Edexcel A-Level Chemistry Paper 1, June 2018: Question 4

9 marks · Medium difficulty · Multiple Choice

Answer multiple-choice, structure-drawing, and explanatory questions about transition metals covering electronic configurations, oxidation numbers, bonding types, colour, complex stereoisomerism, and autocatalytic reaction kinetics.

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Question

Question text

4 This question is about transition metals.

(a) Which of these ions has the electronic configuration [Ar]3d5?

(1)

A Cr3+

B Fe2+

C Mn2+

D Mn3+

(b) In which of these complex ions does the transition metal have the

oxidation number +3?

(1)

A [Ag(CN) ]−

B [CuCl ]2−

C [Fe(CN) ]3−

D [Ni(EDTA)]2−

(1)

A dative covalent only

B dative covalent and covalent only

C dative covalent and ionic only

D dative covalent, covalent and ionic

(d) Which best explains why [Cu(NH )2]+ ions are colourless?

(1)

A all complex ions having a metal ion with a +1 charge are colourless

B no electronic transitions can take place between d-orbitals

C the d-orbitals cannot split in energy

D there are no electrons in the d-subshell

(e) Glycinate ions are bidentate ligands and can be represented by the structure

H2N CH2 C

O−

Complete the diagram below to show the structure of the [Cu(NH*P52302RA0628*2CH2COO)2] complex,

which is square planar.

(2)

(f ) Manganate(VII) ions, MnO−, react with ethanedioate ions in acid solution.

2MnO− + 5C O2− + 16H+ → 2Mn2+ + 10CO + 8H O

42 4 2 2

The reaction starts slowly, the rate of reaction then increases, before it decreases again.

Explain this sequence.

(3)

(Total for Question 4 = 9 marks)

Mark scheme

Show the mark scheme

Question Answer

Mark

Number

4(a) The only correct answer is C (1)

A is not correct because it is 3d3 not 3d5

B is not correct because it is 3d6 not 3d5

D is not correct because it is 3d4 not 3d5

Mark

Number

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

A is not correct because it is +1 not +3

B is not correct because it is +2 not +3

D is not correct because it is +2 not +3

Mark

Number

4(c) The only correct answer is B (1)

A is not correct because covalent is missing

C is not correct because it has ionic is incorrect

D is not correct because it has ionic is incorrect

Mark

Number

4(d) The only correct answer is B (1)

A is not correct because it is not an explanation

C is not correct because the d-orbitals can be split in energy

D is not correct because there are ten electrons in the d-subshell

Question

Acceptable Answer Additional Guidance Mark

Number

4(e) Example of structure (2)

2 glycinate ligands attached to Cu through

nitrogen atoms (1)

2 glycinate ligands attached to Cu through

single bonded oxygen atoms

and Allow the two ligands attached to any 2 pairs of

rest of structure correct (1) adjacent bonds

Allow cis or trans isomer / delocalised carboxylate

groups / skeletal formulae

Ignore bond lengths and bond angles

Ignore lone pairs of electrons, charge on the copper or

oxygen ions and direction of dative covalent bonds

Do not award M1 if bond between Cu and H of NH2

Question

Acceptable Answer Additional Guidance Mark

Number

4(f) An explanation that makes reference to the following points: (3)

(the initial rate of reaction is slow)

because both reacting species are negatively charged / Allow because there is no catalyst / no

repel each other Mn2+ ions present at the start

the reaction has a high activation energy / few particles

have energy greater than (or equal to) the activation

energy (1)

(the rate of reaction increases) because Mn2+ ions (are Allow a description of how the Mn2+

formed) ions are acting as a catalyst e.g. the

and idea of Mn2+ ions reacting and being

they act as a catalyst / are autocatalytic / provide an regenerated

alternative route with a lower activation energy (1)

Do not award ‘enzyme’

(the rate decreases) because the concentrations /amounts Allow example of one of the reagents

of the reactants decrease / the reactants are used up (1) used up / becoming a limiting factor

Do not award ‘the Mn2+ ions are used

up’

(Total for Question 4 = 9 marks)

How to answer it

Transition Metals Study Guide

📌 What this question tests

This comprehensive question assesses core transition metal chemistry concepts: electron configurations of ions, determination of oxidation numbers, identification of bonding types within complex ions (dative covalent and covalent), reasons for colourless complex ions (incomplete d-subshells / d-d transitions), drawing 3D structures of square planar complexes with bidentate ligands, and explaining autocatalysis in kinetics.

