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Nature’s Chemistry May 2017

1

DUNCANRIG SECONDARY SCHOOL

CHEMISTRY DEPARTMENT

National 5

Chemistry

Unit 2

Nature’s Chemistry

Key Facts and Theory

Hydrocarbons, Homologous Series, Alcohols,

Carboxylic Acids, and Energy from fuels May 2017 version


2

Hydrocarbons

A hydrocarbon is a compound made up of only hydrogen and carbon atoms, e.g. hexane, C6H14, is a

hydrocarbon but acetone, C3H6O, is not a hydrocarbon because acetone contains atoms of oxygen as

well.

Since both carbon and hydrogen are non-metal atoms, hydrocarbons are made up of molecules with

the atoms joined by covalent bonds, i.e. by the sharing of electron pairs.

Carbon is in Group 4 of the Periodic Table and so atoms of carbon have four

electrons in the outer shell (energy level) and form four bonds.

Hydrogen atoms have one electron in the first shell (energy level) and forms

one bond.

What is meant by a hydrocarbon?

Complete the following table.

Name Formula Hydrocarbon (yes or no)

octane C8H16

ethylamine C2H5NH2

glucose C6H12O6

natural gas CH4

water H2O

carbon monoxide CO

alcohol C2H5OH

acetylene C2H2

How many covalent bonds are formed by … carbon atoms? … hydrogen atoms?


3

The alkanes Activity 2.1 Methane, ethane and propane are the first three members of a series of hydrocarbons called the

alkanes. The alkanes are a subset of the set of hydrocarbons.

Each member of the alkane series has a name that ends in –ane and a prefix that indicates the number

of carbon atoms in the molecule, e.g. methane is the alkane with one carbon atom per molecule.

Prefix

Number of carbon atoms

in the molecule

meth - 1

eth - 2

prop - 3

but - 4

pent - 5

hex - 6

hept - 7

oct - 8

All the hydrocarbons in the alkane series are known as saturated hydrocarbons. A saturated

hydrocarbon is one in which all the carbon to carbon bonds are single covalent bonds.

What is meant by a saturated hydrocarbon?


4

Alkanes continued

The hydrocarbon with just one carbon atom is called methane.

In a molecule of methane, one carbon atom is joined to four

hydrogen atoms to give a stable electron arrangement for all the

atoms in the molecule.

The formula for methane is CH4. This gives the number of atoms of

each element in the molecule but CH4 does not give any information

about the arrangement of atoms in the molecule. Information about

the arrangement of atoms is given by the full structural formula.

The full structural formula is ‘flat’. The actual structure of methane

is based on the three-dimensional tetrahedral arrangement of

electrons in a carbon atom:

Molecules of ethane have two carbon atoms joined by a single covalent

bond.

Each carbon atom can form three bonds with hydrogen atoms.

Molecules of propane have three carbon atoms joined by single

covalent bonds.

Three hydrogen atoms are attached to the end carbon atom;

two hydrogen atoms are attached to the middle carbon atom.


5

Methane, ethane and propane A shortened structural formula can be used to show the grouping of hydrogen atoms round each carbon

atom. The different ways of representing alkanes are shown below.

Name of

alkane

Full structural

formula

Shortened

structural formula

Formula

Methane

Ethane

CH4

CH3CH3

CH4

C2H6

What information is given by the full structural formula?

Complete the table to show the different ways of representing propane.

Name of

alkane

Full structural

formula

Shortened structural

formula

Formula

Propane


6

Alkanes continued

For the alkanes with four, five, six, seven and eight carbon atoms per molecule:

write the name, write the formula, draw the full structural formula, and draw

the shortened structural formula.

5 carbons

Name:

Formula:

Full structural formula:

Shortened structural formula:

6 carbons

Name:

Formula:



4 carbons

Name:

Formula:




7

Alkanes continued

7 carbons

Name:

Formula:



8 carbons

Name:

Formula:



Alkanes are an example of a homologous series. A homologous series is a family of compounds

that can be represented by a general formula and have similar chemical properties.

The general formula for the alkanes is CnH2n+2. Each of the alkanes has two hydrogen atoms

for every carbon atom plus two hydrogen atoms at the end.

