Question

Distinguish between the following pairs of compounds using the test given within brackets:
(i) Iron(II) sulphate and iron (III) sulphate (using ammonium hydroxide).

Answer 1

Reaction with iron(III) sulphate:

• When ammonium hydroxide $\left({\mathrm{NH}}_4\mathrm{OH}\right)$ is treated with iron(III) sulphate solution $\left({\mathrm{Fe}}_2{\left({\mathrm{SO}}_4\right)}_3\right)$, it produces a reddish brown precipitate of ferric hydroxide $\left(\mathrm{Fe}{\left(\mathrm{OH}\right)}_3\right)$, which is insoluble in excess of ${\mathrm{NH}}_4\mathrm{OH}$.
• ${\mathrm{Fe}}_2{\left({\mathrm{SO}}_4\right)}_3\left(\mathrm{aq}\right)+3{\mathrm{NH}}_4\mathrm{OH}\left(\mathrm{aq}\right)⟶\mathrm{Fe}(\mathrm{OH}{)}_3\left(\mathrm{s}\right)\downarrow +3{\left({\mathrm{NH}}_4\right)}_2{\mathrm{SO}}_4(\mathrm{aq})$

Reaction with iron(II) sulphate:

• Ammonium hydroxide reacts with iron(II) sulphate $\left({\mathrm{FeSO}}_4\right)$ solution to form a dirty green colored precipitate of $\mathrm{Fe}{\left(\mathrm{OH}\right)}_2$, which is insoluble in excess of ammonium hydroxide.
• ${\mathrm{FeSO}}_4\left(\mathrm{aq}\right)+2{\mathrm{NH}}_4\mathrm{OH}\left(\mathrm{aq}\right)⟶\mathrm{Fe}(\mathrm{OH}{)}_2\left(\mathrm{s}\right)\downarrow +{\left({\mathrm{NH}}_4\right)}_2{\mathrm{SO}}_4(\mathrm{aq})$