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Topic 7 – National 4 & 5 Chemistry Summary Notes

Hydrocarbons

A vast number of different hydrocarbons exist and so chemists have grouped

them into “sub-sets” to make them easier to study. In this topic we will study

three different hydrocarbon sub-sets.

The Alkanes

The alkanes make up the simplest sub-set of all the hydrocarbons.

Most of the FUELS we use are made from alkanes. They are also useful starting

materials for a huge range of other substances such as PLASTICS and DRUGS.

Name of Alkane Uses

Methane (natural gas) for cooking, heating

propane Used in gas cylinders in homes

butane Sold in blue cylinders as camping gas

octane A component of petrol

All the alkanes end in –ANE. A prefix tells you how many carbon atoms are

present in the molecule.

Prefix Number of Carbon

Atoms

Meth 1

Eth 2

Prop 3

But 4

Pent 5

Hex 6

Hept 7

Oct 8

The alkanes all contain COVALENT bonds. They are different from other

compounds since they are able to form long, chain like molecules containing many

carbon atoms joined together.

LI 1

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Physical State of Alkanes

Name of

Alkane

Boiling Point

(⁰C)

Physical State at Room

Temperature (25⁰C)

Methane

Ethane

Propane

Butane

Pentane

Hexane

Heptane

Octane

*Complete the table using the data book pg ______

General Formula for the Alkanes

If you look at the full structural formula for the alkanes you should see that

there is a pattern.

Each carbon atom is joined to two hydrogen atoms with two extra hydrogens at

the end of the molecule. This gives the general formula in mathematical terms

as:

LI 2

Where n = the number of carbon atoms in the molecule

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Drawing and Naming Alkanes

We can represent the alkanes in a variety of different ways:

Molecular Formula

This is the simplest type of formula used for alkanes and other hydrocarbons.

This type of formula shows only the number of carbon and hydrogen atoms

present.

e.g. ethane C2H6

Full Structural Formula

The full structural formula gives a simplified picture of the structure of the

molecule. All the bonds present between the carbon atoms and the carbon and

hydrogen atoms are shown.

e.g. ethane

Shortened Structural Formula

This type of formula shows how many CH2 and CH3 groups are present in the

molecule.

e.g. ethane CH3CH3

propane CH3CH2CH3

LI 3

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ALKANE

MOLECULAR

FORMULA

FULL

STRUCTURAL FORMULA

SHORTENED

STRUCTURAL FORMULA

Methane

CH4

Ethane

C2H6

Propane

C3H8

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Butane

C4H10

Pentane

C5H12

Hexane

C6H14

*The names and molecular formulae of the first eight alkanes are found in the data book pg _______

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Saturation and Unsaturation

All the carbon to carbon bonds in the alkanes are single C-C bonds. For this

reason they are described as being SATURATED molecules.

In other hydrocarbon families not all the carbon to carbon bonds are single C-C

bonds and are therefore described as being UNSATURATED molecules.

Homologous Series

A HOMOLOGOUS SERIES is a family of compounds with similar chemical

properties and can be represented by a general formula.

The alkanes form a homologous series because they share the following

characteristics:

they all fit the same general formula

they all have similar chemical properties i.e. they all burn

they show a gradual change in physical properties as the molecules get

bigger in size e.g. boiling point increases as molecular size increases.

Reactions of Alkanes

Apart from COMBUSTION the alkanes undergo few other reactions.

Alkanes burn in a plentiful supply of oxygen to produce carbon dioxide and

water.

alkane + oxygen carbon dioxide + water

e.g. CH4 + 2O2 CO2 + 2H2O

It is for this reason that the alkanes are mainly used as FUELS.

ALKANES ARE SATURATED MOLECULES

LI 4

LI 5

LI 6

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The Alkenes

The alkenes are another homologous series of hydrocarbons

containing covalent bonds. Alkenes are similar to alkanes

in many ways but the main difference is the presence of the carbon-to-carbon

double bond in alkenes, represented as C=C.

All alkenes end in –ENE and the first member of the family is ethene.

Can you think why there is no methene?

Each alkene contains only one C=C

*Each carbon atom only forms 4 bonds so be careful not to add too many

hydrogen atoms when drawing full structural formula!

Since the alkenes are not full of carbon to carbon single bonds they are

described as being UNSATURATED molecules.

General Formula for the Alkenes

All alkenes fit the same general formula. It should be easy to see a pattern as

the number of carbon atoms increase.

