases.

Aluminium oxide reacts with both acids and alkalis so it is classed as an amphoteric oxide.

Reactions of alkaline oxides with acids

The alkaline metal oxides Na₂O and MgO neutralise acids to form salts and water.

For example, sodium oxide reacts with sulfuric acid to produce sodium sulfate and water:

• Na₂O_(s) + H₂SO_4(aq) ➔ Na₂SO_4(aq) + H₂O_(l)

For example, magnesium oxide reacts with hydrochloric acid to produce magnesium chloride and water:

• MgO_(s) + 2HCl_(aq) ➔ MgCl_2(aq) + H₂O_(l)

Reaction of aluminium oxide with acids and alkalis

The amphoteric aluminium oxide neutralises both acids and alkalis.

The reaction with acids produces a salt and water.

For example, Al₂O₃ reacts with hydrochloric acid to produce aluminium chloride and water:

• Al₂O_3(s) + 6HCl_(aq) ➔ 2AlCl_3(aq) + 3H₂O_(l)

The reaction with alkalis produces an aluminate salt.

For example, Al₂O₃ reacts with hot, concentrated sodium hydroxide to produce sodium aluminate:

• Al₂O_3(s) + 2NaOH_(aq) + 3H₂O_(l) ➔ 2NaAl(OH)_4(aq)

Reaction of acidic oxides with alkalis

The acidic oxides SiO₂, P₄O₁₀, SO₂, and SO₃ neutralise alkalis to produce salt solutions.

Silicon dioxide reactions

Silicon dioxide reacts as a weak acid with strong bases to produce a silicate salt.

For example, SiO₂ reacts with hot, concentrated sodium hydroxide to produce sodium silicate:

• SiO_2(s) + 2NaOH_(aq) ➔ Na₂SiO_3(aq) + H₂O_(l)

Phosphorus pentoxide reactions

Phosphorus pentoxide reacts with water to produce phosphoric(V) acid (H₃PO₄). It is actually the phosphoric acid that then reacts with alkalis, not the phosphorus pentoxide itself.

Phosphoric acid reacts with alkalis to produce phosphate salts and water.

For example, H₃PO₄ reacts with sodium hydroxide to produce sodium phosphate and water:

• H₃PO_4(aq) + 3NaOH_(aq) ➔ Na₃PO_4(aq) + 3H₂O_(l)

The reaction occurs in three stages because H₃PO₄ contains three -OH groups with acidic hydrogens. The hydrogens are replaced by sodium in stages forming the intermediate products NaH₂PO₄ and Na₂HPO₄.

Reactions of sulfur oxides

Sulfur dioxide and sulfur trioxide neutralise alkalis to form salts and water.

The reaction with sulfur dioxide produces sulfite salts.

For example, SO₂ reacts with sodium hydroxide to produce sodium sulfite and water:

• SO_2(g) + 2NaOH_(aq) ➔ Na₂SO_3(aq) + H₂O_(l)

The reaction with sulfur trioxide produces sulfate salts.

For example, SO₃ reacts with sodium hydroxide to produce sodium sulfate and water:

• SO_3(g) + 2NaOH_(aq) ➔ Na₂SO_4(aq) + H₂O_(l)