AQA A-Level Chemistry Paper 1, 2019: Question 4

14 marks · Medium difficulty · Long Answer

Identify reaction products, colours, and equations for aqueous iron complexes, suggest a reducing agent, and describe the shapes and isomerism of transition metal complexes with drawn examples.

Practise this question

Question

Question text

04 Figure 3 shows some reactions of aqueous iron ions.

Figure 3

04.1 Give the formula of Precipitate J and state its colour.

Give an equation for Reaction 1.

[3 marks]

Formula of J

Colour

Equation

04.2 Give the formula of L and an equation for Reaction 2.

[2 marks]

Formula of L

Equation

04.3 Suggest a reagent for Reaction 3.

[1 mark]

04.4 Give the formula of Precipitate M and state its colour.

[2 marks]

*07* Formula of M

Colour

04.5 Transition metal complexes have different shapes and many show isomerism.

Describe the different shapes of complexes and show how they lead to different

types of isomerism.

Use examples of complexes of cobalt(II) and platinum(II).

You should draw the structures of the examples chosen.

[6 marks]

Mark scheme

Show the mark scheme

Question Answers Additional Comments/Guidelines Mark

[Fe(OH)3(H2O)3] 1

M2: Allow red-brown

04.1 Brown 1

M3: Allow correct equations with Na2CO3

2 [Fe(H O) ]3+ + 3 CO 2– 2 [Fe(OH) (H O) ] + 3 CO + 3 H O M3: Ignore state symbols 1

26 3 3 2 3 2 2

[FeCl ]− 1

04.2 – −

[Fe(H O) ]3+ + 4 Cl [FeCl ] + 6 H O M2: Allow correct equations with HCl 1

26 4 2

04.3 (XS) Zn (in acid or HCl or H2SO4) Allow KI/potassium iodide 1

[Fe(OH)2(H2O)4] 1

04.4

green 1

This question is marked using levels of response. Refer to the Indicative Chemistry content

Mark Scheme Instructions for Examiners for guidance on how

to mark this question. Stage 1: shapes of complexes

Level 3 1a octahedral or 6 co-ordinate diagram

5–6 marks 1b tetrahedral or square planar or 4 co-ordinate diagram

All stages are covered and the description of each stage is 6

generally correct and virtually complete. Answer is

communicated coherently and shows a logical progression

from stage 1 to stage 2 and stage 3

Stage 2: cis/ trans isomerism (or E-Z or geometric)

Answer is illustrated using diagrams of at least 2 specific

examples of pairs of cobalt or platinum complex isomers. 2a cis/trans isomerism in either square planar and/or

octahedral complexes

Level 2

3–4 marks 2b Diagrams showing cis and trans isomerism in a square

All stages are covered but the description of each stage may planar complex

be incomplete or may contain inaccuracies OR two stages are

covered and the explanations are generally correct and 2c Diagrams showing cis and trans isomerism in both 15

04.5 virtually complete. Answer is mainly coherent and shows

isomers of octahdedral complexes eg draw cis and trans

progression from stage 1 to stage 2 and/or stage 3.

M(H O) (OH) or [M(NH ) (H O) ]2+

24 2 3 4 2 2

Answer is illustrated using diagrams of at least 1 specific

example of a pair of cobalt or platinum complex isomers.

Level 1

1–2 marks Stage 3: optical isomerism

Two stages are covered but the description of each stage may 3a optical isomerism / non superimposable mirror images in

be incomplete or may contain inaccuracies, OR only one stage

is covered but the explanation is generally correct and virtually octahedral complexes

complete. Answer includes isolated statements and these are 2-

presented in a logical order. 3b occurs with a specific bidentate ligands eg.C2O4 or

NH2CH2CH2NH2

Answer is illustrated using at least 1 appropriate diagram or

formula. 3c draw both optical isomers of eg [M(NH CH CH NH ) ]2+

22 2 2 3

Level 0

0 marks Insufficient correct chemistry to gain a mark.

How to answer it

Reactions of Aqueous Iron Ions & Transition Metal Complex Isomerism

📌 What this question tests

This question assesses key Transition Metal chemistry:

Hydrolysis and acidity differences between 3+ and 2+ aqueous metal-aqua ions with carbonate and ammonia.
Ligand substitution involving coordination number change (chloride vs water).
Redox reactions interconverting Fe³⁺ and Fe²⁺.
6-mark extended response on shapes of complexes (octahedral, tetrahedral, square planar) and isomerism (cis/trans and optical) with Cobalt(II) and Platinum(II).

