OxfordAQA International GCSE Chemistry: Carbon compounds as fuels (9202) – Revision Notes

These revision notes condense carbon compounds as fuels for OxfordAQA International GCSE Chemistry (9202). They are aligned to the OxfordAQA International GCSE Chemistry (9202) specification, Version 6.2, for exams May/June 2018 onwards, section 3.10.1 (3.10.1.1 crude oil, 3.10.1.2 hydrocarbons, 3.10.1.3 obtaining useful substances from crude oil) and the required practical on testing for a double bond. The qualification is untiered, so every point applies to every candidate. The study guide has the full explanations and worked examples.

Links: practice questions · course hub · printable checklist · free diagnostics

Key definitions

Term	Meaning
Hydrocarbon	a molecule made of hydrogen and carbon atoms only
Crude oil	a mixture of a very large number of compounds, mostly hydrocarbons
Fraction	a group of molecules with a similar number of carbon atoms, so similar boiling points
Saturated	all carbon–carbon bonds are single covalent bonds
Unsaturated	some carbon–carbon bonds are double covalent bonds
Alkane	saturated hydrocarbon, CₙH₂ₙ₊₂
Alkene	unsaturated hydrocarbon with one C=C, CₙH₂ₙ
Cracking	thermal decomposition of large hydrocarbons into smaller, more useful molecules
Biofuel	a fuel produced from plant material, such as biodiesel or ethanol

3.10.1.1: fractional distillation in steps

1. Heat the crude oil so it evaporates.
2. Feed the vapour into the base of the column.
3. The temperature of the column falls from base to top.
4. Each vapour rises until it reaches a level cooler than its boiling point, then condenses.
5. Liquids are tapped off at different heights as fractions.
6. Small molecules: low boiling point, leave near the top. Large molecules: high boiling point, leave near the bottom.
7. Crude oil enters and fractions leave without stopping: a continuous process.

You are not asked to name particular fractions or fuels.

3.10.1.2 a and b: alkanes at a glance

Name	Formula	Structure
methane	CH₄	one carbon, four C–H bonds
ethane	C₂H₆	H₃C–CH₃
propane	C₃H₈	H₃C–CH₂–CH₃

Test any formula: if H = 2 × C + 2 and there is no other element, it is an alkane. Ethane in displayed form:

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

3.10.1.2 c: size and properties

Bigger molecule → higher boiling point → more viscous → less flammable.

Smaller molecule → lower boiling point → runnier → ignites more easily.

These trends explain why short-chain hydrocarbons are the more useful fuels.

3.10.1.2 d and e: combustion

Combustion releases energy. Carbon and hydrogen in the fuel are oxidised.

Complete (oxygen plentiful): carbon → CO₂, hydrogen → H₂O.

2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O

Incomplete (oxygen short): carbon → CO or C (soot); hydrogen still → H₂O.

2C₂H₆ + 5O₂ → 4CO + 6H₂O

Method in steps: balancing a combustion equation

1. Write the fuel, O₂, and the products.
2. Balance C first, then H.
3. Count O atoms on the right and halve to get the O₂ needed.
4. If you have a half, double every coefficient.
5. Check each element on both sides.

Pollutants: cause and effect

Pollutant	Origin	Problem
SO₂	sulfur impurity in the fuel	acid rain
NOₓ (oxides of nitrogen)	air reacting at high temperature	acid rain
CO₂	carbon in the fuel	increase may cause climate change
Particulates	soot and unburnt fuel	global dimming
CO	incomplete combustion	toxic

Link products to elements: a product containing an element (other than oxygen or nitrogen from the air) means the fuel contained that element.

Small worked reminder: predicting products

A sample of coal contains carbon, hydrogen and a little sulfur. It burns in a furnace where the air supply is poor.

• Carbon: CO₂, plus CO and soot because oxygen is short.
• Hydrogen: water vapour.
• Sulfur: sulfur dioxide.
• High temperature: oxides of nitrogen from the air.
• Poor burning: particulates, including unburnt coal dust.

Every product traces back either to an element in the fuel or to the conditions of burning.

3.10.1.2 f: making ethanol

	Ethene + steam	Fermentation
Equation	C₂H₄ + H₂O → C₂H₅OH	C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
Raw material	from crude oil, non-renewable	sugar, renewable
Catalyst	phosphoric acid	enzymes in yeast
Process	continuous, efficient	batch
Labour	low	higher
Rate	fast	slow
Conditions	high temperature (about 300 °C) and high pressure	20 °C to 35 °C
Purity	pure ethanol	dilute ethanol in water
Energy	high	low for the reaction; more if the ethanol is separated

Small worked reminder

Fermentation of 90 g of glucose (Mr 180): 0.50 mol glucose → 1.0 mol ethanol → 1.0 × 46 = 46 g ethanol.

