Edexcel A-Level Chemistry Paper 1, June 2024: Question 6

6 marks · Medium difficulty · Extended Writing

Explain why aqueous solutions of Cu2+ and Fe2+ ions are coloured with different colours, while aqueous Zn2+ ions are colourless, referring to electronic configurations.

Practise this question

Question

Question text

Explain why aqueous solutions of Cu2+ ions and Fe2+ ions are coloured but have

different colours, whereas aqueous solutions of Zn2+ ions are colourless.

Include any relevant electronic configurations.

(6)

… 14

… *P76895A01428*

(Total for Question 6 = 6 marks)

Mark scheme

Show the mark scheme

Question

Acceptable Answer Additional Guidance Mark

Number

This question assesses a student’s ability to show a Guidance on how the mark scheme should be (6)

coherent and logically structured answer with linkages and fully- applied:

sustained reasoning.

The mark for indicative content should be

Marks are awarded for indicative content and for how the answer is added to the mark for lines of reasoning.

structured and shows lines of reasoning.

For example, an answer with five indicative

The following table shows how the marks should be marking points, which is partially structured

awarded for indicative content. with some linkages and lines of reasoning,

scores 4 marks (3 marks for indicative

Number of indicative marking Number of marks awarded for content and 1 mark for partial structure and

points seen in answer indicative marking points some linkages and lines of reasoning).

5–4 3 If there are no linkages between points, the

3–2 2 same five indicative marking points would

11 yield an overall score of 3 marks (3 marks for

00 indicative content and no marks for linkages).

The following table shows how the marks should be awarded for In general it would be expected

structure and lines of reasoning. that 5 or 6 indicative points would get 2 reasoning

Number of marks awarded marks, and 3 or 4 indicative points would get 1 mark

for structure of answer and for reasoning, and 0, 1 or 2

sustained line of reasoning indicative points would score zero marks for

Answer shows a coherent and logical reasoning.

structure with linkages and fully

sustained lines of reasoning If there is any incorrect chemistry, deduct mark(s)

demonstrated throughout. from the reasoning. If no reasoning mark(s) awarded

Answer is partially structured with

1 do not deduct mark(s).

some linkages and lines of reasoning.

Answer has no linkages between points

0 Incorrect chemistry

and is unstructured.

Penalise the incorrect mention of a single

d-orbital instead of d-orbitals or d-subshell once

only in whole response

Indicative content:

• IP1 (electron configurations of Cu2+ and Fe2+) Cu 2+ is [Ar] 3d9 / 1s2, 2s2, 2p6, 3s2, 3p6, 3d9

correct electron configuration of both Cu2+ and Fe2+ ions Fe2+ is [Ar] 3d6 / 1s2, 2s2, 2p6, 3s2, 3p6, 3d6

• IP2 (ligands and d orbitals) Allow the energy gap / energy difference

ligand splits the (3) d-orbitals / (3) d-subshell splits (into determines the frequency of absorbed light

higher and lower energy levels) Ignore the energy gap is dependent on the ligand.

• IP3 (energy gap) Allow energy levels for energy gap as long as it

energy gap / H / (between d-orbitals) is different in is clear they mean in the d-subshell / between the

different (metal) ions / between Cu2+ and Fe2+ ions split d-orbitals

• IP4 (electron promoted) Allow transition / promotion between energy

electron is promoted / excited / d−d transition levels

and Do not award transition / promotion between

absorbing (visible) light energy / photons of different d-subshells

wavelength / colour

• IP5 (colour)

complementary colour seen / transmitted / reflected Allow colour / light not absorbed seen

Do not award reference to electrons returning to

the ground state or emission of colour / light

• IP6 (zinc)

2+ Ignore zinc (3)d subshell does not split

(zinc ions / Zn ) colourless because the

2+ Ignore zinc cannot bind ligands

(3)-d subshell / d-orbitals in Zn are

Allow no space for an electron to be promoted

full /

Do not award shell for subshell

and

Zn2+ is [Ar] 3d10 / 1s2, 2s2, 2p6, 3s2, 3p6, 3d10

Penalise the incorrect mention of a single

(and so d-d transitions cannot occur)

d-orbital instead of d-orbitals or d-subshell once

only in whole response

(Total for Question 6 = 6 marks)

How to answer it

Origin of Colour in Aqueous Cu²⁺, Fe²⁺, and Zn²⁺ Ions

📌 What this question tests

This 6-mark extended response question assesses your understanding of crystal field theory and electron transitions in d-block chemistry:

Writing correct electronic configurations of transition metal ions (remembering 4s electrons are lost first).
Explaining d-orbital splitting caused by coordination with ligands (H₂O molecules in aqueous solution).
Linking photon absorption (ΔE = hν) and electron promotion (d–d transitions) to visible complementary colour transmission.
Explaining why different transition metal ions exhibit different colours based on orbital energy differences.
Explaining why species with full d-subshells (like Zn²⁺) are completely colourless.

