Part 3: Acid–base reactions and exam applications

Balance reactions of basic, acidic and amphoteric Period 3 oxides and apply them to unfamiliar acids and bases.

Classify by reactions, not just by location in the period

A basic oxide reacts with acids; an acidic oxide reacts with bases. An amphoteric oxide reacts with both. Na₂O and MgO are basic, Al₂O₃ is amphoteric, and SiO₂, P₄O₁₀, SO₂ and SO₃ are acidic. “Amphoteric” does not mean that the substance is a mixture of an acid and a base.

To predict a salt, identify the cation supplied by the base/metal oxide and the anion supplied by the acid/acidic oxide. Balance their charges first, then balance the equation. Conditions and the amount of base can determine whether a fully neutralised or hydrogen salt forms.

Basic oxides consume hydrogen ions

The oxide part is protonated to water. The metal remains in the same oxidation state: these neutralisations are not redox reactions. Molecular equations can name the salt, while ionic equations show the chemical change more directly.

Aluminium oxide reacts in both directions

With acid, aluminium oxide forms aluminium ions and water. With excess aqueous hydroxide it forms soluble aluminate, represented here as [Al(OH)₄]⁻. Strong conditions may be needed because the oxide layer and lattice resist attack. The hydroxide chemistry of aqueous aluminium is developed in Reactions of Ions in Aqueous Solution.

Aluminium remains +3 in both products. The negative charge on the aluminate complex is the sum of Al³⁺ and four OH⁻ ligands; it does not make aluminium’s oxidation state negative.

Acidic oxides consume hydroxide

These equations show complete neutralisation with sufficient base. Silicon dioxide requires hot concentrated aqueous alkali in this representation; do not describe it as readily dissolving in cold dilute water. Alternative high-temperature fused-alkali equations must use consistent conditions and states.

SO₂ gives sulfite, retaining sulfur at +4; SO₃ gives sulfate, retaining sulfur at +6. Phosphorus(V) oxide can make dihydrogen phosphate or hydrogen phosphate with less alkali, so inspect the named product or reagent ratio before using the fully neutralised equation.

Apply the pattern to unfamiliar reactants

Original worked example: 0.850 g Al₂O₃, Mᵣ = 102.0, is dissolved in excess acid. n(Al₂O₃) = 0.00833 mol. Six moles of H⁺ react per mole of oxide, so 0.0500 mol H⁺ is required. At 0.500 mol dm⁻³ HCl, the stoichiometric minimum is 0.100 dm³ = 100 cm³. An experimental excess would require more.

An acidic oxide can also react with a basic oxide directly. For example, MgO + SO₃ → MgSO₄. This fits acid–base behaviour even without aqueous H⁺ written in the equation. Start from oxidation states and salt charges, not a memorised list alone.

Turn practical observations into justified conclusions

In supervised comparisons, use small suitable samples, fresh portions for water/acid/alkali tests and clean apparatus. Record whether a solid dissolves, the resulting pH and any temperature change; use the specified indicators or calibrated meter. Phosphorus oxides and sulfur oxides require controlled handling because their formation and contact with water can be vigorous and hazardous.

A disappearing solid in acid supports a reaction, but identify the products before deciding whether it is acid–base or redox. No change in cold water is not enough to classify an oxide. To demonstrate amphoterism, show separate reactions with acid and base, not merely one pH reading.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official AQA mark allocations.

Sources