Transition metal ions form hydrated hexaaqua complex ions in aqueous solution. Because of their differing oxidation states and electron structures, they exhibit characteristic precipitation and ligand exchange reactions when treated with bases. This guide summarizes the procedures, equations, and colours needed to identify these ions.
🔑 Core Specification Link
This practical links directly to topic 3.2.5 Transition metals, covering the properties, complexes, precipitation, and amphoteric behavior of transition metal ions.
Precipitate Colours Reference
The diagram below displays the characteristic colours of precipitates formed when a few drops of sodium hydroxide or ammonia are added to the hexaaqua metal complexes:
Experimental Procedures
Part A: Reactions with Sodium Hydroxide (\(\text{NaOH}\))
- Add 10 drops of the hexaaqua metal ion solution into a clean test tube.
- Add dilute sodium hydroxide solution dropwise. Record the initial colour of any precipitate formed.
- Continue to add sodium hydroxide until in excess. Shake the tube and observe whether the precipitate dissolves or remains insoluble.
Part B: Reactions with Ammonia (\(\text{NH}_3\))
- Add 10 drops of the hexaaqua metal ion solution into a clean test tube.
- Add dilute ammonia solution dropwise. Record the initial precipitate colour.
- Continue to add ammonia until in excess. Shake the tube and observe whether the precipitate dissolves.
Summary of Precipitates, Excess Reactions, and Equations
| Metal Cation | Starting Hexaaqua Ion | Add drops of base (\(\text{OH}^-\) or \(\text{NH}_3\)) | Adding excess \(\text{NaOH}\) | Adding excess \(\text{NH}_3\) |
|---|---|---|---|---|
| \(\text{Cu}^{2+}\) | \([\text{Cu(H}_2\text{O)}_6]^{2+}(\text{aq})\) (blue) |
\(\text{Cu(OH)}_2(\text{H}_2\text{O)}_4(\text{s})\) Blue precipitate |
Insoluble. Remains blue precipitate. |
Dissolves to form a deep blue solution. \([\text{Cu(NH}_3)_4(\text{H}_2\text{O)}_2]^{2+}\) |
| \(\text{Fe}^{2+}\) | \([\text{Fe(H}_2\text{O)}_6]^{2+}(\text{aq})\) (pale green) |
\(\text{Fe(OH)}_2(\text{H}_2\text{O)}_4(\text{s})\) Green precipitate |
Insoluble. Precipitate slowly darkens to brown at surface in air. |
Insoluble. Precipitate slowly darkens to brown. |
| \(\text{Fe}^{3+}\) | \([\text{Fe(H}_2\text{O)}_6]^{3+}(\text{aq})\) (yellow-brown) |
\(\text{Fe(OH)}_3(\text{H}_2\text{O)}_3(\text{s})\) Rust-brown precipitate |
Insoluble. Remains brown precipitate. |
Insoluble. Remains brown precipitate. |
| \(\text{Cr}^{3+}\) | \([\text{Cr(H}_2\text{O)}_6]^{3+}(\text{aq})\) (violet-green) |
\(\text{Cr(OH)}_3(\text{H}_2\text{O)}_3(\text{s})\) Grey-green precipitate |
Dissolves to form a dark green solution. \([\text{Cr(OH)}_6]^{3-}\) (amphoteric) |
Insoluble. Remains grey-green precipitate. |
| \(\text{Mn}^{2+}\) | \([\text{Mn(H}_2\text{O)}_6]^{2+}(\text{aq})\) (very pale pink) |
\(\text{Mn(OH)}_2(\text{H}_2\text{O)}_4(\text{s})\) Buff/cream precipitate |
Insoluble. Rapidly darkens to brown in air. |
Insoluble. Rapidly darkens to brown in air. |
| \(\text{Co}^{2+}\) | \([\text{Co(H}_2\text{O)}_6]^{2+}(\text{aq})\) (pink) |
\(\text{Co(OH)}_2(\text{H}_2\text{O)}_4(\text{s})\) Blue-green precipitate |
Insoluble. Precipitate darkens/turns pink. |
Dissolves to form a straw-yellow solution. \([\text{Co(NH}_3)_6]^{2+}\) |
Important Chemical Mechanisms
1. Deprotonation (Precipitation)
Both sodium hydroxide and ammonia act as bases (proton acceptors) to deprotonate the acidic water ligands attached to the metal center. Because neutral complexes carry no charge, they precipitate out of aqueous solution:
