OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2023: Question 22

14 marks · Hard difficulty · Structured Questions

Explain d-block and transition elements using electron configurations, describe precipitation and ligand substitution reactions for copper or chromium, determine complex ion properties, and construct a redox equation for dichromate and vanadium(III).

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Question text

22 This question is about the d-block elements in Period 4 of the periodic table (Sc to Zn).

(a)* Explain, with examples from Period 4, what is meant by the terms d-block element and

transition element.

Explain why some d-block elements are not transition elements.

Use electron configurations to support your explanations.

… [6]

Additional answer space if required.

(b) (i) Describe precipitation reactions using either copper or chromium ions as examples.

Include equations.

… [2]

(ii) Describe ligand substitution reactions using either copper or chromium ions as

examples.

Include equations.

… [2]

A complex ion of cobalt(III) contains two ethanedioate ligands and two water ligands.

Determine the charge of this complex ion and the coordination number of cobalt in the

complex ion.

Charge of complex ion …

Coordination number of cobalt … [2]

(d) An acidified solution containing Cr O 2– ions reacts with vanadium(III) ions in a redox

reaction to form a solution containing Cr3+ ions and VO + ions.

Construct the overall equation for this reaction.

… [2]

Mark scheme

Show the mark scheme

Question Answer Marks Guidance

element

22 (a) Please refer to the marking instructions on page 5 of this 6 AO1.1 Indicative scientific points may include:

mark scheme for guidance on how to mark this question. 4

Terms

Level 3 (5–6 marks) AO1.2 d-block element: element with highest energy/

Explains the terms ‘d-block element’ AND ‘transition element’ 2 valence electron in d-orbital/sub-shell OR d

AND subshell is being filled

Explains why not all d-block are transition elements DO NOT ALLOW d block for d-subshells

AND

At least THREE correct electron configurations (need to be Transition element: element forming one or

one electron configuration of d block atom, transition element more ions (allow atom and ion - IUPAC definition)

ion and zinc (or scandium) ion with incomplete/partially filled d-subshell/d-

orbitals

There is a well-developed line of reasoning which is clear and DO NOT ALLOW d shell

logically structured. The information presented is relevant and

substantiated. d-block element:

ALLOW examples with an ion with an incomplete

Level 2 (3–4 marks) d-subshell, e.g. Fe2+ - [Ar]4s03d6

Explains both the terms ‘d-block element’ and

‘transition element’ ALLOW examples with highest energy electrons in

AND a d-subshell, e.g. Fe - [Ar]4s23d6

Explains why not all d-block are transition elements

OR Not all d-block are transition elements:

Explains both the terms ‘d-block element’ and Sc and Zn form ions with complete or empty d-

‘transition element’ shells ORA

AND

Links terms to at least TWO correct electron For Sc3+, ALLOW Sc+3 OR Sc forms a 3+ ion

configurations For Zn2+, ALLOW Zn+2 OR Zn forms a 2+ ion

Explains the terms ‘d-block element’ OR ‘transition Sc3+ 1s22s22p63s23p6

element’ Sc3+ AND d subshell empty / d orbital(s) empty

AND Zn2+ 1s22s22p63s23p63d10

Explains why not all d-block are transition elements Zn2+ AND d subshell full / ALL d orbitals full

AND

Links terms to at least ONE correct electron

configuration

28 AO

element

ALLOW minor slips on inner shell electron

There is a line of reasoning presented with some structure. configurations

The information presented is relevant and supported by some -----------------------------------------------------------

evidence.

Level 1 (1–2 marks) NOTE: A clear and logically structured response

would link definitions to electron configurations to

Explains the term ‘d-block element’ OR ‘transition support the explanations. If stated, for the level,

element’ there should be clear indication that the d subshell

AND is full/empty or partially full

Attempts to link terms with ONE correct electron

configuration

Explains the term ‘d-block element’ AND ‘transition

element’

Explains the term ‘d-block element’ OR ‘transition

element’

AND Explains why not all d-block are transition

elements

Any TWO out of THREE correct electron

configurations (one element and one ion that is a

transition element and one ion that is not a transition

element)

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

reasoning. The information is in the most part relevant.

