AQA GCSE Combined Science: Trilogy Chemistry Paper 1 (Higher), June 2025: Question 5

9 marks · Standard Demand difficulty · Short Answer

Answer questions on acids, state symbols, covalent dot-and-cross bonding in HOCl, pH and hydrogen ion concentration, weak versus strong acids, and bond energy calculations.

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Question text

05 This question is about hydrogen chloride (HCl) and hydrogen chlorate (HOCl).

Hydrogen chloride and hydrogen chlorate both form acids when dissolved in water.

05.1 Hydrogen chloride gas dissolves in water to form hydrochloric acid.

Give the state symbols for:

• hydrogen chloride

• water

• hydrochloric acid.

[1 mark]

Hydrogen chloride

Water

Hydrochloric acid

05.2 A solution of hydrochloric acid has:

• a pH of 2.5

• a concentration of H+ ions of 3.16 × 10−3 mol/dm3.

What is the concentration of H+ ions in a solution of hydrochloric acid that has

a pH of 3.5?

[1 mark]

Concentration =19 mol/dm3

05.3 Figure 5 shows the outer shells in one molecule of hydrogen chlorate (HOCl).

Complete the dot and cross diagram to show the electrons in the outer shells.

[2 marks]

Figure 5

05.4 Hydrogen chlorate dissolves in water to form a weak acid.

Explain why a weak acid has a higher pH than a strong acid of

the same concentration.

[2 marks]

05.5 Hydrogen chloride is produced in the reaction between methane and chlorine.

Figure 6 represents the equation for the reaction.

Figure 6

Table 4 shows the bond energies.

Table 4

Bond C — H Cl — Cl C — Cl H — Cl

Bond energy in kJ/mol 413 243 346 432

Calculate the overall energy change for the reaction.

[3 marks]

Overall energy change = kJ/mol

Mark scheme

Show the mark scheme

Question 5

AO /

Question Answers Extra information Mark

Spec. Ref.

05.1 (hydrogen chloride) g / (g) 1 AO2

and 5.2.2.2

(water) l / (l)

and

(hydrochloric acid) aq / (aq)

AO /

Spec. Ref.

05.2 3.16 × 10−4 (mol/dm3) 1 AO2

5.4.2.5

AO /

Spec. Ref.

05.3 allow any combination of AO2

x, , e(−), ⚫ for electrons 5.2.1.1

5.2.1.4

1 pair of electrons in both

overlaps

4 non-bonding electrons on do not accept non-bonding 1

oxygen atom and 6 non-bonding electrons on hydrogen

electrons on chlorine atom

an answer of

– scores 2 marks – 8464/C/1H –

AO /

Spec. Ref.

05.4 a weak acid partially ionises in 1 AO1

an aqueous solution 5.4.2.4

5.4.2.5

(so) produces a lower 1

concentration of hydrogen ions

AO /

Spec. Ref.

05.5 (bonds broken = (4 × 413) + 243 1 AO2

=) 1895 5.5.1.3

(bonds made =

(3 × 413) + 346 + 432 =)

2017 1

(energy change = 1895 − 2017 allow correct use of an

=) (−) 122 (kJ/mol) incorrectly determined value for 1

bonds broken and / or bonds

made

alternative approach

(bonds broken = 413 + 243 =)

656 (1)

(bonds made = 346 + 432 =)

778 (1)

(energy change = 656 − 778 =)

(−) 122 (kJ/mol) (1)

allow correct use of an

incorrectly determined value for

bonds broken and / or bonds

made

Total Question 5 9

How to answer it

Acids, Covalent Bonding, and Bond Energy Calculations

📋 What This Question Tests

State Symbols: Identifying the correct physical states for pure substances and aqueous solutions.
Logarithmic pH Scale: Calculating changes in hydrogen ion concentration [H⁺] when pH changes.
Covalent Bonding: Drawing accurate dot-and-cross diagrams with shared pairs and lone pairs.
Acid Strength vs pH: Explaining the link between degree of ionisation and resulting pH.
Bond Energy Calculations: Calculating the overall enthalpy change (ΔH) using reactant and product bonds.

Question 05.1 • 1 Mark

State Symbols for Hydrogen Chloride Dissolving

Identifying states of matter in an acid-forming reaction

✅ Correct Answer

All three state symbols must be correct for 1 mark:

Hydrogen chloride: g or (g)
Water: l or (l)
Hydrochloric acid: aq or (aq)

❌ Common Errors

Writing (aq) for pure water instead of (l) . Water is the liquid solvent, not a solution dissolved in water.
Assuming hydrogen chloride gas is already (aq) before it dissolves.
Missing out one of the three blanks—all three are required for the single mark.

