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

This study guide teaches carbon compounds as fuels for OxfordAQA International GCSE Chemistry (9202). It follows the OxfordAQA International GCSE Chemistry (9202) specification, Version 6.2, for exams May/June 2018 onwards. It covers every statement in section 3.10.1: crude oil (3.10.1.1 a to c), hydrocarbons (3.10.1.2 a to g) and obtaining useful substances from crude oil (3.10.1.3 a to e), plus the required practical on testing for a double bond. The qualification is untiered, so all of this applies to every candidate. Either paper may test this unit.

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What this unit covers

Statement	Skill you need
3.10.1.1 a, b	Describe crude oil as a mixture made mostly of hydrocarbons
3.10.1.1 c	Explain continuous fractional distillation in a fractionating column
3.10.1.2 a, b	Use the alkane general formula CₙH₂ₙ₊₂; recognise and draw alkanes; name methane, ethane and propane
3.10.1.2 c	Describe how boiling point, viscosity and flammability change with molecule size
3.10.1.2 d	Link combustion products to the elements in a fuel and to complete or incomplete burning; link pollutants to acid rain, climate change and global dimming
3.10.1.2 e	State that combustion releases energy and oxidises carbon and hydrogen
3.10.1.2 f	Compare making ethanol from ethene and steam with fermenting sugar
3.10.1.2 g	Weigh up biofuels: renewable resources, land use, carbon footprint
3.10.1.3 a, b	Describe cracking and its products; use the alkene general formula CₙH₂ₙ
Required practical	Test an unknown hydrocarbon for a carbon–carbon double bond
3.10.1.3 c to e	Recognise and draw alkenes; name ethene and propene; bromine water test; cracking products as fuels

Mole calculations use the methods in the quantitative chemistry guide.

3.10.1.1: crude oil and fractional distillation

Crude oil is a mixture of an enormous number of compounds, not chemically joined, so a physical method can separate them.

Most of those compounds are hydrocarbons, whose molecules contain hydrogen and carbon atoms only. A compound containing any other element, such as oxygen, is not a hydrocarbon.

How continuous fractional distillation works

The aim is to split the oil into fractions. Each fraction holds molecules with a similar number of carbon atoms, so they have similar boiling points.

1. Crude oil is heated strongly so that most of it vaporises.
2. The vapour passes into the base of a tall fractionating column.
3. The column is hottest at the bottom and gets cooler towards the top. This is a temperature gradient.
4. The vapours rise. As each substance reaches a level where the temperature is below its boiling point, it condenses to a liquid.
5. Trays at different heights collect the liquids, which are piped out as separate fractions.
6. Large molecules (high boiling points) condense low down or leave at the base as a residue. Small molecules (low boiling points) leave the top as gases.
7. Oil goes in and fractions come out all the time, so the process is continuous.

The specification says you do not need the names of particular fractions or fuels. Concentrate on the process and the link between molecule size and boiling point.

3.10.1.2 a and b: alkanes

Most hydrocarbons in crude oil are alkanes. Alkanes are saturated, meaning they contain only single carbon–carbon covalent bonds. They form a homologous series with the general formula CₙH₂ₙ₊₂.

You must know three names:

Name	Molecular formula
methane	CH₄
ethane	C₂H₆
propane	C₃H₈

Each line in a displayed formula stands for one covalent bond. Propane looks like this:

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

Worked example 1: recognising alkanes

Which of these is an alkane: C₁₁H₂₄, C₈H₁₆, C₄H₁₀O?

• C₁₁H₂₄: n = 11, so 2n + 2 = 24. This fits CₙH₂ₙ₊₂, so it is an alkane.
• C₈H₁₆: 2n + 2 = 18, not 16. It fits CₙH₂ₙ instead, so it is not an alkane.
• C₄H₁₀O contains oxygen, so it is not a hydrocarbon at all.

