3.3.2 Alkanes for A-Level Chemistry (AQA)

What type of hydrocarbons are alkanes classified as?

Saturated hydrocarbons

They contain only single carbon–carbon bonds.

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What natural resource is the main source of alkanes used as fuels?

Crude oil

Also called petroleum, it is a complex mixture of hydrocarbons.

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Fill in the blank:

Crude oil is separated into fractions by _______ distillation.

fractional

This process separates hydrocarbons based on boiling points.

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True or False:

Alkanes contain carbon–carbon double bonds.

False

Alkanes only contain single C–C bonds.

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What property allows hydrocarbons in crude oil to be separated during fractional distillation?

Different boiling points

Larger molecules have stronger intermolecular forces and higher boiling points.

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True or False:

Short-chain alkanes have lower boiling points than long-chain alkanes.

True

Smaller molecules have weaker London dispersion forces.

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Fill in the blank:

The boiling point of alkanes increases as the length of the carbon _______ increases.

chain

Longer chains have stronger intermolecular forces.

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What is a fraction in crude oil processing?

Group of hydrocarbons with similar boiling points.

Each fraction condenses at a different level in the column.

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True or False:

Larger hydrocarbons condense higher up in the fractionating column.

False

Larger molecules condense lower in the column where temperatures are higher.

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Why are hydrocarbons heated before entering the fractionating column?

To vaporise them.

The vapours then rise and condense at different temperatures.

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Fill in the blank:

In fractional distillation, cooler temperatures are found toward the _______ of the column.

top

Temperature decreases as vapours rise up the column.

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Why is crude oil described as a mixture rather than a pure substance?

Contains many hydrocarbons.

It consists mainly of different alkane molecules of varying chain lengths.

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What process breaks long-chain alkanes into smaller molecules?

Cracking

Cracking breaks C–C bonds to produce smaller hydrocarbons.

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What type of bond is broken during the cracking of alkanes?

Carbon–carbon single bond

Breaking these bonds produces smaller alkanes and alkenes.

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Fill in the blank:

Cracking converts long-chain hydrocarbons into _______ hydrocarbons.

shorter

Smaller molecules are often more useful as fuels or chemical feedstocks.

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True or False:

Thermal Cracking often produces alkenes as well as alkanes.

True

The process commonly forms unsaturated hydrocarbons.

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What type of cracking occurs at high temperature and high pressure?

Thermal cracking

This method produces a high proportion of alkenes.

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True or False:

Catalytic cracking requires a catalyst such as a zeolite.

True

Zeolite catalysts provide active sites for the reaction.

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Fill in the blank:

Catalytic cracking is carried out at high temperature and _______ pressure.

slight

It operates at lower pressure than thermal cracking.

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What useful product is mainly produced by catalytic cracking for transportation?

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Motor fuels

This helps produce petrol and other useful fuels.

True or False:

Catalytic cracking is commonly used to produce aromatic hydrocarbons.

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True

These are important feedstocks in the petrochemical industry.

Why is cracking economically important in the petrochemical industry?

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Matches supply and demand.

It converts surplus long-chain hydrocarbons into more valuable shorter ones.

Fill in the blank:

Cracking increases the supply of smaller hydrocarbons such as _______ used in polymer production.

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alkenes

Alkenes are important raw materials for making plastics.

Why are shorter-chain hydrocarbons more valuable than longer ones?

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They are more useful fuels.

Shorter-chain hydrocarbons are more volatile and burn efficiently in engines.

What is the **main use** of **alkanes** in modern society?

Fuels ## Footnote Alkanes release energy when combusted and are widely used for heating and transport.

What type of **reaction** occurs when **alkanes react with oxygen** to release energy?

Combustion ## Footnote Combustion reactions are exothermic oxidation reactions.

# Fill in the blank: Complete **combustion of alkanes** produces carbon _\_\_\_\_\_\_ and water.

dioxide ## Footnote When sufficient oxygen is present the products are CO₂ and H₂O.