Question 04.1 [3 Marks]

Precipitate J and Reaction with Aqueous Sodium Carbonate

Reaction 1: [Fe(H₂O)₆]³⁺ with Na₂CO₃(aq)

✅ Mark Scheme Answers

Formula of J: [Fe(OH)₃(H₂O)₃] (or Fe(OH)₃)

Colour: Brown (allow red-brown)

Equation:
2[Fe(H₂O)₆]³⁺ + 3CO₃²⁻ → 2[Fe(OH)₃(H₂O)₃] + 3CO₂ + 3H₂O
(Equations using full formula Na₂CO₃ are also accepted: 2[Fe(H₂O)₆]³⁺ + 3Na₂CO₃ → 2[Fe(OH)₃(H₂O)₃] + 3CO₂ + 3H₂O + 6Na⁺)

💡 Key Knowledge

Fe³⁺ has a high charge density, polarising O–H bonds in water ligands strongly. This makes [Fe(H₂O)₆]³⁺ acidic enough to react with weak base CO₃²⁻.
Instead of forming an insoluble iron(III) carbonate, it undergoes an acid-base reaction yielding a neutral hydroxide precipitate and CO₂ gas (effervescence).

❌ Common Errors

Writing "Fe₂(CO₃)₃" as the precipitate — iron(III) carbonate does not exist in aqueous solution.
Forgetting CO₂ gas as a product in the equation.
Misbalancing the stoichiometry: remember the 2 : 3 ratio (2Fe³⁺ : 3CO₃²⁻ produces 3CO₂ and 3H₂O).

🧠 Exam Technique

Mark Breakdown: 1 mark for correct formula of J; 1 mark for brown/red-brown; 1 mark for balanced equation. State symbols are not required.

Question 04.2 [2 Marks]

Ligand Substitution with Concentrated HCl

Reaction 2: Formation of Complex Ion L

✅ Mark Scheme Answers

Formula of L: [FeCl₄]⁻

Equation:
[Fe(H₂O)₆]³⁺ + 4Cl⁻ → [FeCl₄]⁻ + 6H₂O
(Equation using concentrated HCl directly is also permitted: [Fe(H₂O)₆]³⁺ + 4HCl → [FeCl₄]⁻ + 6H₂O + 4H⁺)

💡 Key Knowledge

Chloride ions (Cl⁻) are larger than neutral H₂O molecules and bear negative charges, causing electrostatic repulsion.
Only 4 chloride ligands can coordinate around Fe³⁺, resulting in a change from 6-coordinate octahedral to 4-coordinate tetrahedral.
Overall charge calculation: Fe³⁺ + 4(Cl⁻) = -1, hence [FeCl₄]⁻ .

❌ Common Errors

Giving an incorrect charge like [FeCl₄]²⁻ or neutral FeCl₄.
Assuming substitution is 1-to-1 without changing coordination number (e.g. writing [FeCl₆]³⁻).
Forgetting to release all 6 H₂O molecules on the right-hand side.

🧠 Exam Technique

Mark Breakdown: 1 mark for correct formula of L (including charge); 1 mark for balanced equation.

Question 04.3 [1 Mark]

Reduction of Iron(III) to Iron(II)

Reaction 3: [Fe(H₂O)₆]³⁺ → [Fe(H₂O)₆]²⁺

✅ Mark Scheme Answers

Acceptable Reagents:

Zn (in acid, e.g. dilute HCl or H₂SO₄)
KI / Potassium iodide (or any soluble iodide like NaI / I⁻)

💡 Key Knowledge

Fe³⁺ is reduced to Fe²⁺ (oxidation state drops from +3 to +2). You need a suitable reducing agent whose E° is more negative than E°(Fe³⁺/Fe²⁺ = +0.77 V):

Zn(s) → Zn²⁺ + 2e⁻ (in dilute acid)
2I⁻ → I₂ + 2e⁻ (brown solution of iodine forms)

Question 04.4 [2 Marks]

