AQA GCSE Chemistry Chemistry Paper 2 (Higher), November 2020: Question 7

12 marks · Standard Demand difficulty · Short Answer

Explain the properties and reactions of carboxylic acids, including their pH, reaction rates with zinc carbonate, and ester formation.

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Question

Question text

07 This question is about carboxylic acids.

Carboxylic acids belong to a homologous series.

Table 3 shows information about the first three carboxylic acids in this homologous

series.

Table 3

Name Formula pH of a 0.01 mol/dm3 solution

Methanoic acid 2.91

Ethanoic acid CH3COOH 3.39

CH3CH2COOH 3.44

07.1 Complete Table 3.

[2 marks]

07.2 Ethanoic acid ionises in water.

The equation for the reaction is:

CH COOH(aq) ⇌ CH COO–(aq) + H+(aq)

Explain how the equation shows that ethanoic acid is a weak acid.

[2 marks]

07.3 A student adds a solution of ethanoic acid to zinc carbonate in an open flask on

a balance.

*18* Explain what happens to the mass of the flask and its contents during the reaction.

[3 marks]

07.4 The student compares the rates of the reaction of zinc carbonate with:

• 0.01 mol/dm3 methanoic acid

• 0.01 mol/dm3 ethanoic acid.

The rate of the reaction with methanoic acid is greater than the rate of the reaction

with ethanoic acid.

Explain why.

You should refer to ions in your answer.

Use Table 3.

[3 marks]

Ethanoic acid reacts with ethanol to produce an ester.

*0197.*5 Give the name of the ester produced when ethanoic acid reacts with ethanol.

[1 mark]

07.6 Hexanedioic acid and ethanediol join together to produce a polyester.

Ethanoic acid and ethanol join together in the same way to produce an ester.

Which is the displayed structural formula of the ester produced when ethanoic acid

reacts with ethanol?

[1 mark]

Tick ( ) one box.

Mark scheme

Show the mark scheme

Question 7

AO /

Question Answers Extra information Mark

Spec. Ref.

07.1 HCOOH allow HCO2H 1 AO1

4.7.2.4

propanoic acid 1

incomplete / partial ionisation allow incomplete / partial 1 AO3

07.2 dissociation 4.7.2.4

(because) reaction is reversible allow (because) reaction is in 1

equilibrium

07.3 mass (of flask and contents) 1 AO1

decreases

(because) carbon dioxide is 1 AO2

produced allow 1 mark for the gas

produced escapes (from the

(and) carbon dioxide escapes flask) 1 AO2

(from the flask)

4.3.1.3

4.7.2.4

allow converse argument for

07.4 ethanoic acid AO2

(0.01 mol/dm3) methanoic acid allow (0.01 mol/dm3) methanoic 1

has a lower pH acid is a stronger acid AO2

(so 0.01 mol/dm3) methanoic 1

acid has a higher concentration

of hydrogen ions AO3

(therefore) more collisions per 1 4.6.1.2

unit time 4.6.1.3

4.7.2.4

AO1

07.5 ethyl ethanoate 1 4.7.2.4

Question 7 (continued)

16 AO /

Spec. Ref.

07.6 AO2

1 4.7.2.4

4.7.3.2

Total 12

How to answer it

Carboxylic Acids, Weak Acids & Esters

What this question tests

This question assesses core understanding of organic chemistry and acids from Topic 7 (Organic Chemistry) and Topic 4/6 (Chemical Changes & Rates of Reaction):

Recalling carboxylic acid formulas, names, and homologous series conventions.
Explaining the difference between strong and weak acids using reversible ionisation equations.
Applying the law of conservation of mass to open gas-producing reactions.
Connecting pH values, H⁺ ion concentration, collision theory, and reaction rates.
Naming simple esters and identifying ester functional groups (-COO-) in displayed formulas.