3.10.1.2 c: properties depend on molecule size

As the number of carbon atoms in a hydrocarbon molecule increases:

Property	Trend with larger molecules
Boiling point	increases
Viscosity (how thick and slow-flowing the liquid is)	increases
Flammability (how easily it catches fire)	decreases

Larger molecules have stronger forces between them, so more energy is needed to separate them. Small molecules flow easily, ignite easily and burn with a cleaner flame, so they make good fuels. Large, viscous, hard-to-ignite molecules are less useful as fuels, which is one reason for cracking (below).

3.10.1.2 d and e: burning fuels

When a hydrocarbon burns, its carbon and hydrogen are oxidised (they combine with oxygen), and energy is released.

Complete combustion

With plenty of oxygen, the carbon becomes carbon dioxide and the hydrogen becomes water.

Worked example 2: balancing complete combustion of dodecane

Dodecane, C₁₂H₂₆, burns completely.

1. Balance carbon: 12 C gives 12CO₂.
2. Balance hydrogen: 26 H gives 13H₂O.
3. Count oxygen atoms on the right: 24 + 13 = 37, which needs 18½O₂.
4. Double everything to remove the half: 2C₁₂H₂₆ + 37O₂ → 24CO₂ + 26H₂O

Check: 24 C, 52 H and 74 O on each side.

Incomplete combustion

With too little oxygen, some carbon forms carbon monoxide (a toxic gas) or soot (carbon particles). Water still forms. For methane:

• 2CH₄ + 3O₂ → 2CO + 4H₂O
• CH₄ + O₂ → C + 2H₂O

Unburnt fuel can also escape as tiny particles.

Other products and their effects

Many fuels, including coal, contain some sulfur. When the fuel burns, the sulfur is oxidised: S + O₂ → SO₂. At the high temperatures inside engines and furnaces, nitrogen and oxygen from the air react to make oxides of nitrogen, even though the fuel contains no nitrogen.

Product	Where it comes from	Effect
Carbon dioxide	carbon in the fuel, complete combustion	an increase may result in climate change
Water vapour	hydrogen in the fuel	—
Carbon monoxide	carbon, incomplete combustion	toxic gas
Sulfur dioxide	sulfur impurity in the fuel	acid rain
Oxides of nitrogen	nitrogen in air, at high temperature	acid rain
Particulates (soot, unburnt fuel)	incomplete combustion	global dimming

To predict products, list the elements in the fuel, then ask whether oxygen was plentiful.

3.10.1.3: cracking and alkenes

Fractional distillation gives more large hydrocarbons than are needed and fewer of the smaller, more useful ones. Cracking breaks large molecules into smaller ones.

1. The hydrocarbons are heated so they vaporise.
2. The vapour is then either passed over a hot catalyst, or mixed with steam and then heated very strongly.
3. Thermal decomposition reactions break the molecules apart.

The products include smaller alkanes, some of which are useful as fuels, and alkenes.

Alkenes

Alkenes are unsaturated: some carbon–carbon bonds are double covalent bonds. Their general formula is CₙH₂ₙ. You need two names: ethene, C₂H₄, and propene, C₃H₆. In a displayed formula a double line is a double bond:

H       H
 \     /
  C = C
 /     \
H       H

That is ethene. Propene has a C=C bond and a CH₃ group on one end.

Worked example 3: balancing a cracking equation

Nonadecane, C₁₉H₄₀, is cracked into C₁₀H₂₂, propene and one other hydrocarbon. Find the other product.

• Carbon: 19 − 10 − 3 = 6
• Hydrogen: 40 − 22 − 6 = 12
• The product is C₆H₁₂, which fits CₙH₂ₙ, so it is an alkene.

Equation: C₁₉H₄₀ → C₁₀H₂₂ + C₃H₆ + C₆H₁₂

A single product can be found the same way: C₂₃H₄₈ → C₁₅H₃₂ + C₈H₁₆.