Part (a): Electronic Configuration

Which of these ions has the electronic configuration [Ar]3d⁵?

✅ Correct Answer: C (Mn²⁺)

Manganese has an atomic number of 25. Its neutral atom configuration is [Ar] 4s² 3d⁵ . Removing two electrons from the outer 4s orbital gives [Ar] 3d⁵ .

❌ Common Errors & Distractors

A (Cr³⁺): Chromium (Z=24) is [Ar] 4s¹ 3d⁵ . Removing 3 electrons gives [Ar] 3d³ .
B (Fe²⁺): Iron (Z=26) is [Ar] 4s² 3d⁶ . Removing 2 electrons gives [Ar] 3d⁶ .

Part (b): Oxidation Numbers in Complexes

In which of these complex ions does the transition metal have the oxidation number +3?

✅ Correct Answer: C ([Fe(CN)₆]³⁻)

Let iron be x . Each cyanide ligand has a charge of -1. Therefore: x + 6(-1) = -3 , which means x = +3 .

💡 Key Knowledge

[Ag(CN)₂]⁻ : Ag is +1
[CuCl₄]²⁻ : Cu is +2
[Ni(EDTA)]²⁻ : Ni is +2 (EDTA⁴⁻ ligand)

Part (c): Bonding Within Complex Ions

Which type or types of bonding exist within the complex ion [Cr(H₂O)₆]³⁺?

✅ Correct Answer: B (dative covalent and covalent only)

Water molecules bind to the chromium ion via coordinate (dative covalent) bonding through the oxygen lone pair. Within the water molecules themselves (H-O), there are standard covalent bonds.

❌ Common Errors

Students often incorrectly assume ionic bonding is present because the whole entity is a charged complex ion. However, the bonds within the complex ion are strictly covalent and dative covalent.

Part (d): Colour in Transition Metal Complexes

Which best explains why [Cu(NH₃)₂]⁺ ions are colourless?

✅ Correct Answer: B (no electronic transitions can take place between d-orbitals)

Copper(I) has a 3d¹⁰ electronic configuration. Because its d-subshell is completely full, no electrons can transition between split d-orbitals, meaning light is not absorbed in the visible spectrum.

🧠 Exam Technique

Look out for full or empty d-subshells ( 3d⁰ or 3d¹⁰ ) when asked why transition metal ions are colourless. Sc³⁺ and Cu⁺ are classic examples.

Part (e): Drawing Complex Structures

Complete the diagram to show the structure of the [Cu(NH₂CH₂COO)₂] complex, which is square planar.

✅ Correct Answers & Mark Scheme

1 mark: 2 glycinate ligands attached to Cu through nitrogen atoms.
1 mark: 2 glycinate ligands attached to Cu through single bonded oxygen atoms (and rest of structure correct).

Examiner Guidance: Accept cis or trans isomers, skeletal formulae, and simplified ring structures. Do not award M1 if the bond goes directly from Cu to H in the NH₂ group.

💡 Structural Layout Tip

Draw the square planar framework around Cu . Show the bidentate glycinate ligand coordinating via the nitrogen ( N ) of the amine group and the oxygen ( O ) of the carboxylate group to form two 5-membered chelate rings.

Part (f): Kinetics and Autocatalysis

Explain this sequence: The reaction starts slowly, the rate of reaction then increases, before it decreases again. (2Mn²⁺ + 5C₂O₄²⁻ + 16H⁺ → 2Mn²⁺ + 10CO₂ + 8H₂O)

✅ Marking Points (3 Marks total)

Mark 1: Initial rate is slow because both reacting species (MnO₄⁻ and C₂O₄²⁻) are negatively charged and repel each other (leading to a high activation energy).
Mark 2: Rate increases because Mn²⁺ ions are formed, which act as a catalyst (autocatalysis) / provide an alternative pathway with a lower activation energy.
Mark 3: Rate decreases towards the end because reactant concentrations/amounts decrease as they are used up.

🧠 Top-Level Examiner Advice

This is a classic autocatalysis question. You must explicitly mention: (1) Initial repulsion / high activation energy, (2) Formation and catalytic role of Mn²⁺ , and (3) Depletion of reactants causing the final slowdown. Avoid using imprecise terms like "the catalyst gets used up".

Topics

Inorganic Chemistry · Physical Chemistry · Topic 15: Transition Metals · Topic 9: Kinetics I

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 1, June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.