Question 07.1 • 2 Marks

Naming & Formulas of Carboxylic Acids

Completing Table 3 for methanoic and propanoic acid

✅ Correct Answers

Formula for methanoic acid: HCOOH (or HCO₂H ) [1 mark]
Name for CH₃CH₂COOH: propanoic acid [1 mark]

💡 Key Knowledge

Carboxylic acids contain the functional group -COOH . The prefixes follow standard carbon numbers:

Meth- = 1 Carbon: HCOOH
Eth- = 2 Carbons: CH₃COOH
Prop- = 3 Carbons: CH₃CH₂COOH
But- = 4 Carbons: CH₃CH₂CH₂COOH

Marking Insight: Ensure spelling is correct: write propanoic, not "propanol" or "propene". Capitalisation matters for chemical symbols ( HCOOH , not hcooh ).

Question 07.2 • 2 Marks

Weak Acid Ionisation

CH₃COOH(aq) ⇌ CH₃COO⁻(aq) + H⁺(aq)

✅ Correct Answer

Ethanoic acid only partially / incompletely ionises (or dissociates) in water [1 mark]
Because the reaction is reversible (shown by the ⇌ sign / reaction reaches equilibrium) [1 mark]

❌ Common Errors

Confusing weak with dilute. "Weak" refers to incomplete ionisation; "dilute" refers to low solute concentration per dm³.
Stating "it has a low pH" or "it produces H⁺ ions" without explicitly identifying the reversible symbol ( ⇌ ) or incomplete nature.

Examiner Tip: When an exam question asks "how the equation shows...", look closely at the reaction arrow. The reversible arrow ⇌ indicates that both reactants and products exist together, meaning complete ionisation does not take place.

Question 07.3 • 3 Marks

Acid + Carbonate in an Open System

Explaining the change in balance reading

✅ Correct Answer (3 Marks Breakdown)

The mass of the flask and contents decreases [1 mark]
Because carbon dioxide gas is produced [1 mark]
And the carbon dioxide escapes from the open flask into the surroundings [1 mark]

🧠 Exam Technique

Acid + Metal Carbonate → Salt + Water + Carbon Dioxide.

Always state three distinct points when explaining balance loss in open vessels:

Direction of change (decreases).
Name of the specific gas produced (CO₂).
What the gas actually does (escapes into the atmosphere).

Mark Scheme Note: You only get 1 of the explanation marks if you merely say "gas escapes" without identifying the gas as carbon dioxide.

Question 07.4 • 3 Marks

Rate of Reaction & Ion Concentration

Methanoic acid (pH 2.91) vs Ethanoic acid (pH 3.39)

✅ 3-Step Linked Explanation

pH comparison: Methanoic acid has a lower pH (2.91 vs 3.39) / is a stronger acid [1 mark]
Ion concentration: Methanoic acid has a higher concentration of hydrogen ions (H⁺) [1 mark]
Collision rate: Therefore, there are more frequent collisions (more collisions per unit time / per second) with zinc carbonate [1 mark]

❌ The Classic Collision Trap

Never write just "there are more collisions". You will lose the mark!

You must specify rate: "more collisions per unit time", "more collisions per second", or "more frequent collisions".

Alternative: A fully correct converse argument comparing ethanoic acid (higher pH → lower H⁺ concentration → fewer collisions per unit time) is also awarded full marks.

Questions 07.5 & 07.6 • 2 Marks Total

Ester Formation & Structure

💡 Question 07.5: Ester Naming (1 Mark)

Reactants: ethanol + ethanoic acid

Rule: Alcohol gives the first word (-yl), carboxylic acid gives the second (-oate).

Answer: ethyl ethanoate

🧠 Question 07.6: Structural Formula (1 Mark)

Looking for the ester functional group -COO- (a carbon double-bonded to one oxygen and single-bonded to another oxygen).

Option 1: Alcohol (butan-2-ol) ❌
Option 2: Ether (C-O-C) ❌
Option 3: Ketone (C=O only) ❌
Option 4 (Bottom box): Contains the C(=O)-O- ester linkage! ✅

Displayed Formula of Ethyl Ethanoate:

H O H H | || | | H — C — C — O — C — C — H | | | H H H

Tick the fourth (bottom) box.

Topics

Chemistry · C3: Quantitative Chemistry · C4: Chemical Changes · C6: The Rate and Extent of Chemical Change · C7: Organic Chemistry

Question and mark scheme from the AQA GCSE Chemistry examination, Chemistry Paper 2 (Higher), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.