AQA A-Level Chemistry Paper 3, 2024: Question 13

Difficulty: Hard

Hydrogen bonding: which liquid forms the most H-bonds?

What this question tests

AO2 focus: apply hydrogen bonding rules

Recognising hydrogen bond donors (O–H, N–H, F–H) vs acceptors (lone pairs on O, N, F).
Comparing functional groups to decide which molecule can form the greatest number of intermolecular hydrogen bonds per molecule in the liquid state.
Avoiding the trap of counting “number of oxygens” without checking for an O–H bond.

Exam part: Q13 (multiple choice, 1 mark)

Q13: Greatest number of hydrogen bonds per molecule (liquid)

Marks [1] Select the correct option. Mark scheme answer: A (CH₃CH₂COOH).

✅ Correct answer (from mark scheme)

A — CH₃CH₂COOH (propanoic acid)

Reason: a carboxylic acid has both a strong hydrogen bond donor (O–H) and multiple hydrogen bond acceptor sites (lone pairs on oxygen atoms), allowing a higher H-bonding capacity per molecule in the liquid.

💡 Key knowledge: what counts as a hydrogen bond?

Donor requirement: H directly bonded to O, N or F (e.g. O–H).
Acceptor requirement: lone pair on O, N or F.
Hydrogen bonds form between molecules: O–H···O , N–H···O , etc.
Important: an oxygen atom without an O–H bond (e.g. ether, aldehyde) can accept H-bonds but cannot donate them.

🧠 Exam technique: quickest way to do this MCQ

Scan each option for an O–H or N–H bond (donor). Only molecules with donors can build extensive H-bond networks in a pure liquid.
If more than one has an O–H, choose the one with the most/best combination of donor + acceptor sites.
Carboxylic acids ( –COOH ) are standout because they have an O–H donor and a carbonyl oxygen acceptor (plus the hydroxyl oxygen can also act as an acceptor).

What top responses do: they don’t just “spot an O”; they explicitly check donor + acceptor.

❌ Common errors (why students lose this 1 mark)

Choosing D (alcohol) because it has O–H, but forgetting acids can form more H-bond interactions per molecule due to the additional carbonyl oxygen acceptor.
Choosing B or C because they contain oxygen, but they have no O–H so they cannot donate hydrogen bonds in the pure liquid.
Confusing “polar bonds” with “hydrogen bonding”: not all polar molecules hydrogen bond.

Option-by-option breakdown (how to justify the choice)

A: CH₃CH₂COOH (carboxylic acid)

Donor: O–H present → can donate H-bonds.
Acceptors: carbonyl O (C=O) is a strong acceptor; hydroxyl O has lone pairs too.
Result: can form multiple H-bonds per molecule in the liquid (often extensive association).

Examiner-style note: acids are commonly the “maximum H-bonding” option in a set like this.

B: CH₃CH₂OCH₃ (ether)

Donor: none (no O–H).
Acceptors: O has lone pairs → can accept.
Limitation: in the pure liquid, with no donor sites, it cannot form strong H-bond networks like alcohols/acids.

C: CH₃CH₂CHO (aldehyde)

Donor: none (no O–H).
Acceptors: carbonyl O accepts H-bonds.
Limitation: cannot donate, so fewer/less extensive H-bonding in the pure liquid compared with A or D.

D: CH₃CH₂CH₂OH (alcohol)

Donor: O–H present.
Acceptors: O lone pairs accept.
Why not the greatest here: compared to a carboxylic acid, it has fewer distinct strong acceptor sites (no carbonyl oxygen).

🧠 Examiner insight: what this 1-mark MCQ is really checking

It’s an application question: you must apply the definition of hydrogen bonding to unfamiliar molecules quickly. Students often lose marks by using “oxygen = hydrogen bonding” as a shortcut. The mark scheme rewards the correct selection only, so your best strategy is a fast donor/acceptor scan.

💡 If you had to draw something: what the H-bonding would look like (A)

Draw two propanoic acid molecules with an O–H···O=C hydrogen bond between them. Show:

a dotted line from the H of one molecule’s O–H to the carbonyl O of the other,
δ⁺ on the H and δ⁻ on the acceptor O (optional but helps clarity),
clear –COOH functional group on each molecule.

This is the kind of diagram that supports a “greatest hydrogen bonding” choice if it were an explanation question.