AQA GCSE Combined Science: Trilogy Chemistry Paper 1 (Higher), 2022: Question 6

✅ Correct answer

• Show one shared pair of electrons in the overlap.
• Show 6 non-bonding electrons on the outer shell of chlorine.
• Use dots and crosses in any sensible way, but only outer-shell electrons.

💡 Key knowledge

• Hydrogen has 1 outer electron.
• Chlorine has 7 outer electrons.
• They form a single covalent bond by sharing one pair of electrons.
• Chlorine keeps 3 lone pairs after bonding.

🧠 Exam technique

• Draw the overlap clearly.
• Put one electron from each atom in the shared pair.
• Make sure hydrogen has no extra electrons around it.
• Ignore inner-shell electrons completely.

❌ Common errors

• Putting extra electrons on hydrogen.
• Showing two shared pairs instead of one.
• Forgetting the 6 non-bonding electrons on chlorine.
• Drawing inner-shell electrons when the question says only outer shells.

✅ Correct answer

It completely ionises in aqueous solution.

💡 Key knowledge

• “Strong” means all acid particles split into ions in water.
• For hydrochloric acid: HCl(aq) → H⁺(aq) + Cl⁻(aq)
• Do not confuse strong with concentrated.

❌ Common errors

• Saying it is “very acidic” — that is too vague.
• Describing a strong acid as one with a high concentration.
• Saying it “partially ionises” — that is a weak acid.

✅ Correct answer

• Magnesium will fizz / effervesce as hydrogen gas is made, or the magnesium disappears.
• It reacts at a greater rate with a strong acid than with a weak acid.

💡 Key knowledge

• Acids react with magnesium to produce hydrogen.
• Stronger acids have more H⁺ ions in solution, so the reaction is faster.
• Same concentration means the comparison is about acid strength, not amount of acid dissolved.

🧠 Exam technique

• To gain full marks, give a visible observation and a comparison.
• A strong answer can mention faster fizzing, faster disappearance of magnesium, or greater temperature rise.

❌ Common errors

• Saying only “it reacts” without saying how.
• Writing about temperature only, unless you compare rates properly.
• Mixing up strong and concentrated.

✅ Correct answer

pH decreases by 2.

💡 Key knowledge

• pH is a logarithmic scale.
• A 100 times increase in hydrogen ion concentration means pH changes by 2 units.
• Because more H⁺ ions means a lower pH.

📐 Calculation idea

1. A factor of 10 change in concentration changes pH by 1.
2. A factor of 100 = 10 × 10, so pH changes by 2.
3. Because the concentration increases, the pH goes down.

❌ Common errors

• Saying pH increases — the question says concentration increases, so pH must decrease.
• Giving the answer as just “2” without saying decreases by 2.
• Forgetting that pH is logarithmic, not linear.

📐 Step-by-step calculation

1. Work out bonds broken:
   4 × H–C = 4 × 413 = 1652
   1 × C=C = X
   1 × H–Cl = 431
   Total bonds broken = 2083 + X
2. Work out bonds made:
   1 × C–C = 346
   1 × C–Cl = 339
   5 × H–C = 5 × 413 = 2065
   Total bonds made = 2750
3. Use the energy change:
   The reaction is exothermic, and energy released forming new bonds is 56 kJ/mol greater than the energy needed to break existing bonds.
4. Set up the relationship:
   2750 = (2083 + X) + 56
   or
   56 = 2750 - (2083 + X)
5. Solve:
   2083 + X = 2694
   X = 611 kJ/mol

✅ Correct answer

X = 611 kJ/mol

🧠 Exam technique

• Always count the bonds broken and bonds made separately.
• Check the reaction carefully so you include the correct number of each bond.
• Use the given “56 kJ/mol greater” statement exactly as the mark scheme expects.
• Include the correct unit: kJ/mol.

❌ Common traps

• Forgetting that ethene has 4 C–H bonds.
• Missing the C–Cl bond in chloroethane.
• Using the wrong sign for an exothermic reaction.
• Not showing the working, which can lose method marks.
• Giving the answer without the unit kJ/mol.

Hydrogen chloride bonding

One shared pair of electrons, plus 3 lone pairs on chlorine.

Strong acid

Completely ionises in aqueous solution.

Acid reactions

Stronger acids react faster with magnesium because they produce more H⁺ ions.

pH and concentration

100× concentration increase means pH decreases by 2.

Bond energies

Break bonds, make bonds, then use the energy change to find the missing bond energy.