OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), November 2021: Question 17

11 marks · Hard difficulty · Structured Questions

Explain the definitions of d-block and transition elements using electron configurations, define a bidentate ligand, determine the formulae of a hydrated chromium(III) salt and a complex ion formed by ligand substitution, write the ionic equation, and draw 3D structures for the three stereoisomers.

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Question text

17 This question is about d-block elements.

(a) Most d-block elements are also classified as transition elements.

Explain why scandium and zinc are classified as d-block elements but are not also transition

elements.

Your explanations should include full electron configurations.

… [4]

(b) Compound A is a hydrated chromium(III) salt with a molar mass of 608.3 g mol–1 and the

following percentage composition by mass:

Cr, 17.10%; H, 3.94%; O, 63.13%; S, 15.83%.

A student dissolves compound A in water.

The resulting solution contains the complex ion [Cr(H O) ]3+.

The student mixes this solution with aqueous sodium ethanedioate, Na2C2O4.

A ligand substitution reaction takes place forming a solution containing three stereoisomers of

a complex ion B of chromium(III).

Complex ion B is six-coordinate and contains two ethanedioate ligands and two water ligands.

The ethanedioate ion is the bidentate ligand shown below.

O O

–O O–

Ethanedioate ligand

(i) What is meant by a bidentate ligand?

… [1]

(ii)* Determine the formulae of A and B. Write the ionic equation for the ligand substitution

and show 3D structures for the three stereoisomers of B.

… [6]

Additional answer space if required.

Mark scheme

Show the mark scheme

Question Answer Marks Guidance

element

17 (a) 4 AO1.1 FULL ANNOTATIONS MUST BE USED

Transition element: ×4 ------------------------------------------------------

Has an ion with an incomplete/partially-filled d sub- DO NOT ALLOW d shell

shell/d-orbital

IGNORE d block

d-block

d sub-shell/d-orbital is being filled/has highest energy

OR IGNORE outer electron

Electron configurations shown for

Sc: 1s22s22p63s23p63d14s2

AND electron configurations

Zn:1s22s22p63s23p63d104s2 ALLOW 4s0

ALLOW 4s2 before 3d, i.e … 4s23d1; 4s23d10

IGNORE other Sc and Zn ions

Electron configurations of ions

Sc3+: 1s22s22p63s23p6 ALLOW ECF for short hand notation.

AND

d sub-shell empty / d orbital(s) empty For Sc3+, ALLOW Sc+3 OR Sc forms a 3+ ion;

Zn2+: 1s22s22p63s23p63d10 For Zn2+, ALLOW Zn+2 OR Zn forms a 2+ ion;

AND

d sub-shell full / d-orbitals full

(b) (i) 1 AO1.1 ALLOW lone pairs for electron pairs

Donates two electron pairs (to a metal ion) x1

AND ALLOW dative (covalent) bonds for coordinate

forms two coordinate bonds (to a metal ion) bonds

TWO is only needed once if bonds are plural, e.g.

Donates 2 electron pairs to form coordinate bonds

Donates electron pairs to form 2 coordinate bonds

element

(ii)* Please refer to the marking instructions on page 4 of this mark 6 Indicative scientific points:

scheme for guidance on how to mark this question.

Level 3 (5–6 marks) AO2.2 1. Formula of the hydrated salt A

Reaches a comprehensive conclusion with most detail ×2

and few errors to obtain: Formula of A:

the formulae of A and B Cr2H24O24S3

AND ionic equation for ligand substitution

AND the 3D structures of B stereoisomers Example of working

Cr : H : O : S

There is a well-developed line of reasoning which is clear 17.10 3.94 63.13 15.83

and logically structured. The information presented is 52.0 : 1.0 : 16.0 : 32.1

relevant and substantiated.

AO2.6 There may be other methods

Level 2 (3–4 marks) ×2

Reaches a sound conclusion with some detail and some Detail Hydrated salt = Cr2(SO4)3•12H2O

errors for

the formula of A OR B 2. Formula of B and ionic equation

AND

ionic equation for ligand substitution Formula of B:

OR the 3D structures of B stereoisomers [Cr(H O) (C O ) ]–

22 2 4 2

AO3.1

There is a line of reasoning presented with some ×2 Ionic equation

structure. The information presented is relevant and

supported by some evidence. 3+ 2– –

[Cr(H2O)6] + 2C2O4 → [Cr(H2O)2(C2O4)2] +

4H2O

Level 1 (1–2 marks)

Obtains the correct formula of A OR B OR 3D structures

ALLOW ligands in any order, e.g.

of B stereoisomers which are mostly correct. –

[Cr(C2O4)2(H2O)2]

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

reasoning. The information is in the most part relevant. Detail Use of charges and brackets

0 marks No response or no response worthy of credit. 3. 3D structures of B stereoisomers

element

• Consistent use of

2 ‘out wedges’, 2 ‘in wedges’, 2 lines in plane of paper OR 4 lines, 1

‘out wedge’ and 1 ‘in wedge’

ALLOW following orientations

Detail

• Most bonding shown from Cr to O of H O and O–

C O 2–

Total 11

How to answer it

Transition Elements & Complex Ions Comprehensive Study Guide

What this question tests

This multi-part synoptic question assesses your understanding of d-block versus transition element definitions using electron configurations, empirical formula calculations from percentage composition, ligand definitions (bidentate), writing ionic ligand substitution equations, and visualizing complex 3D stereoisomers (cis/trans and optical isomers).

