9.2: Hydrocarbons

Hydrocarbons

Compounds made only of carbon and hydrogen atoms.

Types of hydrocarbons

Alkanes and alkenes.

Alkanes

Saturated hydrocarbons containing only single covalent bonds.

General formula for alkanes

CnH2n+2

First four alkanes

Methane, ethane, propane, butane.

Alkenes

Unsaturated hydrocarbons containing at least one carbon-carbon double bond.

General formula for alkenes

CnH2n

First four alkenes

Ethene, propene, butene, pentene.

Unsaturated

The molecule contains at least one double bond between carbon atoms.

Functional group of alkenes

The carbon-carbon double bond (C=C).

Testing for alkenes

Using bromine water.

Bromine water and alkenes

It turns from orange to colourless.

Why bromine water turns colourless

Because alkenes undergo an addition reaction with bromine.

Reaction type of alkenes with bromine

Addition reaction.

Polymer

A long-chain molecule made from many repeating units (monomers).

Addition polymers

Made from alkenes.

Addition polymerisation

The reaction in which many alkene monomers join together to form a polymer.

Monomer

A small molecule that joins with others to form a polymer.

Repeating unit in a polymer

The part of the polymer structure that repeats over and over.

Drawing repeating units

With square brackets and extending bonds through the brackets.

Properties of polymers

Strong, flexible, unreactive — but non-biodegradable.

Disposal of polymers problem

They are not biodegradable and can cause environmental damage.

Disposal methods for polymers

Landfill, incineration, or recycling.

Cracking

A reaction that breaks long-chain hydrocarbons into shorter alkanes and alkenes.

Usefulness of cracking

It produces more useful, smaller hydrocarbons and alkenes for making polymers.

Conditions for catalytic cracking

A high temperature and a catalyst.

Products of cracking

A shorter alkane and an alkene.

Type of reaction for cracking

A thermal decomposition reaction.