OCR A-Level Chemistry Unified chemistry (03), November 2020: Question 2
6 marks · Medium difficulty · Extended Response
Explain the terms ligand, coordination number and ligand substitution using examples of complex ions with different shapes, limited to monodentate ligands.
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Question text
Transition metal ions can bond to ligands to form complex ions with different shapes.
Explain what is meant by the terms ligand, coordination number and ligand substitution, using
suitable examples of complex ions with different shapes, limited to monodentate ligands.
Your answer should include diagrams and equations where appropriate. [6]
Additional answer space if required
Mark scheme
Show the mark scheme
AO
Question Answer Marks Guidance
element
Refer to marking instructions on page 5 of mark scheme for 6 1.1×4 Indicative scientific points may include:
guidance on marking this question. 2.1×2 Terms
Level 3 (5-6 marks) • Ligand: Donates a lone pair to metal ion
Comprehensive explanation of the terms, ligand and Forms dative covalent (coordinate) bond with metal ion
coordination number and ligand substitution • Coordination number: Number of coordinate bonds
AND to metal ion. Could be implicit in annotated diagrams
3D diagrams of suitable examples of 6 AND 4 coordinate NOTE: For monodentate ligands, ‘number of ligands’
complex ions with different shapes
AND is the same as the number of coordination number
Ligand substitution illustrated with a balanced equation • Ligand substitution: One ligand replacing another
Suitable examples of complex ions with different shapes
There is a well-developed line of reasoning which is clear • Coordination no 6
and logically structured. The information presented is 2+ 3+
Octahedral e.g. [Cu(H2O)6] , [Fe(H2O)6]
relevant and substantiated.
Level 2 (3-4 marks) • Coordination no 4
Explanation of the terms, ligand and coordination number Tetrahedral e.g. CuCl 2–, CoCl 2–
and ligand substitution with some errors or omissions OR Square planar Pt complexes, e.g. Pt(NH3)2Cl2
AND: Diagrams and equations
Diagrams of suitable examples of 6 AND 4 coordinate • Diagrams of complex ions (may be 3D)
complex ions with different shapes
• Equation for ligand substitution
OR
e.g. [Cu(H O) ]2+ + 4Cl– → CuCl 2– + 6H O
A 3D wedged diagram of a suitable example of 6 OR 4 2 6 4 2
coordination [Cu(H O) ]2+ + 4NH → [Cu(NH ) (H O) ]2+ + 4H O
26 3 3 4 2 2 2
OR NOTE: A clear and logically structured response
A diagram of a suitable example of 6 OR 4 coordination would link shapes with some of: coordination number,
AND ligand substitution illustrated with an equation names of shapes, connectivity, involvement of lone
OR pairs, bond angles, etc. (not inclusive)
Ligand substitution illustrated with a balanced equation
There is a line of reasoning presented with some structure. ALLOW minor slips
The information presented is relevant and supported by
some evidence NOTE: Levels and the mark within a level is a
‘best-fit’, not perfection
AO
element
Level 1 (1-2 marks)
Explanation of some terms: ligand, coordination number and
ligand substitution with some errors or omissions.
AND
A suitable example of a complex ion OR Ligand substitution
illustrated with an equation with some errors
There is an attempt at a logical structure with a line of
reasoning. The information is in the most part relevant.
0 marks
No response or no response worthy of credit.
Total 6
How to answer it
Transition Metal Complexes, Ligands, and Substitution
What this question tests
This 6-mark extended response question assesses your precise scientific definitions of core transition metal terminology (ligand, coordination number, ligand substitution), your ability to illustrate different coordination geometries (specifically 6-coordinate octahedral and 4-coordinate tetrahedral/square planar) using 3D structural representations, and your ability to write balanced equations for ligand substitution reactions involving monodentate ligands.
Complete Examination Breakdown (6-Mark Level Response)
💡 Key Knowledge: Definitions
- Ligand: A species that donates a lone pair of electrons to a metal ion, forming a dative covalent (coordinate) bond.
- Coordination Number: The total number of coordinate bonds formed between ligand donor atoms and the central metal ion.
- Ligand Substitution: A reaction in which one ligand in a complex ion is replaced by another ligand.
✅ Correct Exemplar Examples
- Coordination Number 6 (Octahedral): [Cu(H₂O)₆]²⁺ or [Fe(H₂O)₆]³⁺
- Coordination Number 4 (Tetrahedral): [CuCl₄]²⁻ or [CoCl₄]²⁻
- Substitution Equation: [Cu(H₂O)₆]²⁺ + 4Cl⁻ → [CuCl₄]²⁻ + 6H₂O
🧠 Exam Technique & Level Descriptor
Marked via a Level of Response (Levels 1–3) 6-mark scheme:
- Level 3 (5–6 marks): Comprehensive definitions, correct 3D diagrams for both 6- and 4-coordinate systems, and a fully balanced substitution equation with clear logic.
- To secure top marks, make sure your diagrams clearly demonstrate 3D spatial arrangement (using wedges and dashes) and that coordination numbers match your chosen examples.
❌ Common Errors & Pitfalls
- Vague Definitions: Stating a ligand "attaches to a metal" rather than specifically mentioning the donation of a lone pair to form a dative covalent bond.
- Imbalanced Equations: Forgetting stoichiometry during chloride substitution (e.g., failing to write 4 Cl⁻ yielding a tetrahedral complex).
- Poor 3D Drawings: Drawing flat, 2D structures for tetrahedral or octahedral complexes without indicating bonds coming out of (wedges) or going into (dashes) the plane.
Topics
Module 5: Physical chemistry and transition elements · 5.3 Transition elements
Question and mark scheme from the OCR A-Level Chemistry examination, Unified chemistry (03), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.