3.10.1.2 g: biofuels

• Renewable: plant crops regrow; crude oil does not.
• Land use: fuel crops can take land from food crops or lead to forest being cleared.
• Carbon footprint: growing plants absorb CO₂, offsetting some of what is released when the fuel burns. Farming, fertiliser, processing and transport add emissions, so the footprint is reduced, not removed.
• Ethanol for biofuel is made by fermenting plant material at 20 °C to 35 °C, giving a dilute solution.

3.10.1.3: cracking

• Heat the large hydrocarbons to vaporise them.
• Then either pass the vapour over a hot catalyst, or mix it with steam and heat to a very high temperature.
• Thermal decomposition gives smaller alkanes (some useful as fuels) and alkenes.

Method in steps: completing a cracking equation

1. Add up carbons on each side; the missing product takes the difference.
2. Do the same for hydrogens.
3. Check the product against CₙH₂ₙ (alkene) or CₙH₂ₙ₊₂ (alkane).

Example: C₁₃H₂₈ → C₇H₁₆ + 2C₃H₆. Left: 13 C, 28 H. Right: 7 + 6 = 13 C; 16 + 12 = 28 H.

Alkenes

Ethene C₂H₄ and propene C₃H₆ are the two names you need. Propene in displayed form:

    H   H   H
    |   |   |
H — C — C = C
    |       |
    H       H

Each carbon still makes four bonds; the double line counts as two.

Required practical: double bond test

• Add bromine water to the hydrocarbon and shake.
• Alkene: orange → colourless.
• Alkane: stays orange.
• Safety: eye protection, gloves, small volumes, good ventilation, no flames near hydrocarbons.
• For a gaseous sample, bubble the gas through the bromine water instead of adding the liquid.
• Run a known alkane alongside, so you can compare colours directly.
• Report what you see, then what it means: “the bromine water turned from orange to colourless, so the hydrocarbon contains a C=C double bond”.

Must-know distinctions

• Mixture vs compound: crude oil is a mixture; each hydrocarbon in it is a compound.
• Saturated vs unsaturated: only single C–C bonds vs at least one C=C.
• Fractional distillation vs cracking: physical separation by boiling point vs a chemical reaction that breaks molecules.
• Continuous vs batch: hydration runs non-stop; fermentation is done in loads.
• Acid rain vs global dimming vs climate change: SO₂ and NOₓ; particulates; CO₂.

Quick self-test

1. Write the formula of the alkane containing 20 carbon atoms.
2. Give the molecular formula of the alkene with 9 carbon atoms.
3. Is C₆H₁₄ saturated or unsaturated?
4. Why does a fractionating column have a temperature gradient?
5. Which has the higher viscosity, a hydrocarbon with 8 carbons or one with 18?
6. Complete: C₁₄H₃₀ → C₈H₁₈ + ……
7. Balance: C₃H₈ + …O₂ → …CO₂ + …H₂O
8. A fuel burns to give carbon dioxide, water and sulfur dioxide. Which elements did the fuel contain?
9. Why do oxides of nitrogen form in a car engine?
10. State the catalyst for hydration of ethene and the catalyst for fermentation.
11. What mass of ethanol forms when 0.25 mol of glucose ferments completely?

Answers

1. C₂₀H₄₂
2. C₉H₁₈
3. Saturated (it fits CₙH₂ₙ₊₂).
4. So that each substance condenses at a different height, according to its boiling point.
5. The one with 18 carbons.
6. C₆H₁₂
7. C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
8. Carbon, hydrogen and sulfur (it may also contain oxygen, but you cannot tell from these products alone).
9. The engine runs hot, and at that temperature atmospheric nitrogen combines with oxygen.
10. Phosphoric acid; enzymes in yeast.
11. 0.50 mol ethanol × 46 = 23 g.

Where marks are usually lost

• Saying crude oil is “made of hydrocarbons” without saying it is a mixture.
• Describing the column as hottest at the top, or saying fractions “evaporate” at each level instead of condensing.
• Writing a trend for only one property when the question asks about boiling point, viscosity and flammability.
• Forgetting water as a product of incomplete combustion.
• Linking sulfur dioxide to climate change, or carbon dioxide to acid rain.
• Saying the fuel contains nitrogen to explain oxides of nitrogen.
• Writing “clear” for the bromine water result instead of colourless.
• Giving only the advantages of biofuels when the question asks you to evaluate.
• Leaving out the type of process (batch or continuous) in an ethanol comparison.
• Cracking equations that balance carbon but not hydrogen.

Official syllabus

OxfordAQA International GCSE Chemistry (9202) specification, Version 6.2, for exams May/June 2018 onwards, published by OxfordAQA. These notes cover section 3.10.1 Carbon compounds as fuels and the required practical on testing for a double bond.