AQA A-Level Chemistry Paper 3, 2019: Question 23
1 mark · Easy difficulty · Multiple Choice
Identify which row correctly matches the organic compound with the observations seen when reacted separately with sodium hydrogen carbonate, acidified potassium dichromate(VI), and Tollens' reagent.
Practise this questionQuestion
Question text
23 Three reagents are added separately to four organic compounds.
Which row shows the correct observations?
[1 mark]
Acidified
Sodium hydrogen Tollens’
potassium
carbonate reagent
dichromate(VI)
no
orange solution
A Propan-1-ol effervescence visible
turns green
change
orange solution silver
B Propanal no visible change
turns green mirror
no visible silver
C Propanone no visible change
change mirror
Propanoic no visible silver
D effervescence
acid change mirror
Mark scheme
Show the mark scheme
23 B 1
How to answer it
Distinguishing Organic Functional Groups via Chemical Tests
This question assesses knowledge of characteristic qualitative diagnostic tests used in organic chemistry to identify functional groups:
- Carboxylic acids: Reaction with carbonates / hydrogencarbonates ( NaHCO₃ ) producing carbon dioxide gas ( CO₂ ).
- Alcohols & Aldehydes: Oxidation reactions using acidified potassium dichromate(VI) ( K₂Cr₂O₇ / H₂SO₄ ).
- Aldehydes vs Ketones: Mild oxidation using Tollens' reagent ( [Ag(NH₃)₂]⁺ ) producing a silver mirror.
Multiple Choice: Reagents and Observations
Identify which row correctly matches the compound to its chemical test results
| Option | Compound | Sodium hydrogencarbonate | Acidified potassium dichromate(VI) | Tollens' reagent |
|---|---|---|---|---|
| A | Propan-1-ol (primary alcohol) | effervescence ❌ | orange solution turns green ✔ | no visible change ✔ |
| B | Propanal (aldehyde) | no visible change ✔ | orange solution turns green ✔ | silver mirror ✔ |
| C | Propanone (ketone) | no visible change ✔ | no visible change ✔ | silver mirror ❌ |
| D | Propanoic acid (carboxylic acid) | effervescence ✔ | no visible change ✔ | silver mirror ❌ |
✅ Correct Answer
B: Propanal
- Sodium hydrogencarbonate: Aldehydes are not acidic enough to react with carbonates; therefore, no visible change occurs.
- Acidified dichromate(VI): Propanal is easily oxidized to propanoic acid. The Cr₂O₇²⁻ (orange) is reduced to Cr³⁺ (green).
- Tollens' reagent: Propanal reduces silver(I) ions to elemental silver, forming a characteristic silver mirror.
💡 Key Knowledge Breakdown
- NaHCO₃ test: Tests specifically for carboxylic acids ( -COOH ). Carboxylic acids release CO₂(g) (fizzing/effervescence). Alcohols, aldehydes, and ketones do not react.
- Acidified K₂Cr₂O₇ : Orange to green indicates oxidation. Occurs with primary alcohols, secondary alcohols, and aldehydes. Tertiary alcohols and ketones cannot be oxidized.
- Tollens' reagent: Contains [Ag(NH₃)₂]⁺ . Positive test (silver mirror) occurs only with aldehydes. Ketones and alcohols give no visible change.
❌ Common Errors & Eliminating Distractors
- Why A is incorrect: Propan-1-ol is neutral in this context and cannot react with NaHCO₃ to produce CO₂ . There is no effervescence.
- Why C is incorrect: Propanone is a ketone; ketones cannot be oxidized by mild oxidizing agents like Tollens' reagent. It should show no visible change with Tollens'.
- Why D is incorrect: Propanoic acid is fully oxidized. It does not react with Tollens' reagent; it should show no visible change instead of forming a silver mirror.
🧠 Exam Technique: Rapid Elimination
In multiple choice questions with comparison tables, use the fastest diagnostic tests first to eliminate wrong rows quickly:
- Look at Tollens' reagent: Only aldehydes give a silver mirror. Propanal is the only aldehyde in the list. This instantly eliminates rows A, C, and D!
- Double-check Row B across the other two reagents to confirm accuracy before moving on.
Topics
Organic Chemistry · Required Practicals · 3.3.6 Organic Analysis · 3.3.8 Aldehydes and Ketones · Required Practical 6: Tests for alcohol, aldehyde, alkene and carboxylic acid
Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.