OCR A-Level Chemistry Unified chemistry (03), November 2020: Question 6

10 marks · Hard difficulty · Structured Questions

Predict bond angles and shapes, explain acid strength using pKa values, and draw a reaction mechanism involving curly arrows for carboxylic and sulfonic acids and their derivatives.

Practise this question

Question

Question text

6 This question is about two different types of acid found in organic compounds, carboxylic acids

and sulfonic acids, as shown in Fig. 6.1.

O O

R C R S OH

OH O

Carboxylic acid Sulfonic acid

Fig. 6.1

(a) Complete Table 6.1 to predict bond angles a and b and name the shapes which makes these

bond angles in the functional groups of carboxylic acids and sulfonic acids.

Type of acid Acid Bond angle Name of shape

Carboxylic acid R C a

Sulfonic acid R S O

b …

O H

Table 6.1

[2]

(b) Ethanoic acid, CH3COOH, and methanesulfonic acid, CH3SO2OH, are both monobasic acids.

The pKa values are shown in the table.

Acid pKa

Ethanoic acid CH3COOH 4.76

Methanesulfonic acid CH3SO2OH –1.90

A student suggests that 1.0 mol dm−3 CH SO OH should have a lower pH value than

1.0 mol dm−3 CH COOH.

Write an equation, showing conjugate acid–base pairs, for the equilibrium of CH3SO2OH with

water and explain, with reasons, whether the student is correct.

Label the conjugate acid–base pairs: A1, B1 and A2, B2.

… [4]

Sulfonic acid esters can be hydrolysed by aqueous alkali.

The equation shows the alkaline hydrolysis of a sulfonic acid ester.

CH SO OCH + OH− → CH SO O− + CH OH

32 3 3 2 3

In the 3 boxes below, add curly arrows to show the mechanism for this reaction.

In the first box, the hydroxide ion acts as a nucleophile.

O O –

H3C S OCH3 H C S OCH

O O OH

– OH

O H

H C S O – OCH

H C S O – + HOCH

[4]

Mark scheme

Show the mark scheme

Question Answer Marks Guidance

element

6 (a) Bond angle Name of shape 2 1.2×2

120(°) Trigonal planar

104–105(°) Non-linear For non-linear,

ALLOW bent, v-shaped, angular

Mark by row OR by column to give higher mark IGNORE planar, ‘not straight’

i.e. 2 bond angles correct

2 shapes correct

i.e. bond angle AND shape correct in 1st row

bond angle AND shape correct in 2nd row

(b) CH3SO2OH + H2O CH3SO2O– + H3O+ 4 2.1×2 ALLOW → for

A1 B2 B1 A2 ALLOW acid–base pairs labelled other way round.

i.e. CH3SO2OH + H2O CH3SO2O– + H3O+

For an equilibrium shown using CH3COOH instead of A2 B1 B2 A1

H2O, mark acid–base pairs by ECF, i.e. ALLOW small slip

CH3SO2OH + CH3COOH CH3SO2O– + CH3COOH2+

A1 B2 B1 A2 ECF If ONE charge is missing from equilibrium.

ALLOW ECF for acid–base pairs mark

CH3SO2OH dissociates more (than CH3COOH)

OR CH3SO2OH is a stronger acid IGNORE ‘more acidic’

3.1 Response needs strength/dissociation

ORA in terms of CH3COOH being a weaker acid

ALLOW maths explanation for final 2 marks, e.g.

Ka(CH3COOH) = 10–(4.76) = 1.74 × 10–5

Student is correct

AND [H+] = √(1.74 × 10–5) × 1) = 4.17 × 10–3

(sulfonic acid has) lower pK /higher K OR greater [H+] pH = –log 4.17 × 10–3 = 2.38

a a

ORA

3.2 Ka(CH3SO2OH) = 10–(–1.90) = 79.4

[H+] = √(79.4) × 1) = 8.91

pH = –log 8.91 = –0.95

BOTH pH calcs subsumes ‘Student is correct’

21 AO

element

Marks solely for curly arrows

IGNORE any curly arrows on bottom structures

(not in boxes):

6 curly arrows correct

5 curly arrows correct

4 curly arrows correct

3 curly arrows correct

Total 10

OCR (Oxford Cambridge and RSA Examinations)

The Triangle Building

Shaftesbury Road

Cambridge

CB2 8EA

OCR Customer Contact Centre

Education and Learning

Telephone: 01223 553998

Facsimile: 01223 552627

Email: general.qualifications@ocr.org.uk

www.ocr.org.uk

For staff training purposes and as part of our quality assurance programme your call may be

recorded or monitored

How to answer it

Organic Acid Structures, pKₐ & Mechanisms

OCR A-Level Chemistry • Organic Chemistry & Analysis

What this question tests

This question assesses your understanding of functional group geometry (VSEPR theory), acid strength comparisons via pKₐ values, Brønsted-Lowry acid-base equilibria including conjugate pairs, mathematical links between pKₐ and pH, and nucleophilic substitution mechanisms (alkaline ester hydrolysis) using curly arrow notation.