0 marks No response or no response worthy of credit

29 AO

element

(b) (i) 2 ALLOW any one precipitation reaction

any one ligand substitution

AO1.1 ALLOW other correct equations linked to correct

colour change -check with TL

Precipitation with OH–/NH 2 marks AO1.2 IGNORE state symbols

(Pale) Blue (precipitate) AND Cu(OH)2 (can be seen in DO NOT ALLOW dark/royal blue (complex ion

the equation)✓ colour)

Cu2+ + 2OH– → Cu(OH) ✓ ALLOW Cu(H O) (OH)

22 4 2

ALLOW

[Cu(H O) ]2+ + 2OH– → Cu(OH) (H O) + 2H O

26 2 2 4 2

OR [Cu(H O) ]2+ + 2OH– → Cu(OH) + 6H O

26 2 2

OR [Cu(H O) ]2+ + 2NH → Cu(OH) (H O) + 2NH +

26 3 2 2 4 4

Precipitation with I– 2 marks

White (precipitate) AND CuI ✓

2Cu2+ + 4I- → 2CuI + I ✓ ------------------------------------------------------------------

-------------------------------------------------------------------------------

Precipitation with OH–/NH 2 marks

(Dark) Grey-Green (precipitate) AND Cr(OH)3 ✓ ALLOW Green

ALLOW Cr(H2O)3(OH)3

ALLOW

[Cr(H O) ]3+ + 3OH– → Cr(OH) (H O) + 3H O

26 3 2 3 2

Cr3+ + 3OH– → Cr(OH) ✓ OR [Cr(H O) ]3+ + 3NH → Cr(OH) (H O) + 3NH +

32 6 3 3 2 3 4

OR [Cr(H O) ]3+ + 3OH– → Cr(OH) + 6H O

26 3 2

30 AO

element

(b) (ii) Cu 2 ALLOW other correct equations linked to correct

Ligand substitution with NH /Cl– 2 marks colour change -check with TL

NH3 Deep/dark/royal blue (solution) AO1.1

AND [Cu(NH ) (H O) ]2+ ✓ ALLOW ECF on any incorrect charges of the

34 2 2

complex ions when linked to colour via an

[Cu(H O) ]2+ + 4NH → [Cu(NH ) (H O) ]2+ + 4H O ✓ AO1.2 equation.

26 3 3 4 2 2 2

Cl – yellow (solution)

AND [CuCl ]2– ✓

[Cu(H O) ]2+ + 4Cl – → [CuCl ]2– + 6H O ✓

26 4 2

-------------------------------------------------------------------------------

Ligand substitution with NH3 2 marks

NH3

Purple (solution) AND [Cr(NH ) ]3+ ✓

[Cr(H O) ]3+ + 6NH →[Cr(NH ) ]3+ + 6H O ✓

26 3 3 6 2

Dark Green (solution) AND [Cr(OH) ]3- ✓

[Cr(H O) ]3+ + 6OH- →[Cr(OH) ]3- + 6H O ✓

26 6 2

24 2 2 2

DO NOT ALLOW Co–

Coordination number: 6 ✓ IGNORE sign

31 element

(d) 3V3+ + Cr O 2– + 2H+ → 3 VO + + 2Cr3+ + H O 2

27 2 2

ALL reactant and product species correct ✓ AO2.5 IGNORE Balancing and electrons for first mark

Correct balancing (of correct equation) AND cancelling of AO2.6 DO NOT ALLOW electrons in final answer

species ✓

How to answer it

Transition Metals and Complex Ions Study Guide

What this question tests

This question assesses your understanding of Period 4 d-block elements, the strict chemical definition of transition elements, electron configurations, precipitation reactions, ligand substitution mechanisms, complex ion formulas, coordination numbers, and balancing complex redox equations in acidic conditions.

Part (a): Defining d-Block vs Transition Elements

6-Mark Level of Response Question

💡 Key Knowledge

d-block element: An element whose highest energy/valence electron is in a d-orbital/subshell.
Transition element: An element that forms at least one stable ion with an incompletely/partially filled d-subshell/d-orbitals.
Why Sc and Zn aren't transition elements: Sc³⁺ has an empty d-subshell ([Ar]3d⁰). Zn²⁺ has a completely full d-subshell ([Ar]3d¹⁰). Neither forms stable ions with incomplete d-orbitals.

🧠 Exam Technique

This is a Level of Response (LoR) question. To hit Level 3 (5–6 marks), you must cover all three pillars: definitions of both terms, why non-transition elements exist, and include at least three correct electron configurations (e.g., a d-block atom, a transition element ion, and Zn or Sc ion).
Use full subshell notation (e.g., 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹ ) or noble gas shorthand ( [Ar] 4s² 3d¹ ).

❌ Common Errors

Saying "d-block has incomplete d-subshell" — this is the definition for a transition element, not a d-block element!
Forgetting to remove 4s electrons first when writing ion configurations (e.g., writing Zn²⁺ as [Ar] 4s² 3d⁸ instead of [Ar] 3d¹⁰ ).

