AQA A-Level Chemistry Paper 3, 2024: Question 5

9 marks · Medium difficulty · State/Explain/Numerical

Explain why some transition metal complexes are coloured, list factors that affect the colour and describe how colorimetry can be used to determine concentration of a coloured complex; calculate the energy change for an electron absorbing radiation at the peak wavelength from the spectrum.

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Question text

05.1 Some complexes containing transition metal ions are coloured.

• Explain why some complexes containing transition metal ions are coloured.

• List the factors that affect the colour.

• Describe how colorimetry can be used to determine the concentration of a

coloured complex.

[6 marks]

05.2 Figure 5 shows the visible spectrum of [Cu(H O) ]2+

*17* Figure 5

Use the wavelength at the peak of the curve in Figure 5 to calculate the change in

energy, in J, of an electron when it absorbs radiation with this wavelength.

the Planck constant, h = 6.63 × 10–34 J s

speed of light, c = 3.00 × 108 m s–1

[3 marks]

Change in energy J

Section B

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CORRECT METHOD WRONG METHODS

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Mark scheme

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Question Answers Additional comments/Guidelines Mark

This question is marked using Levels of Response. Refer to the Indicative Chemistry content

Mark Scheme Instructions for Examiners for guidance. Stage 1 absorption of light

Level 3 All stages are covered, and the explanation of each (3/4 virtually complete, 1/4 for covered)

stage is correct and virtually complete 1a d orbitals have different energy / d orbital (energies) are

5–6

marks Answer communicates the whole explanation split

coherently and shows a logical progression through 1b electrons move to higher (energy) (d) orbitals / electrons

all three stages. move to excited state

Level 2 All stages are covered but the explanation of each 1c absorb visible/white light

stage may be incomplete or may contain

3–4 1d colour seen is that from complementary colours / colours

inaccuracies.

marks transmitted/reflected/not absorbed

Stage 2 reasons for different colours

two stages are covered, and the explanations are (3/4 virtually complete, 1/4 for covered)

generally correct and virtually complete. 6

5.1

2a the metal (6 x AO1)

Answer is coherent and shows some progression

through all three stages. Some steps in each stage 2b the oxidation state (of the metal) / charge of metal (ion)

may be incomplete. 2c the ligand(s)

Level 1 Two stages are covered but the explanation of each 2d the co-ordination number / shape

stage may be incomplete or may contain

1–2 Stage 3 Colorimetry

inaccuracies.

marks (2/3 virtually complete, 1/3 for covered)

3a measure the absorbance for a range of (known)

only one stage is covered but the explanation is concentrations

generally correct and virtually complete.

3b plot graph of absorbance v concentration / calibration

Answer shows some progression between two curve (of absorbance v concentration)

stages. – A-LEVEL CHEMISTRY – –

3c measure absorbance of the coloured complex and find

0 mark Insufficient correct chemistry to gain a mark. concentration from graph

Question Answers Additional comments/Guidelines 29 Mark

M1 wavelength = 800 (nm) ± 5 –19 –19

Range 2.47 x 10 to 2.502 x 10 = 3/3

Range 2.47 x 10–28 to 2.502 x 10–28 = 2/3

hc 6.63 × 10−34× 3.00 × 108

M2 ΔE ( = ) = −9 800 nm on bottom of expression scores M1 3

5.2 λ M1 × 10

(3 x AO3)

M3 2.49 × 10–19 (J) (allow ECF from M1 or M2) at least 2sf

NOT ECF from M2 if equation re-arranged incorrectly

How to answer it

Colours of transition metal complexes + a quick spectroscopy calculation

What this question tests

Transition metal complex colour

d orbitals split in energy in a ligand field
Absorption of visible light promotes an electron to a higher-energy d orbital
Observed colour is the complementary colour to the absorbed wavelength

Explaining “different colours”

Link colour to changes in ΔE (splitting) caused by: metal, oxidation state, ligand, coordination/shape
Write a coherent, staged explanation (levels of response)

Spectroscopy calculation

Read λ from a spectrum
Use ΔE = hc/λ with correct unit conversion nm → m
Give answer to ≥ 2 significant figures

Question 05.1 (6 marks): Why complexes are coloured + factors + colorimetry method

Marking uses Levels of Response. To hit the top band (5–6), you must cover all three stages clearly: Stage 1: absorption of light → Stage 2: reasons for different colours → Stage 3: how colorimetry finds concentration .

Part (a): Explain why some complexes containing transition metal ions are coloured

✅ Correct answer points (Stage 1: absorption of light)

In a complex, the d orbitals split into different energy levels (they are no longer all the same energy).
Electrons can absorb visible/white light and are promoted to a higher-energy d orbital (excited state).
The colour observed is the complementary colour to the light absorbed; the remaining light is transmitted/reflected.

💡 Key knowledge (what the examiner is looking for)

Use the language of the mark scheme: “d orbitals split”, “electrons move to higher-energy d orbitals / excited state”, “absorb visible/white light”, “complementary colour”.
Make it explicit that the energy gap (ΔE) between split d orbitals matches the energy of visible photons.