Alkanes

are a homologous series of saturated hydrocarbons

can be represented by the general fomula CnH2n+2

are very flammable and are commonly used as fuels

are insoluble in water

slowly decolourise bromine water (substitution reaction).


8

The Alkenes Activity 2.2

Ethene and propene are the first two members of a series of hydrocarbons called the alkenes. The

alkenes are another subset of the set of hydrocarbons.

Each member of the alkene series has one carbon to carbon double bond. The name ends in –ene and a

prefix (same as for alkanes) indicates the number of carbon atoms in the molecule, e.g. ethene is the

alkene with two carbon atoms per molecule.

All the hydrocarbons in the alkene series are known as unsaturated hydrocarbons. An unsaturated

hydrocarbon has (at least) one carbon to carbon double bond in each molecule.

What is meant by an unsaturated hydrocarbon?

Ethene and propene Carbon atoms can also join up by the sharing of two electron

pairs, i.e. by forming two covalent bonds.

The two covalent bonds between two carbon atoms is called a

double covalent bond.

Each carbon atom can only form two bonds with other atoms

The hydrocarbon with two carbon atoms joined by a double covalent

bond is called ethene.

Molecules of propene have three carbon atoms joined by covalent

bonds. One is a double covalent bond.

Two hydrogen atoms are attached to carbon atom 1; one hydrogen

atom is attached to carbon atom 2; three carbon atoms are

attached to carbon atom 3.

The different ways of representing ethene and propene are shown

below.


9

Full structural formula Shortened structural

formula Formula

C2H4

Full structural formula Shortened structural

formula Formula

CH2=CHCH3

C3H6

For the alkenes with four, five, six, seven and eight carbon atoms per molecule: write

the name, write the formula, draw the full structural formula, and draw the shortened

structural formula

4 carbons

Name:

Formula:



5 carbons

Name:

Formula:



http://www.google.co.uk/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&ved=&url=http://www.bbc.co.uk/bitesize/standard/chemistry/plasticsandothermaterials/plastics/revision/4/&psig=AFQjCNGaSfmZe09EJPA9fSFrohl3PQCLqw&ust=1498135817528235


10

6 carbons

Name:

Formula:



7 carbons

Name:

Formula:



8 carbons

Name:

Formula:



Alkenes are another example of a homologous series. The general formula for the alkenes is CnH2n.

Each of the alkenes has two hydrogen atoms less than the alkanes due to the double (rather than the

single) covalent bond.

Alkenes

are a homologous series of unsaturated hydrocarbons

can be represented by the general formula CnH2n

are very flammable

used to make alcohols and polymers


immediately decolourise bromine water (addition reaction)


11

The cycloalkanes

Carbon atoms can join together with covalent bonds to form closed chains or ‘rings’.

Alkanes with a ring of carbon atoms are called cycloalkanes.

Cycloalkanes are another subset of the set of hydrocarbons. Each member of the cycloalkane

series has a name beginning with ‘cyclo’ to indicate that there is a ring of carbon atoms. The

name ends in –ane to indicate that all the carbon to carbon bonds are single covalent

bonds and a prefix (same as for alkanes) indicates the number of carbon atoms in the

molecule.

The first member of the series is cyclopropane with three carbon atoms

joined in a ring by single covalent bonds.

Each carbon atom is able to form two covalent bonds with hydrogen

atoms.

The different ways of representing cyclopropane are shown below.

Full structural formula Shortened structural formula Formula

C3H6

What is meant by a cycloalkane?


12

The cycloalkanes continued

For the cycloalkanes with four, five, six, seven and eight carbon atoms per molecule:

write the name, write the formula, draw the full structural formula, and draw the

shortened structural formula

4 carbons

Name:

Formula:



5 carbons

Name:

Formula:



6 carbons

Name:

Formula:




13

7 carbons

Name:

Formula:



8 carbons

Name:

Formula:



Cycloalkanes are another example of a homologous series. The general formula

for the cycloalkanes is also CnH2n. Each of the cycloalkanes has two hydrogen atoms

less than the corresponding alkane due to the bond that closes the ring.

Cyclolkanes

are a homologous series of saturated, cyclic hydrocarbons

can be represented by the general formula CnH2n

are very flammable and are commonly used as fuels

are used as solvents


slowly decolourise bromine water (substitution reaction).