Where n = the number of carbon atoms in the molecule

LI 7

ALKENES ARE UNSATURATED MOLECULES

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ALKENE

MOLECULAR

FORMULA

FULL

STRUCTURAL FORMULA

SHORTENED

STRUCTURAL FORMULA

Ethene

C2H4

Propene

C3H6

Butene

C4H8

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Pentene

C5H10

Hexene

C6H12

Heptene

C7H14

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Uses of Alkenes

The C=C present in alkenes is very reactive compared to the C-C in alkanes.

The C=C allows alkenes to be changed readily into new products e.g. ethene and

propene are used in the plastics industry to make polyethene and polypropene.

Reactions of Alkenes

1.Combustion

Like all other hydrocarbons the alkenes burn in a plentiful supply of oxygen to

produce carbon dioxide and water.

alkene + oxygen carbon dioxide + water

e.g. C2H4 + 3O2 2CO2 + 2H2O

2. Addition Reactions

The C=C is a reactive bond which breaks easily. This means that small molecules

can add across the double bond, usually with the help of a catalyst e.g.

a) Addition of Hydrogen (H2)

Two hydrogen atoms “add across” the C=C to form the corresponding ALKANE.

e.g. ethene + hydrogen ethane

Alkenes are very important feedstocks for

the PLASTICS INDUSTRY.

LI 8

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b) Addition of Bromine (Br2)

ethene + bromine 1,2-dibromoethane

Using Bromine Water to Test for Unsaturation

A solution of bromine water Br2(aq) can be used to distinguish an alkene from an

alkane.

Bromine water is an ORANGE colour but when bromine reacts with an ALKENE,

the product of the reaction is COLOURLESS. We say that the bromine has been

DECOLOURISED.

ethene bromine

(orange)

1,2-dibromoethane

(colourless)

ALL ALKENES QUICKLY DECOLOURISE BROMINE WATER

ALKANES DO NOT REACT WITH BROMINE WATER

Br2 Br Br

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Colour the boxes below to show the effects of bromine water reacting with:

a) an alkane

+

alkane bromine water stays orange

(colourless) (orange)

b) an alkene

+

alkene bromine water decolourises

(colourless) (orange)

The Cycloalkanes

The CYCLOLKANES are another homologous series of hydrocarbons in which

the carbon atoms are joined to form RINGS.

Cycloalkanes are full of C-C single bonds and are therefore SATURATED

compounds. They will not decolourise bromine water.

The first member of the cycloalkanes is CYCLOPROPANE.

The cycloalkanes all fit the same general formula:

LI 9

Where n = the number of carbon atoms in the molecule

N5

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CYCLOALKANE

MOLECULAR

FORMULA

FULL

STRUCTURAL FORMULA

SHORTENED

STRUCTURAL FORMULA

Cyclopropane

C3H6

Cyclobutane

C4H8

Cyclopentane

C5H10

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Branched Chain Alkanes

Alkanes like propane and butane are described as STRAIGHT CHAIN

ALKANES because each carbon atom is joined to no more than 2 other carbon

atoms. However, crude oil and natural gas both contain another type of alkane

called BRANCHED CHAIN ALKANES. In these alkanes some carbon atoms are

joined directly to three or four carbon atoms.

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Rules for Naming Branched Chain Alkanes

1. Look for the longest continuous chain of carbon atoms and name after the

parent alkane.

2. Number the carbon atoms in the longest chain from the end which gives

the lowest numbers to the carbon atoms that have branches attached.

3. Identify the groups making up the branches ( known as alkyl groups).

Alkyl Group Name of branch

-CH3 methyl

-C2H5 or –CH2CH3 ethyl

-C3H7 or –CH2CH2CH3 propyl

4. Use the prefixes di-, tri-, tetra etc. to indicate how many of a

particular branch are present.

5. Indicate the position of each branch with a number placed in front of its

name.

6. If more than one type of branch is present, the names are put in

alphabetical order e.g. ethyl would come before methyl.

Straight chain Branched chain Branched chain

LI 10

e.g.

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Examples:

Isomers

Butene and cyclobutane are ISOMERS. These two compounds have the same

number of carbon and hydrogen atoms but they are arranged differently.

Butene contains a C=C whereas cyclobutane does not.

Isomers are compounds with the same MOLECULAR FORMULA but

a different STRUCTURAL FORMULA.

LI 11

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*Isomers of straight chain alkanes can be branched alkanes.

