Edexcel A-Level Chemistry Paper 1, November 2021: Question 8

12 marks · Hard difficulty · Open Response

Explain ligand properties, colour, isomerism, and formulas of transition metal complex ions including cobalt and platinum complexes.

Practise this question

Question

A four-part question about transition metal complex ions. Part (a) asks why the ammonium ion cannot act as a ligand, and to explain why [Co(NH3)6]2+ and [Co(H2O)6]2+ are coloured and have different colours. Part (b) is a multiple-choice question showing four platinum complex structures labelled A, B, C, and D to identify which is used in cancer treatment. Part (c) asks to compare and contrast cobalt(III) complex ions formed with ethane-1,2-diamine and EDTA4- ligands, showing their chemical structures. Part (d) provides information about hydrated chromium(III) chloride reacting with silver nitrate and asks to deduce and justify the formula of the chromium(III) complex ion.
Question text

8 Transition metals form complex ions.

(a) Complex ions have a central metal ion surrounded by ligands.

(i) Give a reason why the ammonium ion cannot act as a ligand.

(1)

(ii) Explain why the complex ions [Co(NH )6]2+ and [Co(H O) ]2+ are coloured and

32 6

have different colours.

(4)

(b) Which of these complexes is used in the treatment of cancer?

(1)

Cl OH2 Cl OH2

A Pt B Pt

H2O Cl Cl OH2

Cl NH3 Cl OH2

C Pt D Pt

Cl NH3 H3N Cl

(c) Compare and contrast the complex ions formed by cobalt(III) ions with the ligand

ethane-1,2-diamine and with the ligand EDTA4–.

20 Ignore any difference in colour.

*P65463A02028* O

–O

O

O–

N

N

–O

O O–

NH2

H2N O

ethane-1,2-diamine EDTA4–

(4)

(d) Hydrated chromium(III) chloride, CrCl3.6H2O, dissolves in water to form a number

of different complex ions containing both chloride and water ligands.

The general formula of these complex ions is [Cr(H O) (Cl) ](3–y)+

2 x y

In an experiment, 0.10mol of a complex reacted with excess silver nitrate solution

to produce 0.20mol of silver chloride.

Chloride ions which are ligands within the complex do not react with

silver nitrate.

Deduce the formula of this chromium(III) complex ion. Justify your answer.

(2)

*P65463A02128*

(Total for Question 8 = 12 marks)

Mark scheme

Show the mark scheme The mark scheme for question 8, detailing answer points for parts (a) through (d). Part (a)(i) awards 1 mark for stating ammonium ions lack a lone pair. Part (a)(ii) awards 4 marks for explaining d-orbital splitting, ligand dependence, light absorption differences, and transmitted/reflected light. Part (b) awards 1 mark for selecting structure C. Part (c) awards 4 marks for comparing similarities (dative bonds, coordination number 6, octahedral shape) and differences (denticity, charge, entropy/stability). Part (d) awards 2 marks for justifying the number of free chloride ions and giving the correct formula [Cr(H2O)5(Cl)]2+.

Question

Answer Additional Guidance Mark

Number

8(a)(i) (1)

• ammonium ions do not have a lone pair (of electrons for bonding) Allow ammonium ions are positive and

so are repelled (by the positive metal

cation)

Ignore reference to it already having a

dative/coordinate bond

Question

Answer Additional Guidance Mark

Number

8(a)(ii) An answer that makes reference to (4)

• d orbitals/d sub-shell split (into two different energies) (1) Ignore ‘distort’

Do not award splitting of singular d

orbital

• difference in energy depends on the ligands (1)

• difference in energy leads in different

frequencies/wavelengths/photons of light absorbed (1)

• (so) the unabsorbed frequencies/wavelengths/photons are

reflected/transmitted (1) Allow ‘colour seen’ for

reflected/transmitted

Do not award ‘emission’

Do not award M3 nor M4 if reference

to electron ‘falling’ releases energy is

stated

Question

Answer Mark

Number

8(b) The only correct answer is C ( ) (1)

A is not correct because water is not one of the ligands and the configuration of chloride ions should be cis not

trans

B is not correct because water is not one of the ligands

D is not correct because the configuration should be cis not trans for the chloride ligands and one of the other

ligands is a water molecule rather than ammonia

Question

Answer Additional Guidance Mark

Number

8(c) An answer that makes reference to There must be some comparison. (4)

Hence two separate paragraphs on each

(Similarities) complex without this scores max (3)

At least one from

• both ligands form dative covalent bonds with the cobalt(III) ions (1) Allow both donate lone pairs of electrons

to cobalt(III) ions

• both have coordination number 6 (1) Allow both have 6 coordinate bonds

• both complex ions will be octahedral (1)

(Differences)

At least one from

• EDTA is hexadentate, ethane-1,2-diamine is bidentate Accept EDTA forms 6 bonds and ethane-