14

Naming alkanes Carbon compounds are given a systematic name according to an internationally accepted

convention.

There are three different structures for the compound with molecular formula C5H12.

Their shortened structural formulae are shown below.

Structure A is called a straight chain hydrocarbon. Structures B and C are called

branched chain hydrocarbons.

For naming straight-chain alkanes name them the same way as on pages 5&6.

Branches are named after the corresponding alkane with the –ane ending changed to -yl.

e.g.

To name an alkane:

1. Select the longest continuous chain of carbon atoms and name it after the appropriate alkane.

2. Number the carbon atoms from the end of the chain nearer the branch.

3. Name the branch(es) and indicate the position(s) of the branch(es) on the chain.

4. Use ‘di’ and ‘tri’, etc. when the same branch is present more than once.

e.g.

A

B C

methyl group

ethyl group

2-methylbutane

3-methylpentane

2,3-dimethylbutane


15

Naming alkanes Name each the following alkanes.


16

Naming alkenes Alkenes are named in a similar way to alkanes.

1 Select the longest continuous chain of carbon atoms containing the double bond and

name it after the appropriate alkene.

2. Number the carbon atoms from the end of the chain nearer the double bond and

indicate the position of the double bond.

3. Name any branch(es) and indicate the position(s) of the branch(es) on the chain.

Name each of the following alkenes.

but-1-ene

3-methylbut-1-ene but-1-ene but-2-ene


17

Homologous Series A homologous series is a family of compounds which

fit a general formula

have similar chemical properties

show a gradual change in physical properties


Family General formula

alkanes

alkenes

cycloalkanes

Use the general formulas to complete the following table.

In each of the following lists of hydrocarbons identify the one that is in a different

homologous series from the others.

ethane butene methane octane

C3H8 CH4 C7H14 C12H26

C2H4 C6H12 CH4 C4H8

Number of carbon atoms in the molecule

Formula

alkane 6

alkane 9

alkene 8

alkene 12

cycloalkane 7

cycloalkane 10


18

Cycloalkenes are hydrocarbons with one carbon to carbon double bond. They are another

example of a homologous series.

Try and work out the general formula of the cycloalkenes

Homologous series- Physical changes

Complete the following tables using your data booklet and work out the

state (solid, liquid or gas) of the hydrocarbon at 25oC.

Name Formula b.p. / oC m.p. / oC State at 25 oC

methane

ethane

propane

butane

pentane

hexane


ethene

propene

but-1-ene

pent-1-ene

hex-1-ene

Cyclohexene


19


cyclobutane

cyclopentane

cyclohexane

You can see from the data in the three tables that ‘as size of the molecule get bigger, the

melting and boiling points increase’.

This pattern is due to the fact that as the size of the molecule increases, the strength

of the intermolecular forces increases and so more energy is required to separate the

molecules.

Size of molecule Strength of intermolecular forces

Also note that there is a regular change in melting and boiling points this is because there is

a regular increase in mass of the molecule.

If we look at successive members of ANY homologous series, the formula differs by a –CH2

group, eg

Methane CH4 Ethane C2H6 Propane C3H8 Butane C4H10

and as a result the relative formula masses differ by 14.

Methane CH4 (16g) Ethane C2H6 (30g) Propane C3H8 (44g) Butane C4H10 (58g)

So there is a regular pattern in the melting and boiling points and you are often asked to

predict the melting or boiling point of the next member of the homologous series.


20

Isomers

Isomers are compounds with the same molecular formula but different structural formula.

They usually have different physical properties.

The following flow diagram can be used to decide whether or not two compounds are isomers.

Examples:

1.

2.

NO YES

Do

molecules

have the same

formula?

NOT ISOMERS Do

molecules

have different

structures?

ISOMERS NOT ISOMERS

same molecule

different formulae

NOT isomers

same formula

same structure

NOT isomers

ethane

butane butane

propane

NO YES


21

Isomers continued

3.

4.

5.

6.

Isomers may belong to different homologous series, for example both propene and

cyclopropane have the molecular formula C3H6 but they have a different structure. So they

are isomers.

same formula

same structure

NOT isomers

same formula

different structures

isomers

same formula


isomers

2-methylbutane

same formula

same structure

NOT isomers

2-methylbutane

2,2-dimethylpropane 2-methylbutane

but-1-ene but-2-ene

but-1-ene but-1-ene

propene cyclopropane


22

Isomers continued

Decide whether or NOT each of the following pairs of hydrocarbons are isomers.

