Edexcel A-Level Chemistry Paper 3, November 2021: Question 2

7 marks · Medium difficulty · Short Open Response

Deduce chemical tests to distinguish between isomers of C4H8O2 containing different functional groups and identify expected infrared absorbance ranges for each isomer.

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Question

Question text

2 Analysis shows that a compound has the molecular formula C4H8O2.

A student suggests that the compound could be either A or B.

CH3CH2CH2COOH or HOCH2CH=CHCH2OH

A B

(a) Deduce a chemical test which would give a positive result for A but not for B.

Include the reagent and observation.

(2)

(b) Deduce a chemical test which would give a positive result for B but not for A.

Include the reagent and observation.

(2)

an alcohol functional group, with structure C.

HOCH2CH2CH2CHO

Complete the table to show how the infrared spectra of A, B and C would be

4 expected to differ in the wavenumber range 1800-1600 cm−1.

Use information from the Data Booklet.*P67806A0436*

(3)

Absorbance Wavenumber range / cm−1

Absorbance expected in infrared spectrum

of A but not in B or C

Absorbance expected in infrared spectrum

of B but not in A or C

Absorbance expected in infrared spectrum

of C but not in A or B

(Total for Question 2 = 7 marks)

Mark scheme

Show the mark scheme

Question Answer Additional Guidance Mark

Number

2(a) A description that makes reference to the Examples of reagents and observations (2)

following points:

Reagent Observation

• reagent (1) any carbonate / effervescence / fizzing / bubbles /

NaHCO3 / KHCO3 gas evolved that turns limewater

• observation (1) (added to aqueous acid) milky

magnesium (added to effervescence / fizzing / bubbles /

aqueous acid) gas evolved that gives a pop with a

lighted splint

alcohol and characteristic smell (of an ester)

(concentrated) H2SO4 /

HCl / H+

Allow names or formulae for reagents but if both are given, both

must be correct

Ignore conditions e.g. heat

Do not award PCl5 / Na

If more than one test is given, penalise any incorrect tests

Number

2(b) A description that makes reference to two of Examples of reagents and observations (2)

the following points:

Reagent Observation

• reagent (1) bromine water orange / yellow / brown

Allow bromine (in an organic solution goes colourless

• corresponding observation (1) solvent) Allow bromine water

is decolourised

carboxylic acid and characteristic smell (of an

(concentrated) H2SO4 / HCl / ester)

H+

acidified potassium purple to colourless /

manganate(VII) / decolourised

permanganate

alkaline potassium purple to green

manganate(VII)

(neutral) potassium purple to brown ppt

manganate(VII)

acidified (potassium) orange to green

dichromate((VI)) (ions)

Allow names or formulae for reagents but if both are given, both

must be correct

Ignore conditions e.g. heat

Do not award PCl5 / Na

If more than one test is given, penalise any incorrect tests

Number

2(c) Example of table (3)

Wavenumber range

Absorbance −1

/ cm

• absorbance for A (1) Absorbance expected in infrared 1720 – 1700

spectrum of A but not in B or C

Allow 1725 - 1700

• absorbance for B (1) Absorbance expected in infrared

spectrum of B but not in A or C 1669 – 1645

Absorbance expected in infrared 1740 – 1725

• absorbance for C (1) spectrum of C but not in A or B

Allow 1740 - 1720

Allow values in reverse order e.g. 1700 – 1720 for A

If single values are given instead of a range award (2) for 3 correct values

within the ranges and (1) for 2 correct values

Do not award a single value that occurs in two ranges i.e 1720-1725

If no other mark is awarded, allow (1) for:

A 3300-2500 and C 2900-2820 / 2775-2700

(Total for Question 2 = 7 marks)

How to answer it

Functional Group Identification & IR Spectroscopy

Edexcel A-Level Chemistry — Organic Analysis

What this question tests

This question assesses your ability to distinguish between organic isomers using unique chemical test tube reactions (reagents and specific observations) and interpreting infrared (IR) absorption data from the Edexcel Data Booklet to differentiate specific carbonyl and alkene functional groups.