🧠 Exam technique (how to secure Level 3)

Write in a cause → effect chain: “ligands cause splitting → light absorbed → electron promoted → complementary colour seen”.
Don’t just say “d electrons jump” — state what causes the jump (absorption of light) and what you see (complementary colour).
Keep it coherent: the mark scheme rewards answers that show logical progression through stages.

❌ Common errors (where marks are lost)

Saying complexes are coloured “because they have d electrons” without mentioning d-orbital splitting and absorption.
Stating the observed colour is the colour absorbed (it’s the complementary colour).
Talking about electrons “moving to higher shells” or “ionising” — it’s a d–d transition within split d orbitals.

Part (b): List the factors that affect the colour

✅ Correct factors (Stage 2: reasons for different colours)

The metal ion (which transition metal it is)
Oxidation state of the metal / charge on the metal ion
The ligand(s) present
Coordination number / shape of the complex

These are the four named in the mark scheme (2a–2d). Any explanation should link them to changing the splitting ΔE.

🧠 How to phrase it for full credit

Don’t list random conditions (e.g. “temperature”) unless you can justify via equilibrium/ligand substitution (not required here).
Add one linking sentence: “These factors change the size of the d-orbital splitting, so different wavelengths are absorbed.”

❌ Common errors

Only writing “ligand affects colour” without including the other factors.
Confusing coordination number/shape with “size of the complex” or “number of atoms” without stating coordination.

Part (c): Describe how colorimetry can be used to determine concentration

✅ Correct method (Stage 3: colorimetry)

Measure absorbance for a range of known concentrations (standards).
Plot a calibration curve: absorbance vs concentration.
Measure the absorbance of the unknown coloured complex and read off its concentration from the graph.

This matches 3a–3c in the mark scheme. The key is: standards → graph → use unknown absorbance to find concentration.

🧠 Examiner-style tips that boost clarity

State the graph axes explicitly: absorbance (y) vs concentration (x).
Say “calibration curve” (examiner keyword).
If you add detail: choose a filter/wavelength where absorbance is high (near λmax) — good science, but not required by this mark scheme.

❌ Common errors

Measuring the unknown only, with no standards/calibration graph.
Plotting concentration vs absorbance (axes swapped) and not making it clear how you then read the unknown.
Using “colour intensity” qualitatively instead of absorbance quantitatively.

💡 How Level 3 (5–6 marks) typically reads

A top response covers all three stages in order: (1) d orbitals split and visible light is absorbed to promote an electron; the observed colour is complementary. (2) Different metals/oxidation states/ligands/shapes change the splitting so different wavelengths are absorbed. (3) In colorimetry, measure absorbance for known concentrations, plot a calibration curve, then use the unknown’s absorbance to find its concentration.

Question 05.2 (3 marks): Energy change from the spectrum of [Cu(H₂O)₆]²⁺

Marks are awarded as: M1 read λ at peak, M2 use ΔE = hc/λ with λ in metres, M3 correct numerical answer to ≥ 2 sf.

✅ Read from Figure 5 (M1)

Peak wavelength λ ≈ 800 nm (allowed: 800 ± 5 nm). M1

💡 Key equation (M2)

Energy of a photon absorbed: ΔE = hc/λ M2

Use λ in m. Convert nm → m by ×10⁻⁹.

❌ Common calculation traps

Forgetting nm → m conversion (gives answer 10⁹ too small/large).
Rearranging incorrectly to ΔE = hλ/c (wrong).
Rounding too aggressively; the mark scheme expects ≥ 2 sf.

📐 Step-by-step calculation (show this layout for full marks)

Take λ from the peak: λ = 800 nm = 800 × 10⁻⁹ m = 8.00 × 10⁻⁷ m M1
Substitute into ΔE = hc/λ using the given constants:
ΔE = (6.63 × 10⁻³⁴ J s × 3.00 × 10⁸ m s⁻¹) / (800 × 10⁻⁹ m)
M2
Calculate:
ΔE = 2.49 × 10⁻¹⁹ J
M3

Accepted range (from λ tolerance): about 2.47 × 10⁻¹⁹ to 2.50 × 10⁻¹⁹ J. Answer shown: 2.49 × 10⁻¹⁹ J (≥ 2 sf).

🧠 Examiner insight: what distinguishes full-mark responses

M1 depends on your reading: state the wavelength clearly (with units). If you put 800 nm in the denominator, that also demonstrates correct use of λ.
M2 is method marking: you must show hc/λ with λ in metres. If you set up the wrong relationship, error-carried-forward may not apply.
M3 needs sensible sig figs: write 2.49 × 10⁻¹⁹ J (or within the accepted range).

Topics

Inorganic Chemistry · Physical Chemistry · 3.2.5 Transition Metals · 3.1.1 Atomic Structure · 3.2.6 Reactions of Ions in Aqueous Solution

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.