OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2017: Question 23

13 marks · Hard difficulty · Structured Questions

Deduce formulas and reaction types for copper complex ion reactions and determine the structures and formulas of a nickel complex and its optical isomers from analytical data.

Practise this question

Question

Chemistry exam question with two parts. Part (a) shows a flowchart for reactions of the complex ion [Cu(H2O)6]2+ with HCl, KI, and NH3 resulting in various colored solutions, precipitates, and solids, followed by naming the reaction types. Part (b) is an extended response question providing mass data for heating a hydrated nickel(II) complex, its percentage composition, and properties of its ligands and optical isomers, requiring the determination of formulas A, B, C, and D, and 3D structures.
Question text

23 This question is about reactions of ions and compounds of transition elements.

(a) The flowchart shows reactions of the complex ion [Cu(H O) ]2+.

(i) In the boxes, write down the formulae of the species responsible for the observations.

yellow solution

Reaction 1 HCl(aq)

[Cu(H O) ]2+ NH (aq)

26 3

pale blue solution …

pale blue precipitate

Reaction 2 KI(aq) NH3(aq)

excess

+

white solid brown solution

deep blue solution

[5]

(ii) Name the type of reaction for Reaction 1 and Reaction 2.

Reaction 1 …

Reaction 2 …

[2]

(b)* A hydrated nickel(II) complex, A, is heated in a crucible to remove the water of crystallisation.

The anhydrous complex B is formed. The results are shown below.

Mass of crucible + hydrated complex A = 59.554 g

Mass of crucible + anhydrous complex B = 58.690 g

Mass of crucible = 51.257 g

The anhydrous complex B is analysed and found to have a molar mass of 309.7 g mol–1 and

to contain the following percentage composition by mass:

Ni, 18.95%; C, 23.25%; N, 27.12%; H, 7.75%; Cl, 22.93%.

The anhydrous complex B contains a cation C comprising Ni, C, N and H only.

Cation C is six-coordinate, contains three molecules of the bidentate ligand D, and exists as

optical isomers.

Determine the formula of A, B, C and D and show the 3D structures for the optical isomers

of C.

Show all your working.

… [6]

Mark scheme

Show the mark scheme The official mark scheme showing required chemical formulas for the copper complex reactions, reaction types (ligand substitution, redox), and a 6-mark level-based response rubric for determining the formulas of complexes A, B, C, and ligand D with calculations and 3D optical isomer structures of cation C.

Question Answer Marks Guidance

23 (a) (i) CuCl 2– OR [CuCl ]2– 5 ALLOW Cu(Cl) 2–

44 4

yellow solution

Cu(OH)2 ALLOW Cu(OH)2(H2O)4

pale blue precipitate

2+ Brackets required for [Cu(NH ) (H O) ]2+

[Cu(NH3)4(H2O)2] 3 4 2 2

deep blue solution

NOTE:

CuI I2 Take great care to check that subscripted

white solid brown solution numbers and brackets are correct

(ii) Reaction 1: ligand substitution 2 ALLOW ligand exchange

Reaction 2: redox ALLOW reduction AND oxidation

ALLOW precipitation

(b)* Please refer to the marking instructions on page 5 of this mark 6 Indicative scientific points may include:

scheme for guidance on how to mark this question. 1. Formula of anhydrous complex B

Level 3 (5–6 marks) NiC6N6H24Cl2

A comprehensive conclusion using all data to obtain correct Example of working

formulae for A, B, C and D Ni : C : N : H : Cl

AND optical isomers shown 18.95 23.25 27.12 7.75 22.93

= : : : :

58.7 12.0 14.0 1.00 35.5

There is a well-developed line of reasoning which is clear and There may be other methods

logically structured with use of 3D structures for both optical

isomers of C, use of wedges and bonding to N. 2. Formula of hydrated complex A

The information presented is relevant and substantiated. NiC6N6H24Cl2•2H2O OR NiC6N6H24Cl2(H2O)2

Example of working

Level 2 (3–4 marks) 7.433

Reaches a sound conclusion for the formula of B n(anhydrous salt) = 309.7 = 0.02400 (mol)

AND 0.864

obtains the correct formula of the hydrated complex A n(H2O) = 18.0 = 0.04800 (mol)

OR a 3D diagram of one optical isomer of cation C There may be other methods

There is a line of reasoning and supported by some evidence. 3. Formula of cation C

Calculations are clear and can be followed to obtain correct [NiC N H ]2+ OR [Ni(H NCH CH NH ) )]2+

66 24 2 2 2 2 3

conclusions. 3D diagram, if present, should use wedges (could be in structures

mostly correctly. 2+ charge can be shown on cation OR

Formula of A to show water separately or formula of C to show optical isomers (i.e. seen somewhere)

ligands separately, as appropriate. Bidentate ligand D

H2NCH2CH2NH2 or displayed so that

Level 1 (1–2 marks) structure is clearly unambiguous.

Reaches a simple conclusion to obtain the correct formula of Optical isomers

anhydrous complex B OR shows that A contains 2H2O

There is an attempt at a logical structure with a line of

reasoning. The information is in the most part relevant.