Mark Breakdown: Level 3 (5-6 marks) requires all criteria explained with clear chemical logic and accurate configurations. Level 2 (3-4 marks) covers definitions and partial configurations. Level 1 (1-2 marks) provides fragmented definitions.

Part (b)(i): Precipitation Reactions of Copper or Chromium

Describing Precipitation with Equations (2 Marks)

✅ Correct Answer Examples

Copper with OH⁻ / NH₃:

Forms a pale blue precipitate of Cu(OH)₂ .
Equation: Cu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(s)
(Hexa-aquo equations like [Cu(H₂O)₆]²⁺ + 2OH⁻ → Cu(OH)₂(H₂O)₄ + 2H₂O are also fully accepted).

Chromium with OH⁻ / NH₃:

Forms a dark grey-green precipitate of Cr(OH)₃ .
Equation: Cr³⁺(aq) + 3OH⁻(aq) → Cr(OH)₃(s)

🧠 Exam Technique

Always state the colour of the precipitate alongside the balanced chemical equation. Make sure states or correct charge balancing are managed. Equations can be written using simple metal ions ( Cu²⁺ ) or complex aqua ions ( [Cu(H₂O)₆]²⁺ ).

Mark Breakdown: 1 mark for correct species and colour of precipitate; 1 mark for balanced equation.

Part (ii): Ligand Substitution Reactions

Describing Ligand Exchange (2 Marks)

✅ Correct Answer Examples

Copper with Excess NH₃:

Observation: Deep/dark royal blue solution.
Equation: [Cu(H₂O)₆]²⁺ + 4NH₃ → [Cu(NH₃)₄(H₂O)₂]²⁺ + 4H₂O

Copper with Excess Cl⁻:

Observation: Yellow solution (or green depending on mixture).
Equation: [Cu(H₂O)₆]²⁺ + 4Cl⁻ → [CuCl₄]²⁻ + 6H₂O

❌ Common Errors

Confusing precipitation with ligand substitution. Substitution changes the ligand count/identity in solution without necessarily forming a solid.
Incorrect coordination numbers for chloro complexes (chloride is a large ligand, so only 4 fit around copper: [CuCl₄]²⁻ ).

Mark Breakdown: 1 mark for correct colour change observation with ligand; 1 mark for balanced ligand substitution equation.

Part (c): Complex Ion Charge and Coordination Number

Cobalt(III) Complex with Ethanedioate and Water (2 Marks)

📐 Step-by-Step Calculation

Identify components: Cobalt is in oxidation state +3. It contains two bidentate ethanedioate ligands ( C₂O₄²⁻ ) and two water ligands ( H₂O , which are neutral).
Calculate total charge:
Co = +3
2 × (C₂O₄²⁻) = 2 × (-2) = -4
2 × (H₂O) = 2 × (0) = 0
Overall charge = (+3) + (-4) + (0) = -1
Determine coordination number: Each ethanedioate is bidentate (forms 2 coordinate bonds) = 2 × 2 = 4 bonds. Each water is monodentate = 2 × 1 = 2 bonds. Total coordination number = 4 + 2 = 6 .

✅ Final Answers

Charge of complex ion: -1 (or 1-)

Coordination number of cobalt: 6

Mark Breakdown: 1 mark for charge (-1); 1 mark for coordination number (6). Note: Formula is [Co(C₂O₄)₂ (H₂O)₂]⁻ .

Part (d): Constructing a Redox Equation

Acidified Dichromate and Vanadium(III) Reaction (2 Marks)

✅ Correct Answer

3V³⁺ + Cr₂O₇²⁻ + 2H⁺ → 3VO²⁺ + 2Cr³⁺ + H₂O

🧠 Exam Technique & Steps

Write down reactants ( Cr₂O₇²⁻ , V³⁺ , H⁺ ) and products ( VO²⁺ , Cr³⁺ , H₂O ).
Balance oxidation states: V goes from +3 to +4 (loss of 1 e⁻ per V). Cr goes from +6 to +3 (gain of 3 e⁻ per Cr atom; 6 e⁻ total per Cr₂O₇²⁻ ion).
Multiply V³⁺ and VO²⁺ by 3 to balance electrons with the dichromate reduction half-equation.
Balance oxygen using H₂O and hydrogen using H⁺ .

❌ Common Errors

Leaving free electrons in the final equation or failing to scale the vanadium species to match the 6 electrons released by one dichromate ion.

Mark Breakdown: 1 mark for all correct reactant and product species; 1 mark for correct balancing and cancelled electrons.

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 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.