1.

2.

3.

4.

5.

6.

7.

8.

9.


23

Isomers continued

Decide whether or NOT each of the following pairs of hydrocarbons are isomers.

1. 2-methylpentane butane

2. 2,3-dimethylbutane hexane

3. 2-methylhexane 3,3-dimethylpentane

4. 2-methylbut-1-ene pent-2-ene

5. pent-1-ene methylcyclobutane

Many carbon compounds, other than hydrocarbons, have isomers.

Examples:

1.

2.

3.

same formula


isomers

same formula

same structure

NOT isomers

same formula


isomers


24

Isomers continued

Decide whether or NOT each of the following pairs of carbon compounds are isomers

1.

2.

3.

4.

5.


25

Reactions of alkenes Alkanes, alkenes and cycloalkanes are hydrocarbons.

Alkanes and cycloalkanes are saturated; all the carbon to carbon bonds are single covalent bonds.

Alkenes are unsaturated; the molecules contain at least one carbon to carbon double covalent bond.

What colour change is observed when bromine (in solution) is added to

… a saturated hydrocarbon? …an unsaturated hydrocarbon?

When bromine (in solution) is added to an unsaturated hydrocarbon, the brown colour of the bromine

‘disappears’, i.e. the bromine is immediately decolourised. (It is incorrect to say that the bromine

solution goes clear; it is clear to begin with!)

In the reaction, the carbon to carbon double bond breaks and the bromine atoms add on the carbon

atoms at either side of this bond.

e.g. ethene with bromine

+ Br2

propene with bromine

This kind of reaction is called an addition reaction because of the way that bromine adds on to the

alkene. When a halogen is added on across the double bond in an alkene, the product is called a

dihaloalkane

The reaction with bromine is the way to distinguish an unsaturated hydrocarbon from a saturated

hydrocarbon; the bromine is immediately decolourised by the unsaturated hydrocarbon.

+ Br2


26

Reactions of alkenes

Alkenes can also react with hydrogen in an addition reaction. The corresponding alkane is formed.

e.g. ethene with hydrogen

+ H2

This addition reaction is called hydrogenation.

What is the difference between a saturated and an unsaturated hydrocarbon?

What is the test for an unsaturated hydrocarbon?

Name the type of addition reaction taking place when hydrogen is added on

across adouble bond.

Draw a structural formula formed by the reaction of bromine with each of the

following hydrocarbons.

but-1-ene

but-2-ene


27

Reactions of alkenes continued

Draw a structural formula for the product of the reaction of but-1-ene and hydrogen.


Alkane Cycloalkane Alkene

General formula CnH2n+2 CnH2n CnH2n

Saturated or unsaturated

Reaction with bromine

(yes or no)

cyclohexene

terpinoline


28

Reactions of alkenes continued

Three different hydrocarbons were treated with bromine solution. Each of the

hydrocarbons contained six carbon atoms. The results are shown.

Formula Hydrocarbon Effect on bromine

C6H12 A decolourises quickly

C6H14 B no immediate change

C6H12 C no immediate change

Give the names and draw possible structures for A, B and C.

A Name :

B Name :

C Name :


29

The alcohols

Alcohol is found in alcoholic drinks. However, more than one carbon compound can be classified as

being an alcohol. Like the alkanes, alkenes and cycloalkanes, the alcohols make up a homologous

series. Ethanol (‘eth-’ meaning two) is the alcohol found in alcoholic drinks.

The functional group of the alcohols is the hydroxyl group (-OH). This is the group that gives the

characteristic properties to the alcohols. The atoms in a hydroxyl group are joined by a covalent bond

and the group is part of a covalent molecule (compare with the hydroxide ion found in alkalis).

Each member of the alcohol series has a name which ends in

-anol and a prefix which indicates the number of carbon atoms in the molecule,

e.g. ethanol has the hydroxyl group attached to two carbon atoms.

The different ways of representing ethanol are shown below.