\[ [\text{Cu(H}_2\text{O)}_6]^{2+}(\text{aq}) + 2\text{OH}^-(\text{aq}) \rightarrow \text{Cu(OH)}_2(\text{H}_2\text{O)}_4(\text{s}) + 2\text{H}_2\text{O}(\text{l}) \] \[ [\text{Fe(H}_2\text{O)}_6]^{3+}(\text{aq}) + 3\text{NH}_3(\text{aq}) \rightarrow \text{Fe(OH)}_3(\text{H}_2\text{O)}_3(\text{s}) + 3\text{NH}_4^+(\text{aq}) \]2. Amphoteric Behavior of Chromium(III)
Chromium(III) hydroxide is amphoteric. In excess sodium hydroxide (a strong base), the neutral precipitate undergoes further deprotonation to form a soluble negatively charged hexahydroxochromate(III) complex:
\[ \text{Cr(OH)}_3(\text{H}_2\text{O)}_3(\text{s}) + 3\text{OH}^-(\text{aq}) \rightarrow [\text{Cr(OH)}_6]^{3-}(\text{aq}) + 3\text{H}_2\text{O}(\text{l}) \]3. Ligand Exchange with Excess Ammonia
Ammonia is a nucleophile and can displace water ligands. With excess ammonia, the copper(II) and cobalt(II) precipitates dissolve via a ligand exchange reaction:
\[ \text{Cu(OH)}_2(\text{H}_2\text{O)}_4(\text{s}) + 4\text{NH}_3(\text{aq}) \rightarrow [\text{Cu(NH}_3)_4(\text{H}_2\text{O)}_2]^{2+}(\text{aq}) + 2\text{OH}^-(\text{aq}) + 2\text{H}_2\text{O}(\text{l}) \]Safety & Risk Assessment
| Hazard | Risk | Precaution |
|---|---|---|
| 2.0 mol dm⁻³ Sodium Hydroxide | Corrosive; causes skin burns and severe eye damage. | Wear safety goggles and gloves. Wash splashes off skin immediately. |
| Ammonia solution | Corrosive and has a pungent, irritating vapour. | Handle in a fume cupboard or well-ventilated space. Keep sample bottles stoppered. |
| Cobalt(II) sulfate solution | May cause cancer if inhaled; toxic to aquatic life. | Wear gloves; handle minimum quantities; dispose of in the heavy metals waste container. |
Sources of Error & Improvements
- Oxidation of Fe(II) and Mn(II) precipitates: Iron(II) and manganese(II) hydroxides react with dissolved oxygen in water and air, turning brown rapidly. This can confuse students identifying unknown samples. Improvement: Use freshly prepared solutions and record colours immediately after precipitation.
- Contaminated glassware: Trace metal contaminants in test tubes can yield wrong colours or trigger false precipitates. Improvement: Thoroughly clean test tubes with dilute nitric acid and rinse with deionised water before starting.
Common Exam Questions
1. Write a balanced equation for the reaction of chromium(III) hydroxide precipitate with excess sodium hydroxide.
\[ \text{Cr(OH)}_3(\text{H}_2\text{O)}_3(\text{s}) + 3\text{OH}^-(\text{aq}) \rightarrow [\text{Cr(OH)}_6]^{3-}(\text{aq}) + 3\text{H}_2\text{O}(\text{l}) \]
2. Explain why the green precipitate of Fe(II) hydroxide slowly turns brown at the top of the test tube when left standing in air.
The green precipitate of iron(II) hydroxide, \(\text{Fe(OH)}_2(\text{H}_2\text{O)}_4\), is oxidised by oxygen in the air to form the rust-brown iron(III) hydroxide, \(\text{Fe(OH)}_3(\text{H}_2\text{O)}_3\).
3. State the formula of the complex ion responsible for the deep blue solution formed when excess ammonia is added to a solution of copper(II) ions.
\[ [\text{Cu(NH}_3)_4(\text{H}_2\text{O)}_2]^{2+} \]
CPAC Skills Assessed
- CPAC 1: Follows multi-step written protocols to carry out sequential qualitative analysis.
- CPAC 3: Safely handles corrosive alkalis and heavy metal ion solutions.
- CPAC 4: Records detailed descriptions of precipitate colors and solubility in excess.
Always distinguish clearly between precipitate colours. Copper(II) forms a blue precipitate, iron(II) forms a green precipitate, and chromium(III) forms a grey-green precipitate. Mixing up green and grey-green will lose marks.