Edexcel A-Level Chemistry AS Paper 2, November 2021: Question 7

Mark scheme

Show the mark scheme

Question Answer Additional Guidance

Number Mark

7(a) An answer that makes reference to the following point: (1)

• (a series of compounds each containing the) same functional

group / same chemical properties

and

the same general formula Allow one differs from the next by one CH2

unit

Number Mark

7(b) An answer that makes reference to the following points: Example of diagram (3)

• methanol hydrogen bonds to water (1)

• at least one lone pair on an oxygen atom and an approximate 180o

OHO bond angle (1)

• strength of (all) intermolecular forces between methanol and

water is approximately the same as those in water and methanol

or

strength/extent of H-bonding between methanol and water is

same/> that between water/methanol molecules (1)

Accept one labelled hydrogen bond (min)

between the O or H of methanol and a correct

atom in water.

Minimum of one lone pair must be shown on

the relevant O atom

Ignore reference to the methyl group

Allow any mention of H-bond between

methanol and water for M11720 – 1700 cm-1

(Total for Question 7 = 4 marks)

How to answer it

Study Guide: The Solubility of Alcohols

What this question tests

This question assesses your understanding of organic chemistry definitions (homologous series) and intermolecular bonding (hydrogen bonding). You will be expected to recall core definitions, apply knowledge of intermolecular forces to explain physical properties like solubility, and communicate complex spatial bonding arrangements clearly using labelled diagrams.

Question 7(a) — 1 Mark

Defining a Homologous Series

✅ Correct Answer

A series of compounds containing the same functional group, sharing similar chemical properties, and having the same general formula.

Alternative credit: Stating that each successive member differs from the next by one CH₂ unit.

💡 Key Knowledge

Alcohols share the functional group -OH (hydroxyl group).
General formula for straight-chain saturated alcohols: CₙH₂ₙ₊₁OH .
Members show a gradual trend in physical properties (e.g., boiling points increasing, solubility decreasing as chain length grows).

🧠 Exam Technique

Definitions must be memorised precisely. Examiners look for specific keywords: functional group, chemical properties, and general formula. Vague answers mentioning only "similar formulas" will fail to score.

❌ Common Errors

Students frequently lose this mark by stating that members have the same physical properties (incorrect; physical properties change progressively with molecular mass) or forgetting to mention the general formula.

🎯 Mark breakdown: 1 mark for stating both the same functional group/properties and the same general formula (or CH₂ difference).

Question 7(b) — 3 Marks

Explaining Solubility via Hydrogen Bonding

✅ Correct Answer (Mark Scheme Points)

Point 1: Methanol forms hydrogen bonds with water molecules.
Point 2: The diagram must show at least one lone pair of electrons on an oxygen atom, a linear or near-linear O-H...O arrangement (approx. 180° bond angle), and a clear dashed line representing the hydrogen bond.
Point 3: The strength of the intermolecular forces formed between methanol and water is approximately equal to the forces broken within pure water and pure methanol.

💡 Key Knowledge & Diagram Guide

The Diagram to Draw: Show CH₃-O-H bonded to H-O-H . Include a dashed line labelled "hydrogen bond" between the hydrogen of one molecule and the lone pair on the oxygen of the adjacent molecule.
Show non-bonding electron pairs (dots) on the oxygen atoms.
Polarity arises due to the high electronegativity difference of oxygen relative to hydrogen and carbon.

🧠 Exam Technique

When drawing hydrogen bonds, ensure the dashed line connects the hydrogen atom covalently bonded to an electronegative atom (O, N, F) directly to a lone pair on another electronegative atom. The alignment around the hydrogen bond must look linear (180°).

❌ Common Errors

Omitting lone pairs on the oxygen atoms, drawing the hydrogen bond pointing towards a covalent bond instead of a lone pair, or failing to mention energy/strength equivalence of intermolecular forces. Ignoring the requirement to include a diagram automatically caps your score.

🎯 Mark breakdown: 1 mark for stating methanol H-bonds to water; 1 mark for the accurate annotated diagram (lone pair & approximate 180° angle); 1 mark for explaining that intermolecular bond strengths are comparable/favourable.

Topics

Physical Chemistry · Organic Chemistry · Topic 2: Bonding and Structure · Topic 6: Organic Chemistry I