Question 17 (a) - 4 Marks

Scandium and Zinc: d-block vs. Transition Elements

✅ Correct Answer Framework

Transition Element definition: An element that forms at least one stable ion with an incomplete/partially-filled d sub-shell.
Scandium: Forms only the Sc³⁺ ion with electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶ (empty d sub-shell / 3d⁰). Therefore, it is a d-block element but not a transition element.
Zinc: Forms only the Zn²⁺ ion with electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ (full d sub-shell / 3d¹⁰). Therefore, it is a d-block element but not a transition element.

💡 Key Knowledge

Remember the golden rule of A-Level Chemistry definitions: All transition elements are d-block elements, but not all d-block elements are transition elements. Scandium and Zinc are the textbook counter-examples because their common/only stable ions do not feature a partially filled d orbital.

❌ Common Errors

Stating that the atoms have full or empty d sub-shells instead of focusing on their ions.
Writing outer electron configurations only, omitting full electron configurations when explicitly requested by the command words.

Mark Allocation: 1 mark for transition element definition + 1 mark for d-block definition/configurations + 2 marks for specific Sc and Zn ion configurations showing empty/full d-shells.

Question 17 (b)(i) - 1 Mark

Definition of a Bidentate Ligand

✅ Correct Answer

A species that donates two electron pairs (or two lone pairs) to a metal ion and forms two coordinate (dative covalent) bonds per ligand.

🧠 Exam Technique

Keywords are vital here. You must state both parts to secure the single mark: mentioning electron pairs/lone pairs AND coordinate/dative bonds.

Mark Allocation: 1 mark for correct reference to two lone pairs and two coordinate bonds.

Question 17 (b)(ii) - 6 Marks (Level of Response)

Formulae Determination, Ionic Equation, and 3D Stereoisomers

📐 Step 1: Calculating Empirical Formula of A (Hydrated Salt)

Assume 100g sample: Cr = 17.10g, H = 3.94g, O = 63.13g, S = 15.83g
Divide by relative atomic masses (Ar):
Cr: 17.10 / 52.0 = 0.3288 mol
H: 3.94 / 1.0 = 3.940 mol
O: 63.13 / 16.0 = 3.945 mol
S: 15.83 / 32.1 = 0.4931 mol
Find simplest whole number ratio (divide by smallest, 0.3288):
Cr : H : O : S = 1 : 11.98 : 11.99 : 1.50
Scale up to integers (multiply by 2): Ratio becomes Cr₂ : H₂₄ : O₂₄ : S₃.
Empirical formula = Cr₂H₂₄O₂₄S₃ (or written as hydrated chromium sulfate: Cr₂(SO₄)₃·12H₂O ). Check molar mass matches 608.3 g mol⁻¹.

✅ Step 2: Formula of B and Ionic Equation

Formula of B: [Cr(H₂O)₂(C₂O₄)₂]⁻

Ionic Equation for Ligand Substitution:
[Cr(H₂O)₆]³⁺ + 2C₂O₄²⁻ → [Cr(H₂O)₂(C₂O₄)₂]⁻ + 4H₂O

💡 Step 3: 3D Structures of B Stereoisomers

Complex ion B exhibits stereoisomerism (specifically cis/trans geometric isomerism AND optical isomerism due to the two bidentate ethanedioate ligands and two water ligands):

Trans isomer: The two water molecules are directly opposite each other (180° apart), and the bidentate ligands span across the trans positions. (Achiral / optically inactive).
Cis isomer (Pair of optical isomers / Enantiomers): The two water molecules are adjacent to each other (90° apart). This lack of a plane of symmetry means it exists as a non-superimposable mirror image pair (one d and one l optical isomer).

Drawing tip: Use solid lines for bonds in the plane of the paper, wedged bonds for coming out, and hashed/dotted wedges for going back into the page. Clearly show square brackets and overall negative charge [-] .

❌ Common Errors in Level 6 Questions

Rounding intermediate molar values too early, leading to incorrect empirical ratios.
Failing to include state symbols, correct charges, or square brackets on complex ions.
Forgetting that cis-bidentate octahedral complexes can rotate polarized light, missing the third required optical isomer structure.

Mark Allocation: Level 3 (5–6 marks) requires correct formula for A, formula for B, valid ionic equation, and all three 3D stereoisomers drawn with proper 3D bond conventions.

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), November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.