6.2.3Unit 6. Practical Skills in Chemistry II
Transition Metal Complexes — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 5 parts Lesson 12 of 14 Unit quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5Ligand Substitution
Changing the ligands changes the colour of the complex.
Ligand
- Ligand A molecule or ion that donates a lone pair to a central metal ion by forming a dative covalent bond.
Hexaaqua ions
- Hexaaqua ions In water, transition metal ions exist as octahedral [M(H₂O)₆]ⁿ⁺, such as pale blue [Cu(H₂O)₆]²⁺.
Ammonia
- Ammonia Excess ammonia replaces water to give [Cu(NH₃)₄(H₂O)₂]²⁺, which is a deep blue solution.
Chloride
- Chloride Concentrated hydrochloric acid gives tetrahedral [CuCl₄]²⁻, which is yellow-green, because chloride ions are larger.
| Ion | Ligand change | Colour change |
|---|---|---|
| Copper(II) | H₂O to NH₃ | Pale blue to deep blue |
| Copper(II) | H₂O to Cl⁻ | Pale blue to yellow-green |
| Cobalt(II) | H₂O to Cl⁻ | Pink to blue |
Part 2 of 5Preparing a Complex Salt
1
Dissolve copper(II) sulfate crystals in a small volume of water and add aqueous ammonia dropwise.
2
A pale blue precipitate of copper(II) hydroxide forms first and dissolves in excess ammonia to give a deep blue solution.
3
Add ethanol, in which the complex salt is less soluble, to cause deep blue crystals to separate.
4
Filter the crystals under reduced pressure and wash with a little ethanol.
5
Dry the crystals between filter papers and record the mass to find the percentage yield.
Part 3 of 5Why Colours Occur
Why Colours Occur
d-orbital splitting
- d-orbital splitting Ligands split the d orbitals into two energy levels in the complex ion.
Absorption
- Absorption Light of energy equal to the gap is absorbed by an electron moving to the higher level.
Observed colour
- Observed colour The colour seen is the light that is not absorbed, called the complementary colour.
Ligand effect
- Ligand effect Different ligands cause different splitting, so the same metal ion shows different colours.
Part 4 of 5Evaluating the Results
Coordination number
- Coordination number Water and ammonia give coordination number 6, while large chloride ligands give 4.
Reversibility
- Reversibility The change from [Cu(H₂O)₆]²⁺ to [CuCl₄]²⁻ is reversible, and adding water turns the mixture blue again.
Observation language
- Observation language State both starting and final colours for each change.
Safety
- Safety Concentrated hydrochloric acid and ammonia should be used in a fume cupboard.
Part 5 of 5Exam Checklist & Tips
6.2.3 – Key Points for the Exam
- Complex ion A central metal ion surrounded by ligands bonded by dative covalent bonds.
- Ligand An ion or molecule that donates a lone pair to a metal ion.
- Coordination number The number of dative bonds from ligands to the central metal ion.
- Ligand substitution A reaction in which one ligand is replaced by another.
Common Mistakes
- Writing copper(II) sulfate solution as containing Cu²⁺ without water ligands.
- Saying the complex colour is the colour of light absorbed rather than transmitted.
- Forgetting the intermediate pale blue precipitate before the deep blue ammonia solution.
- Claiming a tetrahedral chloro complex has coordination number 6.
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