Attempts more than one part of the problem.

0 marks No response or no response worthy of credit.

Accuracy of structures

H032/01 Mark Scheme22 June 2017

Bonding shown from Ni to N of

H2NCH2CH2NH2

ALLOW CH3CH(NH2)2 for ligand

For H2NCH2CH2NH2 in optical isomers,

ALLOW C–C without Hs and NH2 NH2

Each structure to contain

2 ‘out wedges’, 2 ‘in wedges’ and 2 lines in

plane of paper OR 4 lines, 1 ‘out wedge’ and

1 ‘in wedge’:

Ni

Bond into paper can be shown as:

Total 13

How to answer it

Transition Elements: Complex Ions & Empirical Analysis

What this question tests

This synoptic exam question assesses your knowledge of transition metal chemistry reactions (ligand substitution, precipitation, and redox), alongside advanced quantitative analysis involving water of crystallisation, empirical/molecular formulas, multidentate ligands, and 3D stereoisomerism (optical isomerism).

Part (a) — Copper(II) Complex Reactions

Complex Ion Flowchart & Reaction Types

✅ Correct Answers (a)(i)

  • Reaction 1 (HCl addition): [CuCl₄]²⁻ (Yellow solution)
  • Reaction with NH₃ (aq): Cu(OH)₂ (Pale blue precipitate)
  • Reaction with excess NH₃ (aq): [Cu(NH₃)₄(H₂O)₂]²⁻ (Deep blue solution - note brackets)
  • Reaction 2 (KI addition): CuI (White solid) + I₂ (Brown solution)

✅ Correct Answers (a)(ii)

  • Reaction 1: Ligand substitution (or ligand exchange)
  • Reaction 2: Redox (or reduction AND oxidation)

💡 Key Knowledge

  • Chloride ions are larger than water molecules, so coordination number changes from 6 to 4 (tetrahedral [CuCl₄]²⁻ ).
  • Addition of aqueous ammonia first acts as a base precipitating copper hydroxide, which then dissolves in excess NH₃ via ligand substitution to form the deep blue amine complex.
  • Iodide reduces Cu²⁺ to Cu⁺ while being oxidised itself to iodine (I₂), forming a white CuI precipitate mixed with brown aqueous iodine.

❌ Common Errors

  • Omitting square brackets around complex ions in part (a)(i), or missing the charge on the overall complex.
  • Confusing ligand substitution with precipitation when writing reaction classifications in (a)(ii).
Part (b) — Extended Quantitative & Structural Analysis

Structure Determination of Hydrated Nickel(II) Complex

📐 Step-by-Step Calculation & Structural Deduction

  1. Find mass of anhydrous complex B:
    Mass of (crucible + B) - Mass of crucible = 58.690 g - 51.257 g = 7.433 g
  2. Find mass and moles of water lost (Hydrated A minus Anhydrous B):
    Mass of water = 59.554 g - 58.690 g = 0.864 g
    Moles of H₂O = 0.864 / 18.0 = 0.0480 mol
  3. Moles of anhydrous complex B:
    Using given molar mass of 309.7 g mol⁻¹:
    Moles of B = 7.433 / 309.7 = 0.0240 mol
  4. Determine water ratio (A : B):
    Ratio of H₂O to B = 0.0480 / 0.0240 = 2 . Therefore, hydrated complex A contains 2H₂O of crystallisation. Formula: NiC₆H₂₄Cl₂ • 2H₂O
  5. Empirical formula of B using percentage composition:
    Ni: 18.95/58.7 = 0.323
    C: 23.25/12.0 = 1.938
    N: 27.12/14.0 = 1.937
    H: 7.75/1.0 = 7.750
    Cl: 22.93/35.5 = 0.646
    Divide by smallest (0.323): Ni : C : N : H : Cl approx 1 : 6 : 6 : 24 : 2 → NiC₆H₂₄Cl₂
  6. Cation C and Bidentate Ligand D identification:
    Cation C has a 2+ charge (balancing 2 Cl⁻ ions) and contains 3 molecules of a bidentate ligand D containing Ni, C, N, H. Ligand D is H₂NCH₂CH₂NH₂ (1,2-diaminoethane). Cation C formula: [Ni(H₂NCH₂CH₂NH₂)₃]²⁺ .

🧠 Exam Technique & Optical Isomer Guidance

  • 3D Structures: To show optical isomers for the octahedral complex containing three bidentate ligands, draw two non-superimposable mirror images using wedges and dashes.
  • Ensure bonds link clearly from nitrogen ( N ) to nickel ( Ni ).
  • Check that each structure shows 2 'out wedges', 2 'in wedges', and 2 lines in the plane of the paper.

❌ Common Calculation Traps

  • Forgetting to subtract the empty crucible mass when finding masses of A and B.
  • Mixing up atomic masses or rounding intermediate values too early, throwing off the empirical formula integer ratios.

Topics

Module 5: Physical chemistry and transition elements · Module 2: Foundations in chemistry · 5.3 Transition elements · 2.1 Atoms and reactions

Question and mark scheme from the OCR A-Level Chemistry examination, Periodic table, elements and physical chemistry (01), June 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.