Reactions of transition metal ions

Chemistry· Unit 21: Transition elements· 15 min read

1. Reactions with Aqueous Sodium Hydroxide★★☆☆☆⏱ 5 min

📘 Definition

Insoluble Transition Metal Hydroxide

When aqueous NaOH provides $O H^{-}$ ions that displace water ligands from hydrated transition metal ions, forming a neutral, insoluble hydroxide precipitate. Characteristic colours allow easy identification of metal ions.

Example:

Aqueous $C u^{2 +}$ forms a pale blue precipitate of $C u (O H)_{2} (s)$

Almost all transition metal hydroxides are insoluble in water, and only chromium(III) hydroxide is amphoteric, meaning it will dissolve in excess strong concentrated NaOH to form a soluble complex ion.

📐 Worked Example

Describe all observations and write balanced equations when dilute NaOH is added dropwise until excess to aqueous $C r^{3 +}$ solution.

1
When NaOH is first added, a green gelatinous precipitate forms:
2
$Cr^{3+}(aq) + 3OH^-(aq) \rightarrow Cr(OH)_3(s)$
3
When excess concentrated NaOH is added, the green precipitate dissolves to form a dark green solution of the hexahydroxochromate(III) complex:
4
$Cr(OH)_3(s) + 3OH^-(aq) \rightarrow [Cr(OH)_6]^{3-}(aq)$

Exam tip:

Always state both the colour and the state (precipitate vs solution) to earn full marks in CIE exams.

2. Reactions with Aqueous Ammonia★★★☆☆⏱ 6 min

Aqueous ammonia acts as both a base (producing $O H^{-}$ from ionization with water) and a ligand. An initial hydroxide precipitate almost always forms first, then in many cases the precipitate dissolves in excess ammonia via ligand exchange to form a soluble amine complex.

📘 Definition

Ligand Exchange

A reaction where one ligand in a transition metal complex is replaced by a different ligand, almost always accompanied by a distinct, characteristic colour change.

📐 Worked Example

State all observations and write equations for the reaction when ammonia is added dropwise to excess to aqueous copper(II) sulfate.

1
Initial addition of ammonia produces a pale blue precipitate of copper(II) hydroxide:
2
$Cu^{2+}(aq) + 2OH^-(aq) \rightarrow Cu(OH)_2(s)$
3
Excess concentrated ammonia dissolves the pale blue precipitate to form a deep royal blue solution of tetraamminecopper(II):
4
$Cu(OH)_2(s) + 4NH_3(aq) \rightarrow [Cu(NH_3)_4]^{2+}(aq) + 2OH^-(aq)$

3. Reactions with Sodium Carbonate★★★☆☆⏱ 4 min

The reaction of transition metal ions with carbonate depends on the charge of the metal ion, due to differences in charge density and acidity of the hydrated ion:

+2 charged transition metal ions form insoluble neutral carbonate precipitates
+3 charged transition metal ions have high charge density, making hydrated ions acidic. They react with carbonate to produce carbon dioxide gas and an insoluble hydroxide precipitate, no carbonate forms.

📐 Worked Example

Compare the reaction of aqueous $F e^{2 +}$ and $F e^{3 +}$ with sodium carbonate solution, including observations and balanced equations.

1
For pale green aqueous $F e^{2 +}$ : A pale green precipitate of iron(II) carbonate forms:
2
$Fe^{2+}(aq) + CO_3^{2-}(aq) \rightarrow FeCO_3(s)$
3
: For yellow-brown aqueous $F e^{3 +}$ : Effervescence of carbon dioxide gas occurs, and a brown gelatinous precipitate of iron(III) hydroxide forms:
4
$2Fe^{3+}(aq) + 3CO_3^{2-}(aq) + 3H_2O(l) \rightarrow 2Fe(OH)_3(s) + 3CO_2(g)$

4. Common Additional Ligand Exchange Reactions★★★★☆⏱ 5 min

CIE 9701 regularly tests two other key ligand exchange reactions for qualitative identification of transition metal ions, and for colorimetry experiments.