CH3CH2OH

C2H5OH

For the straight-chain alcohols with one, three and four carbon atoms per

molecule (and the hydroxyl group at the end of the carbon chain):

write the name, write the formula, draw the full structural formula, and draw the

shortened structural formula

1 carbon

Name:

Formula:




30

Alcohols continued 2 carbons

Name:

Formula:



3 carbons

Name:

Formula:



4 carbons

Name:

Formula:



5 carbons

Name:

Formula:




31

Alcohols continued

6 carbons

Name:

Formula:



7 carbons

Name:

Formula:



8 carbons

Name:

Formula:




32

Alcohols continued

What is the group that gives the characteristic properties to the alcohol family of

carbon compounds?

What is the general formula for the homologous series of alcohols?

Ethanol can be prepared in industry by the reaction of ethene with water. Since the double bond

breaks as atoms are added on to the carbon atoms at either side, this is another example of an

addition reaction.

+ H2O

ethene water ethanol

This kind of reaction is also known as hydration.

Why is the reaction of ethene with water called an addition reaction?


33

Naming alcohols

From propanol onwards, isomerism can occur due to different positions of the hydroxyl group.

e.g.

To name an isomeric alcohol:

1 Select the longest continous chain of carbon atoms containing the hydroxyl group and name it after

the appropriate alcohol.

2 Number the carbon atoms from the end of the chain nearer the hydroxyl group and indicate the

position of the group.

3 Name any branch(es) and indicate the position(s) of the branch(es) on the chain.

e.g.

Some alcohols have more than one hydroxyl group.

e.g.

propan-1-ol

propan-2-ol

3-methylbutan-1-ol

ethane-1,2-diol

(ethylene glycol,

found in antifreeze)

propane-1,2,3-triol

(glycerol)


34

Naming alcohols

Name each of the following alcohols.

Draw a structural formula for each of the following alcohols.

2,2 dimethypentan-1-ol

2-methylhexan-3-ol

Draw the full structural formula for each of the FOUR alcohols that have the formula

C4H9OH. Write the name of each alcohol.


35

Carboxylic acids

Vinegar is an acidic solution. Ethanoic acid is the name of the solution that gives vinegar the sour taste.

Carboxylic acids are used in the manufacture of soaps, preservatives, medicines (aspirin).

Ethanoic acid (‘eth’ indicating two carbons) is the second member of another homologous series called

the carboxylic acids.

The group that gives the characteristic properties to the carboxylic acids is the carboxyl group:

Each member of the carboxylic acid homologous series has a name which ends in -anoic acid and a

prefix which indicates the number of carbon atoms in the molecule,

e.g. ethanoic acid has two carbon atoms including the carbon in the carboxylic acid group.

The characteristic acid group must always be at the end of a carbon chain.

The different ways of representing ethanoic acid are shown below.



36

Carboxylic acids

For the straight-chain carboxylic acids with one, three and four carbon atoms

per molecule: write the name, write the formula, draw the full structural

formula, and draw the shortened structural formula

1 carbon

Name:

Formula:



2 carbons

Name:

Formula:



3 carbons

Name:

Formula:




37

Carboxylic acids continued

4 carbons

Name:

Formula:



5 carbons

Name:

Formula:



6 carbons

Name:

Formula:




38


7 carbons

Name:

Formula:



8 carbons

Name:

Formula:



What is the group that gives the characteristic properties to the carboxylic acid family

of carbon compounds?

What is the general formula for the homologous series of carboxylic acids?


39


With branched chain carboxylic acids, any branch(es) and the position(s) of the branch(es) on the chain

are named in the same way as with alcohols.

e.g.

3-methylbutanoic acid 2-methylbutanoic acid

Name each of the following carboxylic acids.

Draw a structural formula for each of the following carboxylic acids.

3-ethylpentanoic acid

3,3-dimethylhexanoic acid


40

Reactions of Carboxylic Acids

Solutions of carboxylic acids have a pH less than 7. This means they can be involved in

neutralisation reactions, the same as other acids such as hydrochloric, nitric and sulfuric.

Remember from Unit 1 that…Bases are substances which neutralise an acid, moving the pH

towards 7.

Examples of bases are metal oxides, metal hydroxides and metal carbonates.

If the metal hydroxide is soluble in water it is called an alkali.