Precipitate M with Concentrated Ammonia

Reaction 4: [Fe(H₂O)₆]²⁺ with Concentrated NH₃(aq)

✅ Mark Scheme Answers

Formula of M: [Fe(OH)₂(H₂O)₄] (or Fe(OH)₂)

Colour: Green

💡 Key Knowledge

Aqueous ammonia acts as a Bronsted-Lowry base: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻.
Unlike Cu²⁺ or Co²⁺, iron(II) does not undergo ligand substitution in excess or concentrated NH₃. It stops at the neutral, insoluble hydroxide precipitate.
On standing in air, this green precipitate gradually darkens and turns brown at the surface due to aerial oxidation to Fe(OH)₃.

❌ Common Errors

Writing [Fe(NH₃)₆]²⁺ — expecting ammonia substitution to occur as it does with Cu²⁺ or Cr³⁺.
Confusing the colour with Fe³⁺ hydroxide: Fe(OH)₂ is green, whereas Fe(OH)₃ is brown.

🧠 Exam Technique

Mark Breakdown: 1 mark for formula [Fe(OH)₂(H₂O)₄]; 1 mark for stating "green".

Question 04.5 [6 Marks] — Extended Response

Shapes and Isomerism of Cobalt(II) and Platinum(II) Complexes

Level-of-Response Breakdown & Required Chemistry Content

📋 Marking Scheme Criteria (Levels of Response)

To access Level 3 (5–6 marks), all 3 stages must be covered, logically structured, and supported by clear 3D/structural diagrams of at least two specific pairs of isomers involving Co and/or Pt complexes.

Stage 1: Shapes of Complexes

Octahedral: 6 coordinate bonds, bond angles 90° (e.g. Cobalt(II) complexes like [Co(H₂O)₆]²⁺).
Tetrahedral: 4 coordinate bonds, bond angle 109.5° (e.g. [CoCl₄]²⁻ with large chloride ligands).
Square Planar: 4 coordinate bonds, bond angles 90°, characteristic of Pt(II) complexes like cisplatin, [Pt(NH₃)₂Cl₂].

Stage 2: Cis/Trans (E-Z / Geometric) Isomerism

In Square Planar [Pt(NH₃)₂Cl₂]:
• cis-platin: identical ligands (Cl–Cl and NH₃–NH₃) are 90° apart (adjacent).
• trans-platin: identical ligands are 180° apart (opposite each other across the Pt centre).
In Octahedral Complexes (e.g. [Co(H₂O)₄(OH)₂] or [Co(NH₃)₄Cl₂]):
• cis-isomer: the two identical ligands are adjacent (90°).
• trans-isomer: the two identical ligands are opposite (180°).

Stage 3: Optical Isomerism

Occurs in octahedral complexes with bidentate ligands (e.g. 1,2-diaminoethane: H₂NCH₂CH₂NH₂ / "en", or ethanedioate: C₂O₄²⁻ ).
Example: [Co(H₂NCH₂CH₂NH₂)₃]²⁺ or [Co(en)₂Cl₂] (specifically the cis isomer).
Forms non-superimposable mirror images (enantiomers) that rotate the plane of plane-polarised light in opposite directions.

🎨 How to Draw the Isomers in the Exam

1. Cis/Trans Platin [Pt(NH₃)₂Cl₂]:
Draw a flat cross with Pt in the centre.
• Cis: Cl at top, Cl at left (adjacent); NH₃ at bottom, NH₃ at right.
• Trans: Cl at top, Cl at bottom (180°); NH₃ at left, NH₃ at right (180°).

2. Optical Isomers of [Co(en)₃]²⁺:
Draw two octahedral cobalt skeletons separated by a dashed mirror line. Connect adjacent pairs of coordination bonds with curved loops labelled "en" or "NH₂CH₂CH₂NH₂". Show non-superimposable chiral reflection.

Examiner Insight for 6/6 Marks: The rubric strictly requires both shapes and isomerism explained using named/drawn examples of Cobalt or Platinum. Many students lose marks by drawing generic "M" metal centers or by not drawing the pairs of isomers (you must show both cis AND trans, or both mirror images).

Topics

Inorganic Chemistry · 3.2.5 Transition Metals · 3.2.6 Reactions of Ions in Aqueous Solution

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 1, 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.