# True or False: **Incomplete combustion** occurs when there is a **limited supply of oxygen**.

True ## Footnote This leads to formation of CO or carbon instead of CO₂.

What **toxic gas** is formed during **incomplete combustion of hydrocarbons**?

Carbon monoxide ## Footnote CO binds strongly to haemoglobin and prevents oxygen transport.

# True or False: **Incomplete combustion** can produce **solid carbon (soot)**.

True ## Footnote Soot forms when hydrocarbons burn with insufficient oxygen.

# Fill in the blank: **Nitrogen oxides** produced in internal combustion engines are commonly called _\_\_\_\_\_\_.

NOx ## Footnote These gases contribute to air pollution and acid rain.

What **device** is used in **car exhaust systems** to reduce harmful emissions?

Catalytic converter ## Footnote It converts CO, NOx and hydrocarbons into less harmful gases.

# True or False: **Catalytic converters** reduce **carbon monoxide to carbon dioxide**.

True ## Footnote They also reduce nitrogen oxides to nitrogen.

What **pollutant** is formed when **sulfur impurities in fuels** are burned?

Sulfur dioxide ## Footnote SO₂ contributes to acid rain and air pollution.

# Fill in the blank: **Sulfur dioxide** can be removed from flue gases using **calcium oxide** or calcium _\_\_\_\_\_\_.

carbonate ## Footnote These substances react with SO₂ to form calcium sulfite or sulfate.

Why are **calcium oxide and calcium carbonate** used to remove **sulfur dioxide from flue gases**?

Neutralisation reaction ## Footnote They react with acidic SO₂ to form solid calcium salts.

What type of **reaction** occurs when **methane reacts with chlorine** in ultraviolet light?

Free-radical substitution ## Footnote A hydrogen atom in methane is replaced by a chlorine atom.

What highly reactive **species with an unpaired electron** is formed during the **chlorination of methane**?

Radical ## Footnote Radicals are produced during the initiation step.

# Fill in the blank: In **free-radical mechanisms**, an unpaired electron is represented by a _\_\_\_\_\_\_.

dot ## Footnote The dot indicates the presence of a single unpaired electron.

# True or False: **Chlorination of methane** requires **ultraviolet light** to start the reaction.

True ## Footnote UV light provides energy to break the Cl–Cl bond homolytically.

What is the **first stage** of the **free-radical substitution mechanism** called?

Initiation ## Footnote In this step radicals are first formed.

# True or False: The **propagation step** forms new radicals that continue the **chain reaction**.

True ## Footnote Each step generates another radical allowing the reaction to continue.

# Fill in the blank: In the **initiation step**, chlorine molecules split to form two chlorine _\_\_\_\_\_\_.

radicals ## Footnote Cl₂ → 2Cl· under UV light.

What is the name of the **stage** where radicals react together and remove radicals from the reaction?

Termination ## Footnote Two radicals combine to form a stable molecule.

# True or False: **Propagation steps** maintain the **chain reaction** by regenerating radicals.

True ## Footnote Radicals produced in propagation keep the reaction going.

# Fill in the blank: In the **first propagation step**, a chlorine radical reacts with methane to form _\_\_\_\_\_\_ radical.

methyl ## Footnote CH₄ + Cl· → CH₃· + HCl.

What is the **main organic product** formed when methane reacts with chlorine once?

Chloromethane ## Footnote CH₃Cl forms when a hydrogen atom is replaced by chlorine.

Why can **further substitution** occur during **methane chlorination**?

Radicals continue reacting ## Footnote Additional hydrogens can be replaced forming dichloromethane and other products.

Give the **equation** for the **second propagation step** in **methane chlorination**.

CH₃· + Cl₂ → CH₃Cl + Cl·

3.3.2 Alkanes Flashcards

Explain alkane properties and reactions by modelling combustion and free-radical substitution mechanisms. (49 cards)