Required practical: testing for a double bond

Alkenes react with bromine water, turning it from orange to colourless. Alkanes do not react with it, so the orange colour stays.

1. Put a small volume of the unknown liquid hydrocarbon in a test tube. For a gas, bubble it through the bromine water instead.
2. Add a few drops of bromine water.
3. Stopper the tube and shake.
4. Record the colour. Colourless means a C=C double bond is present; orange means no double bond.

Run a known alkane alongside as a comparison. For the risk assessment: bromine water is harmful and irritates skin and eyes, so wear eye protection and gloves and use small volumes; hydrocarbons are flammable, so keep them away from flames; work in a well-ventilated room. The specification’s skills table lists risk assessment, collecting data, analysing data and making conclusions for this practical.

3.10.1.2 f: two ways to make ethanol

Hydration of ethene. Ethene reacts with steam:

C₂H₄ + H₂O → C₂H₅OH

Phosphoric acid is the catalyst. Typical conditions are about 300 °C and 60 to 70 atmospheres. This is a continuous process, which makes it efficient, but ethene comes from crude oil, a non-renewable resource that may run out.

Fermentation of sugar. Yeast converts sugar to ethanol and carbon dioxide:

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

Enzymes in yeast are the natural catalysts. This is a batch process. Sugar comes from plants, so it is renewable.

Feature	Hydration of ethene	Fermentation of sugar
Raw material	ethene, from crude oil (non-renewable)	sugar, from plants (renewable)
Type of process	continuous	batch
Labour	few workers once running	more labour to empty, clean and refill vessels
Rate of reaction	fast	slow
Conditions	high temperature and pressure, phosphoric acid catalyst	20 °C to 35 °C, yeast
Purity of product	pure ethanol	dilute solution of ethanol in water
Energy requirements	high, to keep temperature and pressure up	low for the reaction; extra energy to separate the ethanol

Worked example 4: fermentation masses

What maximum masses of ethanol and carbon dioxide form from 1.26 kg of glucose? (C = 12, H = 1, O = 16)

1. Mr of C₆H₁₂O₆ = 180, so moles of glucose = 1260 ÷ 180 = 7.0 mol.
2. The ratio is 1 : 2 : 2, so 14 mol of ethanol and 14 mol of CO₂ form.
3. Ethanol: 14 × 46 = 644 g. Carbon dioxide: 14 × 44 = 616 g.

Check: 644 + 616 = 1260 g, so mass is conserved.

Hydration works the same way. One mole of ethene (Mr 28) gives one mole of ethanol (Mr 46), so 56 tonnes of ethene gives 56 × 46 ÷ 28 = 92 tonnes of ethanol. No second product forms, so every atom ends up in the ethanol.

3.10.1.2 g: biofuels

Biofuels, including biodiesel and ethanol, are made from plant material. They are possible alternatives to hydrocarbon fuels. Ethanol for use as a biofuel comes from a dilute solution made by fermenting plant materials at 20 °C to 35 °C.

Factor	Benefit	Disadvantage
Renewable resources	crops can be regrown each season	supply depends on harvests and weather
Land use	can use land unsuited to food crops	land for fuel crops may replace food crops or forest
Carbon footprint	plants absorb CO₂ as they grow, offsetting some of the CO₂ released on burning	farming, fertiliser, transport and distillation all use energy, so the footprint is not zero

A balanced judgement uses both columns.

Common errors

• Calling crude oil a compound. It is a mixture.
• Saying fractions are separated by “different melting points”. Fractional distillation uses boiling points.
• Writing that the top of the column is hottest.
• Leaving water out of incomplete combustion products.
• Writing “clear” or “discoloured” instead of colourless for bromine water.
• Calling fermentation continuous or hydration a batch process.
• Calling biofuels carbon neutral with no qualification.

Next: condense this with the revision notes, then work through the practice questions. Alkenes from cracking lead on to synthetic and naturally occurring polymers.

Official syllabus

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