4.2 Hydrocarbons for A-Level Chemistry (OCR B

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?

Motor fuels

This helps produce petrol and other useful fuels.

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True or False:

Catalytic cracking is commonly used to produce aromatic hydrocarbons.

True

These are important feedstocks in the petrochemical industry.

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Why is cracking economically important in the petrochemical industry?

Matches supply and demand.

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

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Fill in the blank:

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

alkenes

Alkenes are important raw materials for making plastics.

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

They are more useful fuels.

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

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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·

What type of **hydrocarbons** are **alkenes** classified as?

Unsaturated hydrocarbons ## Footnote They contain at least one carbon–carbon double bond.

What type of **bond** is present between carbon atoms in **alkenes**?

Double covalent bond ## Footnote The C=C bond consists of one σ bond and one π bond.

# Fill in the blank: The **carbon–carbon double bond** in alkenes is a region of high _\_\_\_\_\_\_ density.

electron ## Footnote This makes alkenes more reactive than alkanes.

# True or False: **Alkenes** are more reactive than **alkanes** due to the presence of a double bond.

True ## Footnote The electron-rich double bond attracts electrophiles.

What are the two types of **bonds** present in a **carbon–carbon double bond**?

Sigma and pi ## Footnote The σ bond forms by head-on overlap and the π bond by sideways overlap.

# True or False: The **π bond** in alkenes is **stronger than the σ bond**.

False ## Footnote The π bond is weaker and more easily broken in reactions.

# Fill in the blank: The **double bond** in alkenes restricts _\_\_\_\_\_\_ about the carbon–carbon bond.

rotation ## Footnote This restriction leads to E–Z stereoisomerism.

Why do **alkenes** undergo many **addition reactions**?

High electron density ## Footnote The π bond attracts electrophiles.

# True or False: The **electron density** in an alkene is concentrated **above and below the plane** of the molecule.

True ## Footnote This is due to the π bond formed by sideways overlap of p orbitals.

# Fill in the blank: The **π bond** is formed by sideways overlap of _\_\_\_\_\_\_ orbitals.

p ## Footnote These orbitals form electron density above and below the bond axis.

What type of **reactions** are typical for **alkenes** due to their double bond?

Addition reactions ## Footnote The π bond breaks allowing new atoms to add across the double bond.

Why is the **double bond** in alkenes considered a **reactive site**?

Electron-rich region ## Footnote The high electron density attracts electron-deficient species.

What type of **reaction** commonly occurs when **alkenes react with small molecules** such as HBr or Br₂?

Electrophilic addition ## Footnote The π bond breaks and atoms add across the C=C bond.

What type of **species** attacks the **electron-rich double bond** in alkenes?

Electrophile ## Footnote Electrophiles are electron-pair acceptors attracted to high electron density.

# Fill in the blank: In **electrophilic addition**, the alkene double bond acts as a source of _\_\_\_\_\_\_.

electrons ## Footnote The π bond donates electrons to the electrophile.

# True or False: **Bromine water** can be used to test for **unsaturation** in organic molecules.

True ## Footnote Alkenes decolourise orange bromine water.

What **observation** indicates the presence of a **carbon–carbon double bond** when bromine water is added?

Decolourisation ## Footnote The orange colour disappears as bromine reacts with the alkene.

# True or False: **Electrophilic addition reactions** proceed through a **carbocation intermediate**.

True ## Footnote The intermediate forms after the electrophile attacks the double bond.

# Fill in the blank: The **positively charged intermediate** formed during electrophilic addition is called a _\_\_\_\_\_\_.

carbocation ## Footnote Its stability influences which product forms.

Which **carbocation** is most stable: **primary, secondary, or tertiary**?

Tertiary ## Footnote Tertiary carbocations are stabilised by surrounding alkyl groups. This is because alkyl groups have a positive inductive effect.

# True or False: The **most stable carbocation** usually leads to the **major product** of the reaction.

True ## Footnote Reaction pathways favour the most stable intermediate.

# Fill in the blank: When an **unsymmetrical alkene** reacts with HBr, the hydrogen attaches to the carbon with the _\_\_\_\_\_\_ hydrogens.

most ## Footnote This forms the more stable carbocation intermediate.

What term describes the **main product formed** in a reaction when multiple products are possible?

Major product ## Footnote It forms through the most stable reaction pathway.

Why can **minor products** form during electrophilic addition reactions?

Alternative carbocation formation ## Footnote Less stable intermediates can still form in smaller amounts.

What type of **polymer** is formed when many **alkene molecules join together**?

Addition polymer ## Footnote The polymer forms when the C=C double bonds open and link monomers together.

What is the **small molecule** that joins repeatedly to form a **polymer** called?

Monomer ## Footnote Monomers are the repeating building blocks of polymers.

# Fill in the blank: In **polymer chemistry**, the repeating section of a polymer chain is called the _\_\_\_\_\_\_ unit.

repeating ## Footnote It shows the smallest part of the polymer structure that repeats.

# True or False: **Addition polymers** are formed **without the loss of small molecules**.

True ## Footnote No by-products are produced during addition polymerisation.

What happens to the **carbon–carbon double bond** during addition polymerisation?

It opens ## Footnote The π bond breaks allowing monomers to join into a long chain.

# True or False: **Addition polymers** are generally **chemically unreactive**.

True ## Footnote They contain strong C–C and C–H bonds that require a lot of energy to break and no reactive functional groups.

# Fill in the blank: **Poly(chloroethene)** is commonly known as _\_\_\_\_\_\_.

PVC ## Footnote It is widely used in pipes, cables and construction materials.

What **substance** can be added to **PVC** to make it more flexible?

Plasticiser ## Footnote Plasticisers reduce intermolecular forces between polymer chains.

# True or False: **Polyalkenes** are held together mainly by **strong ionic bonds** between molecules.

False ## Footnote Intermolecular forces between chains are mainly van der Waals forces.

# Fill in the blank: Intermolecular attractions between **polyalkene chains** are mainly _\_\_\_\_\_\_ forces.

Van der Waals ## Footnote These weak forces act between long hydrocarbon chains.

Why are **addition polymers** generally **chemically inert**?

Strong covalent bonds ## Footnote The polymer backbone contains strong C–C and C–H bonds.

What must be done to an **alkene monomer** to identify the **repeating unit of the polymer**?

Break the double bond ## Footnote The C=C bond opens and forms two single bonds in the polymer chain.

4.2 Hydrocarbons Flashcards

Explain structure, properties, and reactions of alkanes and alkenes using mechanisms. (85 cards)