Part (a): Functional Group Geometry

Predicting bond angles and molecular shapes

✅ Correct Answers

Carboxylic acid bond angle (a): 120°
Carboxylic acid shape: Trigonal planar
Sulfonic acid bond angle (b): 104–105° (Allow 104° to 105°)
Sulfonic acid shape: Non-linear (Allow bent, V-shaped, angular)

💡 Key Knowledge (VSEPR)

Carbon atom a is bonded to 3 atoms with no lone pairs, resulting in trigonal planar geometry.
Oxygen atom b in the -OH group has 2 bonding pairs and 2 lone pairs. Electron pair repulsion gives a non-linear shape, similar to water.

❌ Common Errors & Examiner Guidance

Examiners accept marking either row-by-row or column-by-column to award the highest possible credit. For non-linear shapes, terms like bent, V-shaped, or angular are fully accepted, but terms like planar or straight are strictly ignored/penalised.

Marks available: 2 marks (Awarded per row or per column for correct angle AND shape combination).

Part (b): Acid Strength, pKₐ and Equilibria

Brønsted-Lowry theory and pH comparisons

✅ Correct Answers

1. Equilibrium Equation:

CH₃SO₂OH + H₂O ⇌ CH₃SO₂O⁻ + H₃O⁺

2. Conjugate Acid-Base Pairs Labelling:

CH₃SO₂OH (A1) paired with CH₃SO₂O⁻ (B1)
H₂O (B2) paired with H₃O⁺ (A2)

3. Explanation:

The student is correct. Methanesulfonic acid has a lower pKₐ (-1.90 vs 4.76), meaning it dissociates more extensively and is a stronger acid, producing a higher concentration of H⁺ / H₃O⁺ ions and thus a lower pH.

🧠 Exam Technique & Conjugate Pairs

Ensure double-headed equilibrium arrows ⇌ are used.
Clearly label pairs as requested: A1, B1 and A2, B2. Matching the acid with its conjugate base is essential.
You can use a mathematical calculation of pH using pKₐ = -log(Kₐ) to substantiate your answer and automatically capture full marks.

📐 Alternative Mathematical Proof for Acid Strength

For ethanoic acid: Kₐ = 10⁻⁴°⁷⁶ = 1.74 × 10⁻⁵ mol dm⁻³. [H⁺] = √(1.74 × 10⁻⁵ × 1.0) = 4.17 × 10⁻³ mol dm⁻³. pH = 2.38.
For methanesulfonic acid: Kₐ = 10⁻⁽⁻¹°⁹⁰⁾ = 79.4 mol dm⁻³. [H⁺] = √(79.4 × 1.0) = 8.91 mol dm⁻³. pH = -0.95.
Conclusion: Methanesulfonic acid yields a significantly higher [H⁺], confirming a lower pH.

Marks available: 4 marks (1 for equilibrium equation, 1 for conjugate pair labels, 1 for stating dissociation/strength difference, 1 for referencing pKₐ / Kₐ / [H⁺]).

Part (c): Mechanism of Alkaline Ester Hydrolysis

Curly arrow mechanisms

✅ Correct Answers (Curly Arrow Checklist)

Arrow 1: From lone pair on :OH⁻ oxygen to the sulfur atom ( S=O centre).
Arrow 2: From one of the sulfur-oxygen double bonds ( S=O ) to the oxygen atom, forming a single bond and a negative charge on intermediate oxygen.
Arrow 3: From the oxygen lone pair back down to reform the S=O double bond in the intermediate.
Arrow 4: From the S-OCH₃ bond breaking, moving the electron pair directly onto the oxygen of the leaving methoxide group ( ⁻OCH₃ ).

❌ Common Student Pitfalls

Starting curly arrows in empty space rather than precisely from a lone pair or a covalent bond.
Arrowheads landing haphazardly instead of pointing directly to the target atom or forming a bond between atoms.
Forgetting to show intermediate structures correctly with appropriate charges and lone pairs.

Marks available: 4 marks (Awarded based on the total number of correct curly arrows drawn across the three reaction boxes: 6 arrows correct = 4 marks, 5 arrows = 3 marks, 4 arrows = 2 marks, 3 arrows = 1 mark).

Topics

Module 2: Foundations in chemistry · Module 5: Physical chemistry and transition elements · Module 6: Organic chemistry and analysis · 5.1 Rates, equilibrium and pH · 2.2 Electrons, bonding and structure · 6.1 Aromatic compounds, carbonyls and acids

Question and mark scheme from the OCR A-Level Chemistry examination, Unified chemistry (03), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.