Reactions of Ions in Aqueous Solution (A Level only) (AQA A-Level Chemistry): Revision Notes

6.3.2 Reactions with Bases

Reactions of Metal-Aqua Ions with Bases

Metal-aqua ions undergo distinct reactions with bases, such as hydroxide ions ($\text{OH}^-$), ammonia ($\text{NH}_3$), and carbonate ions ($\text{CO}_3^{2-}$).

These reactions differ based on the metal's oxidation state and acidity.

Reaction with Sodium Hydroxide ($NaOH$)

When sodium hydroxide is added to a solution containing metal-aqua ions, insoluble metal hydroxides precipitate due to the reaction with $\text{OH}^-$ ions:

$[ \text{M}(\text{H}_2\text{O})_6 ]^{2+}$ Ions (e.g., $Fe^{2+}$, $Cu^{2+}$)

Reaction:

$$\text{M}(\text{H}_2\text{O})_6 ]^{2+} + 2\text{OH}^- \rightarrow [ \text{M}(\text{H}_2\text{O})_4(\text{OH})_2 ] + 2\text{H}_2\text{O}$$

Observation:

A coloured precipitate forms (e.g., green for $\text{Fe}^{2+}$, blue for $\text{Cu}^{2+}$).

$[ \text{M}(\text{H}_2\text{O})_6 ]^{3+}$ Ions (e.g., $Al^{3+}$, $Fe^{3+}$)

Reaction:

$$[ \text{M}(\text{H}_2\text{O})_6 ]^{3+} + 3\text{OH}^- \rightarrow [ \text{M}(\text{H}_2\text{O})_3(\text{OH})_3 ] + 3\text{H}_2\text{O}$$

Observation:

A precipitate forms (e.g., white for $\text{Al}^{3+}$, brown for $\text{Fe}^{3+}$).

Amphoteric behaviour:

Some metal hydroxides, like aluminum hydroxide $[ \text{Al}(\text{OH})_3 ]$, are amphoteric and will dissolve in excess $\text{OH}^-$, forming soluble complexes:

$$[ \text{Al}(\text{OH})_3 ] + \text{OH}^- \rightarrow [ \text{Al}(\text{OH})_4 ]^-$$

Reaction with Ammonia ($NH_3$)

Ammonia dissolves in water to produce $\text{OH}^-$ ions, which can also precipitate metal hydroxides in a similar way to $NaOH$.

Additionally, $\text{NH}_3$ can act as a ligand, particularly with transition metals like copper.

Reaction with $[ \text{M}(\text{H}_2\text{O})_6 ]^{2+}$

Initial Reaction:

$$[ \text{M}(\text{H}_2\text{O})_6 ]^{2+} + 2\text{NH}_3 \rightarrow [ \text{M}(\text{H}_2\text{O})_4(\text{OH})_2 ] + 2\text{NH}_4^+$$

Observation:

Precipitate forms, similar to the reaction with $NaOH$.

Ligand Exchange with $\text{Cu}^{2+}$

Excess $\text{NH}_3$ can replace water molecules around $\text{Cu}^{2+}$, forming a deep blue solution of $[ \text{Cu}(\text{NH}_3)_4(\text{H}_2\text{O})_2 ]^{2+}$

Reaction with Sodium Carbonate ($Na_2CO_3$)

The reaction of metal-aqua ions with carbonate ions depends on the metal's oxidation state:

$[ \text{M}(\text{H}_2\text{O})_6 ]^{2+}$ Ions:

Reaction:

$$[ \text{M}(\text{H}_2\text{O})_6 ]^{2+} + \text{CO}_3^{2-} \rightarrow \text{MCO}_3 + 6\text{H}_2\text{O}$$

Observation:

Insoluble metal carbonates precipitate (e.g., green $\text{FeCO}_3$ for $\text{Fe}^{2+}$).

$[ \text{M}(\text{H}_2\text{O})_6 ]^{3+}$ Ions:

Reaction:

$$[\text{M}(\text{H}_2\text{O})_6 ]^{3+} + 3\text{CO}_3^{2-} \rightarrow [ \text{M}(\text{H}_2\text{O})_3(\text{OH})_3 ] + 3\text{CO}_2$$

Observation:

A metal hydroxide precipitate forms with gentle fizzing due to $\text{CO}_2$ release.