AQA GCSE Chemistry Chemistry Paper 2 (Higher), 2022: Question 4

✅ Correct Answer

• Crude oil is heated to vaporise the hydrocarbons. +1 Mark
• There is a temperature gradient in the fractionating column (it is hotter at the bottom and cooler at the top). +1 Mark
• The gases rise and condense at different levels. +1 Mark
• This separation happens because the fractions have different boiling points. +1 Mark

💡 Key Knowledge

Fractional Distillation:
• Small molecules: Low boiling points, highly volatile, condense at the top.
• Large molecules: High boiling points, low volatility, condense at the bottom.

🧠 Exam Technique

To get all 4 marks, your explanation must be sequential. Start with heating/evaporating, describe the column's conditions (temperature gradient), explain what the vapours do (rise and condense), and state the physical property that allows this (different boiling points).

❌ Common Errors

• Saying "crude oil is evaporated" instead of "vaporised" or "heated to vaporise".
• Confusing the gradient (e.g., claiming it is hotter at the top).
• Vaguely stating they "separate" without explaining that they condense at different levels.

✅ Correct Answer

☑️ Detergents +1 Mark
☑️ Solvents +1 Mark

💡 Key Knowledge

The petrochemical industry uses fractions from crude oil (feedstock) to produce modern synthetic materials: polymers (plastics), solvents, lubricants, detergents, and fertilisers. Alloys and ceramics are not organic petrochemical products!

📐 Step-by-Step Balancing

1. Identify Products: Complete combustion of any hydrocarbon always produces carbon dioxide ( CO₂ ) and water ( H₂O ). +1 Mark
2. Balance Carbons: There are 9 Carbons on the left, so we need 9 CO₂ on the right.
3. Balance Hydrogens: There are 20 Hydrogens on the left, so we need 10 H₂O on the right (since 10 × 2 = 20).
4. Balance Oxygens: Count the O atoms on the right: (9 × 2 in CO₂) + (10 × 1 in H₂O) = 28 O atoms. Therefore, we need 14 O₂ on the left (14 × 2 = 28). +1 Mark

❌ Common Calculation Traps

• Writing incorrect formulas: Writing H₂ or O instead of H₂O and O₂ .
• Forgetting to double-check oxygen: Students often forget that oxygen is diatomic ( O₂ ) and write 28 instead of 14.

✅ Correct Answer

• When burned, sulfur impurities react with oxygen to produce sulfur dioxide. +1 Mark
• Sulfur dioxide causes acid rain OR respiratory problems in humans. +1 Mark

💡 Key Knowledge

Sulfur burns in oxygen: S + O₂ → SO₂ .
Sulfur dioxide gas dissolves in water droplets in the atmosphere to form acid rain, which damages buildings, kills aquatic life, and damages forests.

✅ Correct Answer

• As molecular size (number of carbon atoms) increases, viscosity increases. +1 Mark
• Heavy fuel oil has larger molecules (20–40 carbons) than kerosene (11–15 carbons). +1 Mark

🧠 Exam Technique

To "justify" a prediction using data, you must:
1. State the general chemical rule/trend.
2. Explicitly refer to the data provided in the table to show how the rule applies to these specific substances.

✅ Correct Answer

Name of process: Cracking +1 Mark

Conditions: High temperature +1 Mark AND a catalyst OR steam +1 Mark

💡 Key Knowledge

Catalytic Cracking: Uses high temperature (approx. 550°C) and a zeolite catalyst.
Steam Cracking: Uses higher temperature (approx. 800°C) and steam.

✅ Correct Answer

☑️ C₇H₁₄ and C₈H₁₆ +1 Mark

💡 Key Knowledge

• Only alkenes (unsaturated hydrocarbons with C=C double bonds) decolourise orange bromine water.
• Alkenes have the general formula: CₙH₂ₙ .
• C₇H₁₄ and C₈H₁₆ fit this formula perfectly. Alkanes ( CₙH₂ₙ₊₂ ) like C₈H₁₈ do not react with bromine water.