Periodicity: Question 4

Worked solution

Part (a): Classifying the three oxides

From Table 1:

• MgO reacts only with the acid, so it is a basic oxide.
• $\text{Al}_2\text{O}_3$ reacts with both the acid and the alkali, so it is an amphoteric oxide. This dual reactivity with an acid and a base is exactly what defines an amphoteric oxide.
• $\text{SiO}_2$ reacts only with the hot, concentrated alkali (and not with the dilute acid), so it is classified as an acidic oxide.

Part (b): MgO with dilute sulfuric acid

MgO is a basic oxide, so it neutralises the acid to form a salt and water: $\text{MgO(s)} + \text{H}_2\text{SO}_4\text{(aq)} \rightarrow \text{MgSO}_4\text{(aq)} + \text{H}_2\text{O(l)}$

Part (c): $\text{Al}_2\text{O}_3$ with hot, concentrated KOH(aq)

Acting as an acid towards the strong alkali, aluminium oxide forms the aluminate ion: $\text{Al}_2\text{O}_3\text{(s)} + 2\text{KOH(aq)} \rightarrow 2\text{KAlO}_2\text{(aq)} + \text{H}_2\text{O(l)}$

Checking the balance: $2$ Al atoms on each side, $2$ K atoms on each side, and $5$ O atoms on each side ($3$ from $\text{Al}_2\text{O}_3$ $+$ $2$ from $2\text{KOH}$ on the left; $4$ from $2\text{KAlO}_2$ $+$ $1$ from $\text{H}_2\text{O}$ on the right).

Part (d): $\text{Al}_2\text{O}_3$ with dilute sulfuric acid

Acting as a base towards the acid, aluminium oxide forms aluminium sulfate: $\text{Al}_2\text{O}_3\text{(s)} + 3\text{H}_2\text{SO}_4\text{(aq)} \rightarrow \text{Al}_2\text{(SO}_4\text{)}_3\text{(aq)} + 3\text{H}_2\text{O(l)}$

Checking the balance: $2$ Al atoms, $3$ S atoms and $15$ O atoms on each side ($3 + 12 = 15$ on the left; $12 + 3 = 15$ on the right), and $6$ H atoms on each side.

Part (e): Why $\text{SiO}_2$ does not react with dilute acid

$\text{SiO}_2$ has a giant covalent (macromolecular) lattice: every Si and O atom is joined into an extensive three-dimensional network by strong, largely covalent Si–O bonds. There are no discrete oxide ($\text{O}^{2-}$) ions present for $\text{H}^+$ ions from the dilute acid to react with. The acid simply has nothing to attack.

MgO and $\text{Al}_2\text{O}_3$, by contrast, are essentially ionic lattices containing oxide ions. These oxide ions react directly and readily with $\text{H}^+$ ions from a dilute acid to form water, so both oxides dissolve in (and are neutralised by) dilute acid. $\text{SiO}_2$ only reacts at all when attacked by a hot, concentrated alkali, which is able to slowly break the strong Si–O covalent network to form a silicate. A much more forceful condition than a simple dilute acid can provide.

Part (f): pH of MgO with water

MgO reacts with water: $\text{MgO(s)} + \text{H}_2\text{O(l)} \rightarrow \text{Mg(OH)}_2\text{(s)}$

However, $\text{Mg(OH)}_2$ is only sparingly soluble in water, only a small amount dissolves and dissociates into $\text{Mg}^{2+}$ and $\text{OH}^-$ ions. This releases only a low concentration of hydroxide ions into solution, so the resulting mixture is only mildly alkaline, with a pH of around $9$, rather than the strongly alkaline pH ($\approx 13$–$14$) seen with a fully soluble hydroxide such as NaOH or KOH.

Final answers

• (a) MgO: basic. $\text{Al}_2\text{O}_3$: amphoteric (reacts with both acid and alkali). $\text{SiO}_2$: acidic.
• (b) $\text{MgO(s)} + \text{H}_2\text{SO}_4\text{(aq)} \rightarrow \text{MgSO}_4\text{(aq)} + \text{H}_2\text{O(l)}$
• (c) $\text{Al}_2\text{O}_3\text{(s)} + 2\text{KOH(aq)} \rightarrow 2\text{KAlO}_2\text{(aq)} + \text{H}_2\text{O(l)}$
• (d) $\text{Al}_2\text{O}_3\text{(s)} + 3\text{H}_2\text{SO}_4\text{(aq)} \rightarrow \text{Al}_2\text{(SO}_4\text{)}_3\text{(aq)} + 3\text{H}_2\text{O(l)}$
• (e) $\text{SiO}_2$‘s giant covalent lattice has no discrete oxide ions for $\text{H}^+$ to attack; MgO and $\text{Al}_2\text{O}_3$ are ionic lattices whose oxide ions react readily with $\text{H}^+$.
• (f) pH $\approx 9$: $\text{Mg(OH)}_2$ forms but is only sparingly soluble, releasing a low concentration of $\text{OH}^-$.