Edexcel A-Level Chemistry Paper 3, November 2020: Question 7

💡 Key Knowledge: Indicative Content Points (IPs)

• IP1: Formula and colour of first complex ion, e.g., [Co(H₂O)₆]²⁺ (pink) or [Co(NH₃)₆]²⁺ (yellow/brown).
• IP2: Formula and colour of second complex ion, e.g., [CoCl₄]²⁻ (blue).
• IP3: Definition of a ligand (an atom, ion, or molecule that can donate a lone pair of electrons to a central metal ion to form a dative covalent bond).
• IP4: Definition of coordination number (the number of dative covalent bonds to the central metal ion) and examples matching your chosen complexes (6 and 4 respectively).
• IP5: Shapes of the complex ions (octahedral for coordination number 6; tetrahedral for coordination number 4).
• IP6: Explanation for the different shapes/coordination numbers based on ligand size (chloride ions, Cl⁻ , are much larger than water molecules or ammonia molecules, so fewer can fit around the central cobalt ion due to steric hindrance/repulsion).

🧠 Exam Technique & Marking Logic

This question uses a levels-based mark scheme combining Indicative Content (up to 4 marks) with Structure and Lines of Reasoning (up to 2 marks).

• 4 IPs covered = 4 marks for chemistry content.
• 2 marks for structure: To secure both reasoning marks, your answer must be logically structured, use correct scientific terminology, make explicit comparisons (e.g., comparing ligand sizes clearly), and flow coherently without contradictory statements.

✅ Model Answer Elements (Exemplar Content)

"Cobalt(II) forms complex ions such as the octahedral [Co(H₂O)₆]²⁺ , which is pink and has a coordination number of 6. A ligand is defined as a species that donates a lone pair of electrons to a central metal ion. The coordination number is the total number of dative covalent bonds formed with the central metal ion. Conversely, reacting cobalt(II) with concentrated hydrochloric acid forms the tetrahedral [CoCl₄]²⁻ complex ion, which is blue and has a coordination number of 4. The change in coordination number from 6 to 4 occurs because chloride ions are significantly larger than water molecules. Consequently, only four bulky chloride ions can fit around the cobalt(II) ion due to increased steric repulsion, whereas six smaller water molecules can pack around it."

❌ Common Errors to Avoid

• Incorrect bond angles: Quoting bond angles like 90° for tetrahedral complexes will penalise your reasoning mark.
• Vague definitions: Stating a ligand is "just a molecule attached to a metal" without mentioning the lone pair or dative covalent bond loses the definition mark (IP3).
• Confusing coordination number: Stating coordination number is simply "the number of ligands" rather than "the number of dative covalent bonds" (important for multidentate ligands like EDTA or en).
• Omission of size reasoning: Forgetting to explicitly compare the size of chloride ions vs water/ammonia molecules means missing out on IP6.