When an alkali, a metal oxide or an insoluble metal hydroxide react with an acid, a salt and

water are produced.

acid + alkali salt + water

If a metal carbonate reacts with an acid, carbon dioxide gas is produced as well as the salt

and water.

acid + metal carbonate salt + water + carbon dioxide

The name of the salt produced depends on the reactants used.

The first part of the salts name comes from the metal (or positively charged ion) from the

base and the end part comes from the acid.

The following table shows the ending of the salt name produced when the first eight

carboxylic acids are reacted.

Carboxylic acid neutralised Ending of salts name

methanoic methanoate

ethanoic ethanoate

propanoic propanoate

butanoic butanoate

pentanoic pentanoate

hexanoic hexanoate

heptanoic heptanoate

octanoic octanoate


41

Reactions of Carboxylic Acids continued

The word equation for the reaction between methanoic acid and the alkali, sodium hydroxide

is shown below:

methanoic acid + sodium hydroxide sodium methanoate + water

Acids also react with some metals to produce a salt and hydrogen gas.

acid + metal salt + hydrogen This is not classed as a neutralisation reaction as no water is formed. However the salts name is made up the same way. Complete the following word equations propanoic acid + lithium hydroxide + water pentanoic acid + calcium carbonate + + ethanoic acid + zinc oxide + + copper oxide copper octanoate + + magnesium magnesium butanoate +


42

Uses of carbon compounds Alcohols

Alcohols are effective solvents for substances that are insoluble in water. Methylated spirits (or

meths) is ethanol mixed with other chemicals. Alcohols with smaller molecules evaporate easily, making

them ideal for cleaning solvents eg, cleansing wipes, hand gels and handwash.

The high flammability and the very clean flame with which they burn has resulted in alcohols being

used as fuels. Ethanol is mixed with petrol for use as an engine fuel in countries where it can be

economically produced in sufficient quantities, e.g. in Brazil. The ethanol can be obtained by the

fermentation of sugar cane which can be considered as a renewable source of energy. Methanol is also

used as a fuel in car racing.

Carboxylic acids Carboxylic acids are used in the preparation of preservatives, soaps and medicines.

Vinegar is a solution of ethanoic acid. Vinegar is used as a preservative in the food industry. Foods

in vinegar can be stored for a long time because the low pH prevents the growth of harmful bacteria

and fungi.

Vinegar is also used in household cleaning products designed to remove the build-up of insoluble

carbonates found on plumbing fixtures, e.g. taps. The acid reacts with the carbonates in a neutralisation

reaction. It is useful as a household cleaner as it is non-toxic and can be used safely.

Give TWO uses for alcohols.

Why can ethanol be described as a renewable source of energy?

Give a use for vinegar … as a preservative in the food industry … as a household

cleaning product.


43

Uses of compounds continued

What are the THREE products of the reaction between vinegar and a carbonate

compound?

Why is vinegar useful as a household cleaner.

Summary of consumer products

Alkanes

Used as fuels

Alkenes

Used to make alcohols ( in hydration reaction eg/ ethene + water ethanol) and

to make polymers ( eg ethene polythene and propene polypropene)

Cycloalkanes

Used as fuels and as solvents

Alcohols

Used as fuels and solvents

Carboxylic acids

Used in the preparation of preservatives, soaps and medicines.

Vinegar (Ethanoic acid) used in household cleaning products.


44

Energy changes

An exothermic reaction is one in which energy is released, e.g. when a fuel burns (combustion),

heat (mainly) is given out to the surroundings.

The flame in a Bunsen burner is a result of burning methane, i.e. methane and oxygen reacting

to form carbon dioxide and water.

CH4 + 2O2 CO2 + 2H2O

In the chemical reaction, energy is required to break the covalent bonds in the reactant

molecules … to overcome the forces of attraction holding the atoms in the molecules together.

New covalent bonds can then be formed in the product molecules. When these are formed,

energy is released.

In a reaction in which energy is released, i.e. an exothermic reaction, more energy is given out

in the bond making than is required for bond breaking.

The opposite of an exothermic reaction is an endothermic reaction … one in which energy is

taken in during the reaction.

What is meant by … an exothermic reaction? … an endothermic reaction?

Is energy released when covalent bonds … are broken? … are made?


45

Energy changes continued

Which step, bond breaking / bond making, must involve more energy … in an

exothermic reaction? ... in an endothermic reaction?

Give an example of an exothermic reaction.

