8.2.9 Reactions of Metal-Aqua Ions

Practical: Reactions of Metal-Aqua Ions with Bases

Aim:

In this practical, you will investigate the reactions of metal-aqua ions (specifically copper(II), iron(II), iron(III), and aluminium ions) with three different bases: sodium hydroxide ($OH⁻$), ammonia ($NH₃$), and sodium carbonate ($CO₃²⁻$). You will observe any precipitates formed and the changes that occur when these bases are added in excess.

Equipment List

1. Test Tubes: Multiple clean, dry test tubes for each metal-aqua ion reaction (at least four per ion solution).
2. Test Tube Rack: To safely hold test tubes during the experiment.
3. Dropper Pipettes: For accurately adding 10 drops of each reagent to the test tubes.
4. Labeling Markers: To label test tubes for different metal-aqua ions and reagents used.
5. Metal-Aqua Ion Solutions:

• Copper(II) sulfate solution (for $Cu²⁺$ions)
• Iron(II) sulfate solution (for $Fe²⁺$ions)
• Iron(III) chloride solution (for $Fe³⁺$ions)
• Aluminium sulfate solution (for $Al³⁺$ions)

6. Sodium Hydroxide Solution (1.0 mol dm⁻³): For initial reaction and excess testing.
7. Ammonia Solution (1.0 mol dm⁻³): To test reactions and add in excess.
8. Sodium Carbonate Solution (1.0 mol dm⁻³): As the third base reagent for reactions.
9. Gloves and Goggles: For safety when handling potentially harmful chemicals.
10. Waste Disposal Beaker: To safely dispose of used solutions after observing results.

Method:

1. Add Metal-Aqua Ions:

• Begin by placing 10 drops of the metal-aqua ion solution ($Cu²⁺ Fe²⁺ Fe³⁺ or Al³$⁺) into a clean, dry test tube.

2. Add Sodium Hydroxide ($OH⁻$):

• Add 10 drops of 1.0 mol dm⁻³ sodium hydroxide solution to the test tube.
• Shake gently and record any observations of precipitates or solution colour.
• Then add another 10 drops of sodium hydroxide to ensure the solution is in excess, shake again, and note any further changes.

3. Repeat with Ammonia ($NH₃$):

• Repeat the above procedure, but this time using 1.0 mol dm⁻³ ammonia solution.
• Add 10 drops initially, shake, and record observations. Then add 10 more drops of ammonia to put it in excess, and note any additional changes.

4. Repeat with Sodium Carbonate ($CO₃²⁻$):

• Finally, repeat the process with 1.0 mol dm⁻³ sodium carbonate solution.
• Add 10 drops, observe, then add 10 more drops and note the changes in the test tube.

5. Leave for a Few Minutes:

• Allow each test tube to sit for a few minutes before disposing of the contents, as some reactions may develop over time.

Results:

Ion	Colour of Aqueous Ion	Reaction with $OH⁻$	Reaction with $NH₃$	Reaction with $CO₃²⁻$
$Fe²⁺$	Pale green	Dark green precipitate, insoluble in excess, turns orange-brown over time	Dark green precipitate, insoluble in excess, turns orange-brown	Green precipitate
$Cu²⁺$	Pale blue	Light blue precipitate, insoluble in excess	Light blue precipitate, soluble in excess forming a dark blue solution	Blue precipitate
$Al³⁺$	Colourless	White precipitate, redissolves in excess to give a colourless solution	White precipitate, insoluble in excess	White precipitate, some effervescence
$Fe³⁺$	Pale orange	Red-brown precipitate, insoluble in excess	Red-brown precipitate, insoluble in excess	Red-brown precipitate with some effervescence

Key Observations:

• $Fe²⁺$Both $OH⁻$and $NH₃$ form a green precipitate that eventually turns orange-brown at the surface due to oxidation.
• $Cu²⁺$A light blue precipitate is formed with OH⁻ and NH₃, but ammonia in excess dissolves it into a dark blue solution.
• $Al³⁺$A white precipitate forms, but with OH⁻ in excess, it dissolves to form a colourless solution.
• $Fe³⁺$, Produces a red-brown precipitate in all reactions, but with $CO₃²⁻$, effervescence is observed due to $CO₂$ release.