# Reactions of transition metal ions

> Chemistry · CIE A-Level
> Source: https://www.owlsprep.com/study/cie-9701-u21-reactions-of-transition-metal-ions/

This module covers characteristic reactions of common first-row aqueous transition metal ions (Cu²+, Fe²+, Fe³+, Cr³+, Co²+, Mn²+) with NaOH, NH₃, sodium carbonate and common ligands, including colour changes and product identification for CIE 9701 exam preparation.

**Prerequisites:** [Transition element complex formation and electronic configuration](https://www.owlsprep.com/study/cie-9701-u21-introduction-to-transition-elements/)

## Learning objectives

- Describe reactions of aqueous transition metal ions with common reagents (NaOH, NH₃, Na₂CO₃)
- Explain ligand exchange reactions and their characteristic colour changes
- Identify transition metal ions from experimental observations
- Write balanced equations for all reactions covered

## Reactions with Aqueous Sodium Hydroxide

**Insoluble Transition Metal Hydroxide** — When aqueous NaOH provides $OH^-$ 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 $Cu^{2+}$ forms a pale blue precipitate of $Cu(OH)_2(s)$

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 $Cr^{3+}$ 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.

## Reactions with Aqueous Ammonia

Aqueous ammonia acts as both a base (producing $OH^-$ 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.

**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. $$Cu(OH)_2(s) + 4NH_3(aq) \rightarrow [Cu(NH_3)_4]^{2+}(aq) + 2OH^-(aq)$$

> **tip**
>
> Not all transition metal hydroxides dissolve in excess ammonia. Only $Cu^{2+}$, $Co^{2+}$ and $Cr^{3+}$ form soluble amine complexes in CIE 9701. All other common transition metal hydroxides remain as precipitates.

## Reactions with Sodium Carbonate

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 $Fe^{2+}$ and $Fe^{3+}$ with sodium carbonate solution, including observations and balanced equations.

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

## Common Additional Ligand Exchange Reactions

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

> **info**
>
> A common test for $Fe^{3+}$ ions uses thiocyanate ($SCN^-$) ions: addition of $SCN^-$ to $Fe^{3+}$ produces a deep blood-red complex $[Fe(SCN)(H_2O)_5]$, the intensity of which can be measured in colorimetry to find $Fe^{3+}$ concentration.

## Common pitfalls

- **Wrong:** Stating all transition metal hydroxides dissolve in excess ammonia
  - Why it fails: Only a small number of transition metals form stable soluble ammonia complexes; most do not dissolve
  - Correct: Only $Cu^{2+}$, $Co^{2+}$ form soluble complexes in excess ammonia; $Fe^{2+}$, $Fe^{3+}$, $Mn^{2+}$ do not
- **Wrong:** Forgetting to mention the initial precipitate when adding ammonia
  - Why it fails: CIE examiners require full observations, including changes before adding excess reagent
  - Correct: Always describe the initial precipitate formation first, then the change when excess is added
- **Wrong:** Writing that $Fe^{3+}$ forms iron(III) carbonate with sodium carbonate
  - Why it fails: High charge density of $Fe^{3+}$ makes hydrated $Fe^{3+}$ acidic, so carbonate decomposes to $CO_2$
  - Correct: All +3 transition metal ions form hydroxide precipitate and $CO_2$ gas, not carbonate salts
- **Wrong:** Confusing the colour of copper(II) hydroxide and tetraamminecopper(II)
  - Why it fails: CIE requires specific colour descriptions to award full marks for observation questions
  - Correct: Call copper(II) hydroxide a pale blue precipitate, and tetraamminecopper(II) a deep/royal blue solution
- **Wrong:** Stating $Cr(OH)_3$ dissolves in excess ammonia
  - Why it fails: $Cr(OH)_3$ is only amphoteric with strong bases, it does not form a soluble ammonia complex
  - Correct: $Cr(OH)_3$ dissolves only in excess concentrated NaOH, remains as a precipitate in excess ammonia

## 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 |

## 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.

- [Catalysis by Transition Metals](https://www.owlsprep.com/study/cie-9701-u21-catalysis-by-transition-metals/)
- [Arenes](https://www.owlsprep.com/study/cie-9701-u22-overview/)
- [Structure of benzene](https://www.owlsprep.com/study/cie-9701-u22-structure-of-benzene/)

---

From [OwlsPrep](https://www.owlsprep.com) — free study guides for A-Level, IB, AP and IGCSE, written against the official syllabus. Canonical page: https://www.owlsprep.com/study/cie-9701-u21-reactions-of-transition-metal-ions/