*Isomers of alkenes can also be produced by moving the position of the C=C in

molecules with 4 or more carbon atoms.

But-1-ene But-2-ene

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*Note the different positions of the Cl atoms in the two isomers.

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Cracking

In oil refineries, the fractional distillation of crude oil generally produces more

long chain alkanes than are needed by industry. A process called CRACKING

breaks down the long chain alkanes into smaller, more useful molecules which are

in greater demand.

When a long chain alkane is cracked it produces a mixture of ALKANES and

ALKENES. Alkenes are very useful in the plastics industry.

e.g. cracking of decane

C10H22 C6H14 + C4H8

decane hexane butene

(saturated) (unsaturated)

When an alkane is cracked there are not enough hydrogen atoms to produce

two alkanes and so one product is an unsaturated alkene.

When an alkane is cracked the carbon chain may break at different points on

different molecules which means a mixture of products is usually obtained.

N4

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Cracking in the Lab

Cracking can be carried out in the lab using an aluminium oxide catalyst.

To prevent “suck back” the delivery tube should be removed from the

water before heating stops.

To prove the gas is an alkene, bromine water can be added to the test

tube of product gas. The gas decolourises the bromine water.

aluminium oxide catalyst

delivery tube

water

CRACKING is a process used to meet the demand for

shorter chain alkanes and alkenes.

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Number

Learning Intention

Success Criteria

1 I will find out about the simplest subset of

hydrocarbons known as the Alkanes

I can:

State uses for some alkanes

State the physical states of alkanes at room temperature

State that alkanes contain covalent bonds

State that all alkanes end in –ANE.

State that a prefix indicates the number of carbon atoms in the

molecule.

Name the first eight members of the alkanes

2 I will work out the general formula for the

Alkanes after completion of LI3

I can:

State the general formula for the alkanes as CnH2n+2

State that n = a whole number

3 I will find out how to represent alkanes in a

variety of ways

I can:

State what is meant by molecular formula, shortened structural

formula and full structural formula.

Represent the first eight members of the alkane family using

molecular formula, shortened structural formula and full structural

formula

4 I will find out the difference between saturated

and unsaturated molecules

I can:

State that saturated molecules are full of C-C single bonds

State that unsaturated molecules contain a C=C

State that the alkanes are saturated molecules

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I will find out about the meaning of a

homologous series.

I can :

State that all members of a homologous series can be represented by

National 4 & 5 Topic 7 Hydrocarbons

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a general formula

State that all members have similar chemical properties

State that there is a gradual change in physical properties as the

molecules get bigger in size e.g. boiling point increases

6 I will find out about the different types of

reactions alkanes undergo

I can:

State that all alkanes burn and are used mainly as fuels

Write and balance combustion formulae equations for alkanes

7 I will find out about another homologous series

of hydrocarbons called the alkenes

I can

State that alkenes contain one C=C per molecule

Represent the first 6 members of the alkenes using molecular formula,

shortened structural formula and full structural formula.

Work out the general formula for the alkenes and state this as CnH2n

State that alkenes are important feedstocks for the plastics industry.

8 I will find out about the reactions the alkenes

can undergo

I can

State that all alkenes burn to produce CO2 and H2O

State that alkenes undergo addition reactions due to the presence of

the C=C

State that small molecules can add across the C=C

Draw the products of addition reactions

State that alkenes decolourise bromine water whereas alkanes do not.

State that alkenes are unsaturated molecules

State that bromine water is used as the standard test for

unsaturation

9 I will find out about the homologous series of

cycloalkanes

I can:

State that the cycloalkanes are hydrocarbons made up of rings of

carbon atoms

State that cycloalkanes do not react with bromine water and are

therefore saturated compounds

State the general formula as CnH2n

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Represent the first 5 members of the cycloalkanes using molecular

formula, shortened structural formula and full structural formula.

10 I will learn how to recognise and name

branched chain alkanes

I can:

Differentiate a straight chain alkane from a branched chain alkane

Follow a set of rules to name a branched chain alkane

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I will find out the definition of an isomer and

how to draw isomers

I can:

State the definition of an isomer

Draw isomers of straight chain alkanes

State that alkenes are isomers of cycloalkanes i.e. cyclopropane is an

isomer of propene

Draw isomers of alkenes with 4 or more carbon atoms by moving the

position of the C=C

12 I will carry out a cracking experiment I can:

State that cracking is a process used to meet the demand for shorter

chain alkane and alkenes.