📐 Worked Example

Concentrated hydrochloric acid is added to a pale blue solution of aqueous copper(II) sulfate. Describe the observation and write the balanced equation for the reaction.

1
High concentrations of $C l^{-}$ ligands from concentrated HCl substitute the water ligands around $C u^{2 +}$ :
2
The solution changes colour from pale blue to yellow-green, forming the tetrachlorocuprate(II) complex:
3
$[Cu(H_2O)_6]^{2+}(aq) + 4Cl^-(aq) \rightarrow [CuCl_4]^{2-}(aq) + 6H_2O(l)$

5. Common Pitfalls

Wrong move:

Stating all transition metal hydroxides dissolve in excess ammonia

Why:

Only a small number of transition metals form stable soluble ammonia complexes; most do not dissolve

Correct move:

Only $C u^{2 +}$ , $C o^{2 +}$ form soluble complexes in excess ammonia; $F e^{2 +}$ , $F e^{3 +}$ , $M n^{2 +}$ do not

Wrong move:

Forgetting to mention the initial precipitate when adding ammonia

Why:

CIE examiners require full observations, including changes before adding excess reagent

Correct move:

Always describe the initial precipitate formation first, then the change when excess is added

Wrong move:

Writing that $F e^{3 +}$ forms iron(III) carbonate with sodium carbonate

Why:

High charge density of $F e^{3 +}$ makes hydrated $F e^{3 +}$ acidic, so carbonate decomposes to $C O_{2}$

Correct move:

All +3 transition metal ions form hydroxide precipitate and $C O_{2}$ gas, not carbonate salts

Wrong move:

Confusing the colour of copper(II) hydroxide and tetraamminecopper(II)

Why:

CIE requires specific colour descriptions to award full marks for observation questions

Correct move:

Call copper(II) hydroxide a pale blue precipitate, and tetraamminecopper(II) a deep/royal blue solution

Wrong move:

Stating $C r (O H)_{3}$ dissolves in excess ammonia

Why:

$C r (O H)_{3}$ is only amphoteric with strong bases, it does not form a soluble ammonia complex

Correct move:

$C r (O H)_{3}$ dissolves only in excess concentrated NaOH, remains as a precipitate in excess ammonia

6. Quick Reference Cheatsheet

Metal Ion	NaOH (dilute / excess)	NH₃ (dilute / excess)	Na₂CO₃
Cu²+ (pale blue)	Pale blue ppt, insoluble	Pale blue ppt → deep blue solution	Blue-green CuCO₃ ppt
Fe²+ (pale green)	Dirty green ppt, insoluble	Dirty green ppt, insoluble	Pale green FeCO₃ ppt
Fe³+ (yellow-brown)	Reddish-brown ppt, insoluble	Reddish-brown ppt, insoluble	Brown ppt + CO₂ effervescence
Cr³+ (green)	Green ppt → dark green solution	Green ppt, insoluble	Green ppt + CO₂ effervescence
Co²+ (pink)	Blue ppt, insoluble	Blue ppt → brown solution	Pink CoCO₃ ppt
Mn²+ (pale pink)	Pale brown ppt, insoluble	Pale brown ppt, insoluble	Pale pink MnCO₃ ppt

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2022 · 2
Identify transition metal ions from reactions
2023 · 4
Write equations for ligand exchange
2021 · 1
Carbonate reaction multiple choice

Going deeper

syllabusOfficial CIE 9701 SyllabusVerify current learning outcomes

What's Next

Reactions of transition metal ions are a core part of inorganic qualitative analysis for CIE A-Level Chemistry, and these reactions are regularly tested in both multiple choice and structured long answer questions. A solid understanding of characteristic colours and products allows you to easily answer identification questions, which carry multiple marks in exams. This topic also provides the foundation for understanding the redox behaviour and catalytic properties of transition elements, which are covered later in the unit.