Question 05.2 • 1 Mark

Hydrogen Ion Concentration & pH Scale

Calculating [H⁺] from a 1-unit increase in pH

✅ Correct Answer

3.16 × 10⁻⁴ mol/dm³ (or 0.000316 mol/dm³)

[1 mark] for the correct numerical value.

📐 Step-by-Step Relationship

Identify the pH change: The pH increases from 2.5 to 3.5 (+1 pH unit).
Apply the rule: An increase of 1 on the pH scale corresponds to a 10-fold decrease (division by 10) in H⁺ concentration.
Calculate: (3.16 × 10⁻³) ÷ 10 = 3.16 × 10⁻⁴ mol/dm³

🧠 Exam Technique

Remember: Higher pH means lower acid concentration! Many students multiply by 10 instead of dividing because the pH number goes up.

Question 05.3 • 2 Marks

Dot and Cross Diagram: HOCl

Completing the outer electron shells of hydrogen chlorate

✅ Correct Answer & Diagram Details

1 mark: Exactly 1 shared pair of electrons in the H–O overlap AND 1 shared pair in the O–Cl overlap.
1 mark: 4 non-bonding electrons (2 lone pairs) on oxygen AND 6 non-bonding electrons (3 lone pairs) on chlorine.

Examiner Note: Any combination of dots (•), crosses (×), or circles (o) is acceptable.

💡 Shell Target Check

Hydrogen (H): Needs 2 outer electrons (complete with 1 shared pair).
Oxygen (O): Needs 8 outer electrons (2 bonding pairs + 4 non-bonding electrons).
Chlorine (Cl): Needs 8 outer electrons (1 bonding pair + 6 non-bonding electrons).

❌ Common Errors

Adding extra lone electrons onto Hydrogen. Hydrogen can only hold a maximum of 2 electrons total!
Forgetting the non-bonding electrons on chlorine or oxygen and only filling in the overlaps.

Question 05.4 • 2 Marks

Weak vs Strong Acids at Equal Concentrations

Explaining why weak acids have a higher pH

✅ Correct Answer (Mark Breakdown)

Mark 1: A weak acid only partially ionises (dissociates) in aqueous solution.
Mark 2: (So it) produces a lower concentration of hydrogen ions (H⁺) than a strong acid.

🧠 Exam Technique: Two-Part Reasoning

Always connect degree of ionisation to ion concentration to pH:

Partial ionisation → Lower [H⁺] → Higher pH

Do not confuse strength (degree of ionisation) with concentration (amount of solute dissolved per unit volume).

❌ Common Misconceptions

Saying "weak acids are less concentrated" — the question states they are at the same concentration.
Confusing pH direction: forgetting that a higher pH means a weaker / less acidic solution.

Question 05.5 • 3 Marks

Bond Energy Calculation

Reaction: CH₄ + Cl₂ → CH₃Cl + HCl

Bond	C – H	Cl – Cl	C – Cl	H – Cl
Bond energy (kJ/mol)	413	243	346	432

📐 Full Method (Step-by-Step)

Bonds Broken (Reactants):
4 × (C–H) = 4 × 413 = 1652
1 × (Cl–Cl) = 243
Total broken = 1652 + 243 = 1895 kJ/mol [1 mark]
Bonds Made (Products):
3 × (C–H) = 3 × 413 = 1239
1 × (C–Cl) = 346
1 × (H–Cl) = 432
Total made = 1239 + 346 + 432 = 2017 kJ/mol [1 mark]
Overall Energy Change:
ΔH = Bonds Broken − Bonds Made
ΔH = 1895 − 2017 = −122 kJ/mol [1 mark]

💡 Alternative Shortcut

Only calculate the bonds that actually break and form:

Broken: 1 × (C–H) + 1 × (Cl–Cl) = 413 + 243 = 656 kJ/mol
Made: 1 × (C–Cl) + 1 × (H–Cl) = 346 + 432 = 778 kJ/mol
ΔH: 656 − 778 = −122 kJ/mol

Both methods achieve full marks!

❌ Common Errors

Sign Error: Doing Bonds Made − Bonds Broken, giving +122 instead of −122. (Note: The mark scheme allows 122 or −122, but a minus sign correctly identifies it as exothermic).
Miscounting bonds: Counting 4 C–H bonds on both sides instead of noticing only 3 remain in CH₃Cl.
Arithmetic slips: Always re-enter numbers into your calculator twice.

Topics

Chemistry · C2: Bonding, Structure and the Properties of Matter · C4: Chemical Changes · C5: Energy Changes

Question and mark scheme from the AQA GCSE Combined Science: Trilogy examination, Chemistry Paper 1 (Higher), June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.