OR 1,2diamine forms 2

ratio of cobalt(III) to EDTA is 1:1, Ignore multidentate/polydentate

with ethane-1,2-diamine it is 1:3 (1)

• complex with EDTA will be anionic / negatively charged, with ALLOW EDTA is an anion,

ethane-1,2-diamine will be cationic / positively charged (1) ethane-1,2-diamine is neutral

• complex of EDTA is more stable than the complex with Allow molar ratios to illustrate, even if

ethane-1,2-diamine because there is an increase in entropy (1) incorrect

Question

Answer Additional Guidance Mark

Number

8(d) An answer that makes reference to the following points (2)

(Justification)

• two moles of chloride ions in aqueous solution so one mole of chloride

ion is in the complex (1)

[Cr(H O) (Cl)]2+

• complex ion formula (1) 2 5

(Total Question 8 = 12 marks)

How to answer it

Transition Metals and Complex Ions Exam Guide

What this question tests

This question assesses your understanding of transition metal chemistry, specifically ligand definition, d-orbital splitting and origin of colour in complex ions, isomerism/structures of medicinal complexes (Cisplatin), comparing mono/bi/polydentate ligands (entropy effects), and deducing complex ion formulas using stoichiometry and precipitation reactions with silver nitrate.

Question 8 (a) (i)

Ligand Definition & The Ammonium Ion

✅ Correct Answer

The ammonium ion ( NH₄⁺ ) cannot act as a ligand because it does not have a lone pair of electrons available for bonding.

❌ Common Errors

Students often lose this mark by stating that ammonium "already has a dative covalent bond" or mentioning its shape. Keep it strictly focused on the absence of a lone pair.

Mark: 1 mark
Question 8 (a) (ii)

Origin of Colour in Complex Ions

💡 Key Knowledge

  • Ligands cause the partially filled d orbitals to split into two different energy levels.
  • The size of the energy gap ( ΔE ) depends on the identity of the metal ion and the surrounding ligands.
  • Electrons absorb specific frequencies/wavelengths of visible light to promote d-d transitions.
  • The unabsorbed (transmitted/reflected) frequencies combine to create the observed colour.

🧠 Exam Technique (4-Step Chain)

To secure all 4 marks, explain in strict logical sequence:

  1. d orbitals split in energy.
  2. Different ligands cause different ΔE values.
  3. Different ΔE means different frequencies/wavelengths of light are absorbed.
  4. Remaining transmitted/reflected light gives different colours.
Mark: 4 marks
Question 8 (b)

Complexes in Cancer Treatment (Cisplatin)

✅ Correct Answer

C is the correct structure. Cisplatin has a square planar geometry with chloride ligands in the cis position (adjacent to each other) alongside two ammonia molecules.

❌ Common Errors & Distractors

  • A & B: Incorporate water molecules instead of ammonia.
  • D: Features a trans-arrangement or incorrect ligand types. Remember that anticancer Cisplatin specifically features adjacent ( cis ) ligands.
Mark: 1 mark
Question 8 (c)

Comparing Ethane-1,2-diamine and EDTA4−

💡 Similarities (Choose at least one)

  • Both ligands form dative covalent (coordinate) bonds with the cobalt(III) ion using lone pairs.
  • Both result in a coordination number of 6.
  • Both form octahedral complex ions.

💡 Differences (Choose at least one)

  • Denticity: EDTA is hexadentate, whereas ethane-1,2-diamine is bidentate (1:1 vs 1:3 molar ratio with Co(III)).
  • Charge: EDTA complexes are anionic (negative), while ethane-1,2-diamine complexes are cationic (positive).
  • Stability: The EDTA complex is more stable due to an increase in entropy ( ΔS ) when formed.

🧠 Exam Technique

You must structure your answer to explicitly include comparisons (both similarities and differences). Examiners look for distinct comparative points; listing facts about one complex without referencing the other will cap your marks.

Mark: 4 marks
Question 8 (d)

Deducing Chromium(III) Complex Ion Formula

📐 Calculation & Step-by-Step Justification

  1. Find moles of AgCl produced: 0.20 mol of AgCl is formed from 0.10 mol of the complex.
  2. Determine ratio: 0.20 / 0.10 = 2 moles of Cl⁻ ions react in solution per 1 mole of complex.
  3. Account for total chlorine: The starting material formula is CrCl₃·6H₂O (3 chlorine atoms total). Since 2 are free ionic chloride ions outside the complex, exactly 1 chloride ion must be inside the complex as a ligand.
  4. Substitute into general formula: [Cr(H₂O)₅(Cl)]²⁺

❌ Common Calculation Traps

Don't confuse chloride ions acting as ligands (which do not react with silver nitrate) with free chloride ions in the outer sphere (which do precipitate immediately as AgCl(s) ).

Mark: 2 marks

Topics

Inorganic Chemistry · Topic 15: Transition Metals

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 1, November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.