AQA GCSE Chemistry Chemistry Paper 2 (Foundation), June 2024: Question 1

✅ Correct Answers

01.1:

• carbon [1 mark]
• hydrogen [1 mark]

01.2: Tick the box for 20 [1 mark]

💡 Key Knowledge

A hydrocarbon is a compound consisting of hydrogen and carbon only.

In a molecular formula like C₆H₁₄ :

• 6 carbon atoms
• 14 hydrogen atoms
• Total atoms = 6 + 14 = 20 atoms

🧠 Exam Technique: "Name" vs "Symbol"

Always read command words carefully. The question states "Name the two elements", so you must write their full English names.

❌ Common Errors

• Writing "oxygen" or "water" as an element in a hydrocarbon.
• Selecting 14 or 6 in 01.2 by confusing total atoms with the count of an individual element.
• Selecting 2 by mistaking the number of different elements for the total number of atoms.

✅ Correct Answers

01.3: CₙH₂ₙ₊₂ [1 mark]

01.4: Alkanes [1 mark]

01.5: Complete all missing single bonds in Figure 1 [1 mark]:

• Draw horizontal single lines ( — ) between all 6 carbon atoms (5 C—C bonds).
• Draw the single bond between the end H and the first C, and the last C and the right-hand H (2 C—H end bonds).

💡 Alkanes: Saturated Hydrocarbons

Alkanes are saturated hydrocarbons containing only single covalent bonds:

• General formula: CₙH₂ₙ₊₂
• If n = 6: C₆H₍₂ ₓ ₆₎ ₊ ₂ = C₆H₁₄ (hexane)
• Every carbon atom must form exactly 4 covalent bonds.
• Every hydrogen atom forms exactly 1 covalent bond.

🧠 Exam Technique: Spotting Missing Bonds

In Figure 1, vertical bonds between C and H were provided, but the backbone was broken:

Make sure you connect every carbon atom together with a single horizontal line, and do not forget the two outer hydrogens at the ends.

❌ Common Errors

• Confusing alkanes (CₙH₂ₙ₊₂) with alkenes (CₙH₂ₙ).
• Drawing double lines between carbons (which would give carbons 5 bonds).
• Forgetting to draw the two bonds to the terminal hydrogen atoms on the far left and far right.

✅ Correct Answers

01.6: fuel (allow fossil fuel) [1 mark]

01.7:

• Solid pollutant: soot [1 mark]
• Gaseous pollutant: carbon monoxide [1 mark]

💡 Incomplete Combustion

When hydrocarbons burn with an insufficient supply of oxygen:

• Soot (carbon particulates): Tiny solid carbon particles that cause global dimming and respiratory issues.
• Carbon monoxide (CO): A toxic, colourless, and odourless gas that binds irreversibly to haemoglobin.

🧠 Look at the State of Matter

The question provides strong clues:

• "solid atmospheric pollutant" → Soot (pure carbon solid).
• "gaseous pollutant formed during incomplete combustion" → Carbon monoxide.

❌ Common Errors

• Selecting coal as the solid pollutant (coal is an unburned rock fuel, not a combustion product).
• Selecting sulfur dioxide for incomplete combustion (sulfur is not present in pure C₆H₁₄).
• Confusing carbon monoxide with carbon dioxide (carbon dioxide forms during complete combustion).

✅ Correct Answer

C₆H₁₂ [1 mark]

C₁₂H₂₆ → C₆H₁₄ + C₆H₁₂

📐 Calculation: Conservation of Mass

Atoms cannot be created or destroyed:

1. Carbon atoms: 12 on LHS → 6 in C₆H₁₄, so missing = 12 − 6 = 6
2. Hydrogen atoms: 26 on LHS → 14 in C₆H₁₄, so missing = 26 − 14 = 12
3. Formula: C₆H₁₂

💡 Cracking Rule

When a long-chain alkane is cracked, it breaks down into:

Long alkane → Shorter alkane + Alkene

Here, dodecane (C₁₂H₂₆) forms hexane (alkane, C₆H₁₄) and hexene (alkene, C₆H₁₂).

❌ Common Errors

• Arithmetic slip: writing C₆H₁₄ again or C₆H₁₀.
• Writing numbers as normal text instead of subscripts on the question line.

✅ Correct Answer & Marks

• Bar height: Draw a neat bar for Carbon up to exactly 84% [1 mark]
• X-axis labels: Both bars correctly labelled on the x-axis: Carbon / C under the 84% bar, and Hydrogen / H under the existing 16% bar [1 mark]

🧠 Exam Technique: Reading the Scale

• Look at the y-axis: between 80 and 90 there are 5 small grid squares.
• Each small square = (90 − 80) ÷ 5 = 2%.
• 84% is exactly 2 small squares above the 80 line.
• The mark scheme allows a tolerance of ± ½ small square.
• Always use a ruler and pencil to draw bar charts!

❌ Common Errors

• Plotting 84% at 4 small squares above 80 (assuming 1 square = 1%).
• Forgetting to label the existing bar (the bar drawn at 16% must be labelled H / Hydrogen).
• Drawing bars with unequal widths.