Give an example of an endothermic reaction.


46

Combustion of hydrocarbons and alcohols

When any hydrocarbon ( alkane,alkene, cycloalkane) or alcohol is completely combusted,

carbon dioxide and water are formed.

HYDROCARBON OR ALCOHOL + OXYGEN CARBON DIOXIDE + WATER

You can prove water is produced by cooling the gases, using ice, and checking the boiling

point of the liquid produced is 100 0C. Also the freezing point of water and the melting point

of ice is 00C. The carbon dioxide gas will turn limewater from a clear to milky colour.

Word equations and balanced chemical equations can be written for the combustion of

hydrocarbons and alcohols.

Word equation

Ethane + oxygen Carbon dioxide + water

This can be changed into a chemical equation by substituting the words for the formula of

the substances ( remember that oxygen is a diatomic element and is written as O2 ).

Chemical equation

C2H6 + O2 CO2 + H2O

You need to balance the chemical equation by putting numbers in front of the formula.

C2H6 + 3 ½ O2 2 CO2 + 3 H2O

Or

2C2H6 + 7O2 4CO2 + 6H2O


47

Examples for you to try

Write the word and a balanced chemical equation for the complete combustion of

1. Propane

2. Methanol

3. Butene

4. Pentane

5. Ethanol


48

Energy from fuels Activity 2.6 Alkanes and alcohols can be used as fuels, e.g. natural gas is mainly methane, ethanol is mixed with

petrol for use in cars. A fuel releases energy on reaction with oxygen. This type of reaction, known as

burning (or combustion), is exothermic. In an exothermic reaction, more energy is released in bond

making than is required for bond breaking. The following apparatus can be set up to measure how much energy is given out when a fuel burns.

The energy released in the burning of a fuel can be calculated by using the heat energy to raise the

temperature of a known mass of water.

The heat released = c m ΔT

where c = specific heat capacity of water

= 4.18 kJ kg-1 oC-1

m = mass of water (in kg) absorbing heat

(1 cm3 of water has a mass of 1g

So 50cm3 50g 0.05kg)

ΔT = temperature change

Example: Calculate the heat released on the burning of a fuel that raises the temperature of

100 cm3 of water by 10.5 oC.

Heat released = c m ΔT

= 4.18 x 0.1 x 10.5

= 4.39 kJ

In the lab, the calculated energy released is less than the actual energy released because some energy

is lost to the surroundings, e.g. the container for the fuel and the air.

Remember to change

from g kg

you divide by 1000


49

Energy from fuels continued For each of the following results, calculate the heat energy released by the burning of

the fuel.

The temperature of 50 cm3 of water is increased by 12 oC.

The temperature of 250 cm3 of water is increased by 21.8 oC.




50

Energy from fuels continued

The energy released from the burning of different fuels can be compared by calculating the energy

released for the burning of one mole of each.

Alcohol Structural formula

methanol

727

ethanol

1367

propan-1-ol

2020

Butan-1-ol CH3CH2 CH2CH2OH

There is a fairly constant difference between the heat released per mole for any two successive

members of a homologous series. Since each pair differ by a - CH2 - group the bond breaking energy

and energy of bond making with oxygen is approximately constant for this group in different molecules.

Fill in the table by predicting a value for butan-1-ol.

Why is energy released in the burning of a fuel?

When calculating the heat energy released in the burning of a fuel in the lab, why is

the experimental value less than the actual value?

Heat released / kJ mol-1

CH3OH

CH3CH2CH2OH

CH3CH2OH


51

Calculating the specific heat capacity for liquids other than water

You could also be asked to calculate the specific heat capacity of a liquid other than water,

given the temperatures and volume of other liquid.

Try this example,

The energy released when an alcohol burns can be used to heat liquids other than water.

The data below was collected when the energy released, by burning an alcohol, was used to

heat a sodium chloride solution.

Energy released when the alcohol was burned (kJ) 13.3

Initial temperature oC 15

Final temperature oC 49

Mass of sodium chloride solution heated (g) 100

Calculate the specific heat capacity, in kJkg-1 oC-1, of the sodium chloride solution.

You may wish to use the data booklet to help you.

Show your working clearly.

national 5 chemistry - duncanrig secondary school · each carbon atom can form three bonds with...

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