AQA A-Level Chemistry Paper 3, June 2018: Question 26

1 mark · Medium difficulty · Multiple Choice

Identify the correct thermodynamic statement regarding a ligand substitution reaction involving the chelate effect in a cobalt(II) complex.

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Question

Question 26 asks: 'Which statement is correct about this reaction?' followed by the chemical equation: [Co(NH3)6]2+ + 3 H2NCH2CH2NH2 -> [Co(H2NCH2CH2NH2)3]2+ + 6 NH3. Four multiple-choice options are given: A 'The co-ordination number of cobalt decreases.', B 'The enthalpy change is large and positive.', C 'The entropy change is large and positive.', and D 'The shape of the complex changes from octahedral.'
Question text

26 Which statement is correct about this reaction?

[Co(NH ) ]2+ + 3H NCH CH NH [Co(H NCH CH NH ) ]2+ + 6NH

36 2 2 2 2 2 2 2 2 3 3

[1 mark]

A The co-ordination number of cobalt decreases.

B The enthalpy change is large and positive.

C The entropy change is large and positive.

D The shape of the complex changes from octahedral.

Mark scheme

Show the mark scheme Mark scheme for question 26 indicating that the correct answer is C.

26 C

How to answer it

The Chelate Effect & Ligand Substitution

What this question tests

This question assesses understanding of ligand exchange reactions involving transition metal complexes:

  • The Chelate Effect: How substitution of monodentate ligands by multidentate ligands impacts entropy (ΔS).
  • Denticity & Co-ordination Number: Distinguishing monodentate (NH₃) from bidentate (ethane-1,2-diamine) ligands.
  • Enthalpy Changes (ΔH): Why breaking and forming similar coordinate bonds (Co–N) results in near-zero enthalpy changes.
  • Complex Geometry: Deducing the 3D shape based on the total number of coordinate bonds formed.
Question 26 Analysis

Ligand Substitution of Hexaamminecobalt(II)

Reaction: [Co(NH₃)₆]²⁺ + 3H₂NCH₂CH₂NH₂ → [Co(H₂NCH₂CH₂NH₂)₃]²⁺ + 6NH₃

✅ Correct Option: C

"The entropy change is large and positive."

On the reactant side, there are 4 particles in total (1 complex ion + 3 ligand molecules). On the product side, there are 7 particles (1 complex ion + 6 ligand molecules). An increase in the total number of particles in solution leads to a significant increase in disorder, making ΔS large and positive.

💡 Key Knowledge: The Chelate Effect

  • Bidentate Ligand: Ethane-1,2-diamine (often abbreviated as en) has two nitrogen lone pairs and forms 2 coordinate bonds per molecule.
  • Enthalpy Change (ΔH ≈ 0): Six Co–N coordinate bonds are broken and six very similar Co–N coordinate bonds are formed. Hence, ΔH is very small (near zero), neither large and positive nor large and negative.
  • Feasibility: Since ΔG = ΔH - TΔS, a large positive ΔS makes ΔG significantly negative, driving the reaction forward and favouring the chelated complex.

📐 Step-by-Step Option Elimination

  1. Check Option A (Coordination Number):
    Reactant has 6 monodentate NH₃ ligands → CN = 6.
    Product has 3 bidentate ligands → 3 × 2 = 6 coordinate bonds → CN = 6.
    Result: Incorrect (coordination number stays 6).
  2. Check Option B (Enthalpy Change):
    Six Co–N bonds broken; six Co–N bonds made. Bond enthalpies are almost identical.
    Result: Incorrect (ΔH is close to 0, not large and positive).
  3. Check Option C (Entropy Change):
    Particles before = 1 + 3 = 4.
    Particles after = 1 + 6 = 7.
    Result: Correct (ΔS > 0 and large).
  4. Check Option D (Shape):
    Both reactant and product complexes have a coordination number of 6, which corresponds to an octahedral geometry.
    Result: Incorrect (the shape remains octahedral).

❌ Common Errors & Misconceptions

  • Confusing Ligand Count with Coordination Number: Students often see "3" ligands in [Co(en)₃]²⁺ and assume the coordination number dropped to 3 or changed to planar/trigonal. Remember: Coordination number = total number of coordinate bonds , not total number of ligands.
  • Assuming Endothermic Breakdown: Thinking that because 6 NH₃ ligands leave, substantial energy is absorbed (Option B). This overlooks that 6 new coordinate bonds are simultaneously formed.

🧠 Exam Technique: 5-Second Multiple Choice Rule

Whenever you see monodentate ligands being replaced by bidentate or multidentate ligands (like EDTA⁴⁻):

  • Count particles immediately: 4 → 7 .
  • Particle increase = large positive ΔS.
  • This is the textbook definition of the chelate effect. Look directly for the entropy statement.
Mark Scheme: Award 1 mark for correct selection C.

Topics

Inorganic Chemistry · Physical Chemistry · 3.2.5 Transition Metals · 3.1.8 Thermodynamics

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.