Understanding the Marking Structure (Level of Response)

4 Marks for Indicative Content + 2 Marks for Coherent Structure & Reasoning

This is an extended response question marked with a grid:

Indicative Content Points (IP1 to IP6)

6 points: 4 marks
4–5 points: 3 marks
2–3 points: 2 marks
1 point: 1 mark

Reasoning & Structure Marks

2 marks: Coherent, logically structured explanation with sustained lines of reasoning linking splitting, absorption, and transmission.
1 mark: Partially structured answer with some linkages.
0 marks: Unstructured or isolated points.

Question 6: Model Answer & Step-by-Step Breakdown

✅ 1. Electronic Configurations

Cu²⁺: [Ar] 3d⁹ or 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁹ (IP1)
Fe²⁺: [Ar] 3d⁶ or 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ (IP1)
Zn²⁺: [Ar] 3d¹⁰ or 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ (IP6)

Note: Both 4s electrons are lost upon ionisation!

✅ 2. Origin of Colour in Cu²⁺ and Fe²⁺

Ligand Interaction: In aqueous solution, lone pairs on water ligands cause the 3d-orbitals / 3d-subshell to split into two different energy levels (IP2).
d–d Transition: Electrons absorb specific wavelengths of visible light energy and are promoted from the lower to the higher energy d-orbitals (IP4).
Observed Colour: The remaining wavelengths are transmitted/reflected, so the observed colour is the complementary colour to the light absorbed (IP5).

✅ 3. Why Colours Differ & Why Zn²⁺ is Colourless

Why different colours? Cu²⁺ and Fe²⁺ have different nuclear charges and electron configurations, leading to different energy gaps (ΔE) between the split d-orbitals. Thus, different frequencies/wavelengths of visible light are absorbed (IP3).
Why is Zn²⁺ colourless? Zn²⁺ has a completely full 3d-subshell ( 3d¹⁰ ). There is no vacant orbital at higher energy for an electron to be promoted into, so d–d transitions cannot occur and no visible light is absorbed (IP6).

Examiner Standard Model Response (Full 6 Marks):
"In aqueous solution, Cu²⁺ has configuration [Ar] 3d⁹ and Fe²⁺ has [Ar] 3d⁶. Coordination of H₂O ligands splits the 3d-orbitals into two groups of different energy levels. An electron in a lower d-orbital absorbs visible light of energy corresponding to the energy gap (ΔE = hν) and is promoted to an orbital of higher energy (a d–d transition). The light that is not absorbed is transmitted, resulting in the observed complementary colour. Cu²⁺ and Fe²⁺ have different energy gaps (ΔE), meaning they absorb different frequencies of light and display different colours. In contrast, Zn²⁺ has the configuration [Ar] 3d¹⁰. Because the 3d-subshell is completely full, electrons cannot be promoted between split d-orbitals, so no visible light is absorbed and the solution remains colourless."

💡 Key Chemistry Concepts to Remember

Energy Gap Equation: ΔE = hν = hc / λ . A larger energy gap means higher frequency and shorter wavelength of light absorbed.
Degenerate Orbitals: In an isolated, gaseous transition metal ion, all five 3d orbitals have the same energy (degenerate). It is the presence of surrounding ligands that causes them to split.
Octahedral Field: Aqueous transition metal ions exist as hexaaqua complexes, e.g. [Cu(H₂O)₆]²⁺ and [Fe(H₂O)₆]²⁺, forming octahedral shapes where the 5 d-orbitals split into 3 lower and 2 higher energy levels.

🧠 Exam Technique & Structure Tips

Ensure linkages: To get the 2 reasoning marks, do not write a disconnected list of bullet points. Link energy gap → wavelength absorbed → complementary colour transmitted.
Always answer every prompt in the question: The question explicitly asks for electronic configurations and why the colours are different. If you omit either, you automatically cap your marks.
Precision in terminology: Always say d-subshell or d-orbitals (plural). Mark schemes penalise referring to "a d-orbital splitting".

❌ Common Pitfalls & Lost Marks

Emission misconception: Stating that "light is emitted when electrons fall back down to ground state." That applies to flame tests and atomic emission spectra, NOT the colour of complex ions! For complexes, colour is transmitted light after absorption.
Saying "shell" instead of "subshell": Referring to the "3d shell" instead of the 3d subshell or 3d orbitals is penalised.
Incorrect transition ion configurations: Writing [Ar] 4s² 3d⁷ for Cu²⁺. Always remove 4s electrons before 3d electrons when forming ions.
Missing the Zn²⁺ explanation: Simply saying "zinc is not a transition metal" is not enough. You must state that the 3d subshell is completely full, preventing d–d transitions.

Topics

Inorganic Chemistry · Physical Chemistry · Topic 15: Transition Metals · Topic 1: Atomic Structure and the Periodic Table

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 1, June 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.