Part (a): Chemical test for Compound A (Carboxylic Acid)

Deduce a chemical test which would give a positive result for A (CH₃CH₂CH₂COOH) but not for B (HOCH₂CH=CHCH₂OH).

✅ Correct Answer

Reagent: Any metal carbonate or hydrogen carbonate (e.g., Na₂CO₃ , NaHCO₃ , K₂CO₃ ).
Observation: Effervescence / fizzing / bubbles seen; gas evolved turns limewater cloudy.
Alternative: Add magnesium to give effervescence and a pop with a lighted splint.
Alternative: Add an alcohol + concentrated H₂SO₄ to produce a sweet, characteristic smell of an ester.

💡 Key Knowledge

Compound A is a carboxylic acid, meaning it is acidic enough to react with weak bases like carbonates to release carbon dioxide gas.
Compound B contains alcohol and alkene groups, neither of which react with carbonates.

🧠 Exam Technique

Always state both the reagent and the observable change clearly to secure both marks.
If naming a chemical, ensure formula matches if both are given.

❌ Common Errors

Do not use PCl₅ or sodium metal ( Na ), as both will react with both A (carboxylic acid) and B (alcohol).
Forgetting to state what happens to the gas (e.g., limewater turning milky for CO₂ ).

🎯 Mark Breakdown: (1) mark for correct reagent, (1) mark for matching positive observation for A and negative/no reaction for B. Total: 2 marks.

Part (b): Chemical test for Compound B (Alkene / Alcohol)

Deduce a chemical test which would give a positive result for B but not for A.

✅ Correct Answer

Reagent: Bromine water ( Br₂(aq) ).
Observation: Orange / yellow / brown solution goes colourless (or decolourised).
Alternative: Acidified potassium manganate(VII) turns from purple to colourless.
Alternative: Acidified potassium dichromate(VI) turns from orange to green (testing the alcohol group in B).

💡 Key Knowledge

Compound B contains a carbon-carbon double bond (alkene) which undergoes electrophilic addition with bromine water.
Compound A is a saturated carboxylic acid and does not react with bromine water under normal conditions.

🧠 Exam Technique

Specify "bromine water" rather than just "bromine" to get credit for the aqueous reagent and correct starting colour.

❌ Common Errors

Saying the bromine water goes "clear" instead of colourless. (Examiners heavily penalize confusing clear with colourless).

🎯 Mark Breakdown: (1) mark for correct reagent, (1) mark for correct observation. Total: 2 marks.

Part (c): Infrared Spectroscopy Analysis

Complete the table to show how the IR spectra of A, B, and C differ in the wavenumber range 1800–1600 cm⁻¹.

✅ Correct Answer (Table Values)

Absorbance in A (Carboxylic Acid C=O): 1720 – 1700 cm⁻¹ (allow 1725 – 1700)
Absorbance in B (Alkene C=C): 1669 – 1645 cm⁻¹
Absorbance in C (Aldehyde C=O): 1740 – 1725 cm⁻¹ (allow 1740 – 1720)

💡 Key Knowledge

Different functional groups absorb infrared radiation at characteristic wavenumber ranges due to different bond stiffness and masses.
Aldehyde carbonyl peaks appear at slightly higher wavenumbers than carboxylic acid carbonyl peaks.

🧠 Exam Technique

Always quote ranges directly from the Data Booklet provided in your exam. Do not guess values from memory if they are tabulated.

❌ Common Errors

Overlapping ranges: Avoid giving single values that could span across two separate category definitions unless strictly non-overlapping.

🎯 Mark Breakdown: (1) mark for each correct absorbance wavenumber range corresponding correctly to compounds A, B, and C. Total: 3 marks.

Topics

Organic Chemistry · Core Practicals · Core Practical 7: Identify unknown organic liquids and inorganic solids · Topic 6: Organic Chemistry I · Topic 7: Modern Analytical Techniques I

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 3, November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.