AQA A-Level Chemistry Paper 3, 2023: Question 4

6 marks · Hard difficulty · Long Answer

Identification of Iron Salt Solutions This question involves identifying solutions L & M based on observations from chemical tests, explaining effervescence with sodium carbonate for L and writing ionic equations for the reactions observed during the tests.

Practise this question

Question

AQA A-Level Chemistry Paper 3, 2023: Question 4
Question text

04 A student is given two aqueous solutions, L and M, that both contain iron salts.

The student does a series of tests on the solutions.

Table 1 shows these tests and the observations.

Table 1

Test Observations with L Observations with M

Add ammonia solution A red-brown precipitate A green precipitate forms

slowly until in excess. forms that is insoluble in that is insoluble in excess.

excess.

Add sodium carbonate A red-brown precipitate A green precipitate forms.

solution. forms.

Effervescence is seen.

Add dilute nitric acid and No change is seen. No change is seen.

then divide into two

portions.

Add barium chloride No change is seen. A white precipitate forms.

solution to the first portion.

Add silver nitrate solution A white precipitate No change is seen.

to the second portion. forms.

Identify L and M using the results in Table 1.

In your answer:

• identify all precipitates

• explain why effervescence is seen in the reaction of sodium carbonate

with L but not with M

• give ionic equations for all reactions.

[6 marks]

Mark scheme

Show the mark scheme Mark scheme for AQA A-Level Chemistry Paper 3, 2023: Question 4

Question Answers Additional comments/Guidelines Mark

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

Scheme Instructions for Examiners for guidance.

Level 3 All stages are covered and the explanation of each stage Stage 1 IDs (Allow ppts if shown as products

5–6 marks is correct and virtually complete (ie two from stages 2 and in equations or on the table)

3 and three from stage 1). (Allow names – with oxidation states for Fe

compounds)

Answer communicates the whole explanation, including

equations, coherently and shows a logical progression

through all three stages. (1a) BOTH red-brown ppts from L =

Level 2 All stages are covered (NB ‘covered’ means min two from Fe(H2O)3(OH)3 / Fe(OH)3

3–4 marks stage 1) but the explanation of each stage may be

incomplete or may contain inaccuracies. (1b) green ppt from M = Fe(H2O)4(OH)2/Fe(OH)2

OR for NH3

two stages covered and the explanations are generally AND M = FeCO3 with Na2CO3

correct and virtually complete (i.e. two from stages 2

and/or 3 and/or three from stage 1). (1c) white ppt with L = AgCl AND

white ppt with M = BaSO4 6

Answer is coherent and shows some progression through

all three stages. Some steps in each stage may be

incomplete. (1d) L = FeCl3 AND M = FeSO4

Level 1 Two stages are covered (NB ‘covered’ means min two

1–2 marks from stage 1) but the explanation of each stage may be Stage 2 Reactions with sodium carbonate

incomplete or may contain inaccuracies. (ALLOW if any 3+ compared with any 2+)

OR

only one stage is covered but the explanation is generally (2a) carbon dioxide is gas produced from L

correct and virtually complete (i.e. two from stages 2 (could come from equation)

and/or 3 and/or three from stage 1).

(2b) Fe3+ / Fe(H O) 3+ (in L) is more acidic than

Answer shows some progression between two stages. 2+ 2+

Fe / Fe(H2O)6 (in M) ora

0 mark Insufficient correct chemistry to gain a mark.

(2c) Fe3+ is smaller / has higher charge / greater

charge density of Fe3+ / is more polarising

(makes ion a better proton donor)

How to answer it

Step-by-Step Guide: Identifying Solutions L and M

Step 1: Identify the Precipitates

Observations and Deductions

  • L: Red-brown precipitate with ammonia and sodium carbonate — indicates Fe(OH)3.
  • M: Green precipitate with ammonia and sodium carbonate — indicates Fe(OH)2.
  • White precipitate with silver nitrate in L — indicates Cl- (AgCl).
  • White precipitate with barium chloride in M — indicates SO42- (BaSO4).

Answer: L = FeCl3, M = FeSO4

Teacher Tip: Always match the observations to known reactions of ions (e.g., Fe3+ and Fe2+).

Step 2: Explain Effervescence in Sodium Carbonate Reaction

Why Effervescence Occurs in L but Not M

Effervescence (CO2 gas) is observed with L because Fe3+ ions react with carbonate to form CO2, whereas Fe2+ ions form a simple precipitate without gas release.

Chemical Explanation:

  • Fe3+: More acidic, reacts with CO32- to release CO2.
  • Fe2+: Less acidic, does not produce CO2.
Teacher Tip: Remember that Fe3+ has a higher charge density and is more polarising, making it a stronger Lewis acid.

Step 3: Write Ionic Equations

Key Reactions

  • Ammonia with L: Fe(H2O)63+ + 3NH3 → Fe(OH)3 + 3NH4+
  • Ammonia with M: Fe(H2O)62+ + 2NH3 → Fe(OH)2 + 2NH4+
  • Sodium carbonate with L: 2Fe(H2O)63+ + 3CO32- → 2Fe(OH)3 + 3CO2 + 3H2O
  • Sodium carbonate with M: Fe(H2O)62+ + CO32- → FeCO3 + 6H2O
  • Barium chloride with M: Ba2+ + SO42- → BaSO4
  • Silver nitrate with L: Ag+ + Cl- → AgCl
Teacher Tip: Write balanced ionic equations for each observation. Include states (s, l, g, aq) if required in the exam.

Final Answer

L: FeCl3

M: FeSO4

Key Observations: Effervescence with sodium carbonate in L, no effervescence in M.

Key Equations: See step 3 for all reactions.

Topics

Inorganic Chemistry · 3.2.6 Reactions of Ions in Aqueous Solution

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.