OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2025: Question 15

1 mark · Medium difficulty · Multiple Choice

Identify which statements are correct regarding the oxidation states, colours, and coordination numbers of the complex ions [Cr(NH3)6]3+ and [Cr(OH)6]3-.

Practise this question

Question

Multiple choice question asking which statements are correct for the complex ions [Cr(NH3)6]3+ and [Cr(OH)6]3-. Statement 1: Chromium is in the same oxidation state in each complex ion. Statement 2: The complex ions are different colours. Statement 3: The complex ions have different coordination numbers. Options: A (1, 2 and 3), B (Only 1 and 2), C (Only 2 and 3), D (Only 1). Worth 1 mark.

Mark scheme

Show the mark scheme Mark scheme table showing question number 15 with answer B and 1 mark.

How to answer it

Properties of Chromium(III) Complex Ions

What this question tests

This multiple-choice question assesses your core knowledge of transition metal chemistry:

  • Calculating oxidation states of central transition metal ions with neutral vs charged ligands.
  • Understanding factors affecting the colour of transition metal complexes (specifically ligand identity).
  • Determining the coordination number of a complex based on the number of coordinate (dative covalent) bonds formed.
Question 15 Analysis

Evaluating Statements 1, 2, and 3

Complex Ions: [Cr(NH₃)₆]³⁺ and [Cr(OH)₆]³⁻

📐 Statement 1: Oxidation State Calculation

Let the oxidation state of chromium be x:

1. For [Cr(NH₃)₆]³⁺:

  • NH₃ is a neutral ligand (charge = 0).
  • x + 6(0) = +3 ⇒ x = +3

2. For [Cr(OH)₆]³⁻:

  • OH⁻ is a uninegative ligand (charge = −1).
  • x + 6(−1) = −3
  • x − 6 = −3 ⇒ x = +3

Both complexes contain chromium in the +3 oxidation state.

Conclusion: Statement 1 is TRUE.

💡 Statement 2: Why Colours Differ

Colours in transition metal complexes arise from d-d orbital splitting (ΔE):

  • Ligands split the 3d subshell into two different energy levels.
  • Electrons absorb specific visible frequencies ( ΔE = hν ) to promote to higher d-orbitals; the remaining light is transmitted.
  • Different ligands generate different magnitudes of splitting energy (ΔE).
  • Because NH₃ and OH⁻ exert different crystal field splittings, the complexes absorb different wavelengths and thus have different colours ([Cr(NH₃)₆]³⁺ is violet/purple; [Cr(OH)₆]³⁻ is deep green).

Conclusion: Statement 2 is TRUE.

🧠 Statement 3: Coordination Number

Coordination number = number of coordinate bonds to the central metal ion.

  • NH₃ is a monodentate ligand ⇒ 6 × 1 = 6 coordinate bonds.
  • OH⁻ is a monodentate ligand ⇒ 6 × 1 = 6 coordinate bonds.
  • Both complexes have a coordination number of 6 (octahedral).
  • They have the same coordination number, not different.

Conclusion: Statement 3 is FALSE.

Final Decision & Examiner Takeaways

✅ Correct Answer

Option B (Only 1 and 2)

Since Statement 1 and Statement 2 are correct, but Statement 3 is incorrect, the only possible option matching this combination is B.

Mark scheme award: [1 mark] for selecting B.

❌ Common Student Errors

  • Confusing charge with coordination number: Assuming the difference in overall charge (+3 vs −3) means the coordination number must be different.
  • Forgetting ligand charge: Treating OH as neutral, which would incorrectly yield an oxidation state of −3 for chromium.
  • Thinking you must memorise exact complex colours: You do not need to know the specific shade of [Cr(NH₃)₆]³⁺. You only need the conceptual rule: different ligands on the same metal ion cause different d-orbital splitting, producing different colours.

Topics

Module 5: Physical chemistry and transition elements · 5.3 Transition elements

Question and mark scheme from the OCR A-Level Chemistry examination, Periodic table, elements and physical chemistry (01), June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.