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NATIONAL 5 CHEMISTRY
UNIT 2
NATURE’S CHEMISTRY
Contents
Homologous series
• Fuelling Scotland’s future
• Alkanes
• Alkenes
• Cycloalkanes
Fuelling Scotland’s Future
• As North Sea gas and oil runs out Scotland will
become dependent on imported gas unless new
sources can be identified and exploited.
• In the second half of the 19th century and the start
of the 20th century Scotland had a thriving shale oil
industry.
• In 1847 the Scottish chemist James Young prepared
"lighting oil," lubricating oil and wax from cannel
coal and since 1862 from torbanite. In 1850 he
patented the process of cracking oil.
• Commercial scale shale oil extraction in Scotland
started in 1859 by Robert Bell in Broxburn, West
Lothian.
• It was not until 1859 that Amercians
struck free-flowing oil.
• Oil shale was mined in the Lothian’s until
1962 but the industry died out because
it could no longer compete with
imported oil.
Crude oil and hydrocarbons
• Crude oil and its improved industrial
extraction and processing produces a
wide variety of energy-rich compounds
called hydrocarbons.
• Crude oil – is a mixture of unprocessed
hydrocarbons (of varying size/length)
• Hydrocarbons – are compounds that
contain the element carbon and
hydrogen.
Crude oil
Hydrocarbons
How does fractional
distillation work?
Forces between molecules
Fractional distillation
Combustion
The term ‘combustion’ is used to describe
the process of burning (reaction with oxygen).
In order to burn fuels require oxygen.
Increasing the concentration of O2(g)
increases the rate of burning.
When combustion takes place the fuel
reacts with oxygen in the air and energy is
given out.
This means that combustion is an
exothermic reaction.
Hydrocarbons
• Many of the fuels that we use are fossil
fuels.
• Fossil fuels were formed millions of years
ago from material that was once living.
• There are three common fossil fuels - coal,
oil and natural gas.
• The chemical compounds which are found
in oil and natural gas are called
hydrocarbons.
• These compounds are made up using the
elements hydrogen and carbon only.
Combustion of Hydrocarbons
• When hydrocarbons burn in a plentiful
supply of oxygen, the products are carbon
dioxide and water.
• This is complete combustion and the
products will be the same for any
hydrocarbon burning in a good supply of
oxygen.
hydrocarbon  oxygen  carbon dioxide  water
Products of Combustion
A pump is
used to
draw the
gases
through the
apparatus
A burning hydrocarbon
The first test tube
is placed under the
funnel
is surrounded by ice
The second test
tube has lime
water
Tests for products
• Carbon dioxide turns limewater milky.
• Water boils at 100oC and freezes at 0oC.
(or by using cobalt chloride paper which
turns from blue to pink in the presence of
water).
• The fact that water (hydrogen oxide) is
produced proves that the fuel contains
hydrogen. The fact that carbon dioxide is
produced proves that the fuel contains
carbon.
Quick Quiz
What is a fuel?
• Burns to release energy.
What is the test for carbon dioxide?
• Turns limewater cloudy.
What is combustion?
• Burning with oxygen.
What is an exothermic reaction?
• A reaction that releases energy.
What is the proportion of oxygen to nitrogen in
the air?
• 1:4
Starter
1. A mixture of ethanol (boiling point 79oC)
and water is distilled. Which liquid will
Ethanol
boil first?
2. What is a fraction?
A group of hydrocarbons with similar chemical properties and of a similar size
3. Which are smaller, molecule in petrol or
molecules in paraffin? Paraffin
4. What is bitumen used for? Roads and roofing
5. The hydrocarbon butane has the LPG
molecular formula C4H10. In which (liquefied
petroleum
fraction will butane be found?
gas)
Aims:
• Discuss the pollution problems
caused by the burning of
hydrocarbons.
In the presence of oxygen,
hydrocarbons burn to produce carbon
dioxide and water.
Where does the carbon dioxide and
water come from?
CH4 + 2O2
CO2 + 2H2O
Carbon Dioxide
• Carbon dioxide is a
greenhouse gas.
• This means that it lets the
heat from the sun penetrate
through to the earth, but
doesn’t let it back out.
• The level of CO2 produced
every year is increasing.
• Therefore the temperature
of the earth is increasing.
• Effect is increased levels of
seas/rivers causing flooding.
Carbon Monoxide
• Hydrocarbons burned in insufficient
oxygen can produce carbon and carbon
monoxide, which is a very poisonous gas.
• Carbon monoxide destroys blood’s ability
to carry oxygen.
Sulphur Dioxide
• Some fossil fuels contain small traces of
sulphur.
• Sulphur dioxide is also a poisonous gas.
• Sulphur dioxide can dissolve in rain to
produce acid rain.
• Acid rain leads to corrosion of stonework
and metals.
• Most SO2 is produced by coal and oil fired
power stations.
• Removing sulphur compounds reduces air
pollution.
Nitrogen
oxides
•In car engines the air (N
2
and O2) around the spark
plug is provided with
enough energy to cause a
reaction between nitrogen
and oxygen.
•Oxides of nitrogen are
produced (NO2 and NO).
•NO2 dissolves in rain to
produce nitric acid.
How Do We Overcome Pollution?
• Catalytic converters in car exhausts
change harmful gases  less harmful
gases.
• E.g. CO  CO2
NO and NO2  N2
– Catalytic converters contain transition
metals (Pd, Rh or Pt on honey combed
structures).
• Lean burn engines increase the ratio of
air:fuel and so petrol is burned more
efficiently. This causes a decrease in CO
and unburned hydrocarbons.
• Leaded compounds have been removed
from petrol to reduce pollution.
• Low-sulphur petrol produced.
• Sulphur dioxide removed from gases
leaving power stations.
• Lime added to lakes to neutralise the
effect of acid rain.
Quick Quiz
1. Why does the production of carbon dioxide and water
on burning, indicate the presence of carbon and
hydrogen in the original fuel?
2. What does ‘incomplete combustion’ mean?
3. Name a poisonous gas which can be produced as a
result of incomplete combustion of petrol in a car
engine.
4. Describe some of the effects of acid rain.
5. What does a catalytic converter do, and what type
of material is it made from?
Distillation
• This process is used to
separate substances due
to them having different
boiling points.
• E.g. water and alcohol can
be separated using this
process.
• Alcohol boils off first at
78oC and pure water is
left behind.
Products from Crude Oil
• Crude oil (petroleum) is a mixture of
hydrocarbons.
• The hydrocarbons are separated into
smaller mixtures (fractions) by a
technique called fractional distillation.
• The hydrocarbons can be separated into
different fractions using this process
because they have different boiling points.
Uses and Properties of
Fractions from Crude Oil
Refinery gas
Fuel gases
Gasoline
(Petrol)
Fuel for cars
Kerosine
(Paraffin)
Aircraft fuel
Gas Oils
(Diesel oil)
Fuel for
buses/taxis and
cars.
Fuel oil for ships / power
stations / heating
Residue
Lubrication oils and waxes
Bitumen / tar for roads
Boiling
pt. (oC)
Refinery
gas
1-4
Petrol/
Gasoline
(Naptha)
5-12
Petrol and
petrochemicals
Paraffin/
Kerosene
9-15
Heating and fuel
for aeroplanes
180250
Diesel
(Gas oil)
15-25
Fuel for
lorries/trains
250350
Residue
>25
Lubricants,
waxes, road tar
>350
Fuel gas
Flammabillity
Viscosity
Ease of
Evaporation
DECREASES
Properties
INCREASES
Uses
DECREASES
Fraction
Chain
length
(no.of
carbon
atoms)
<20
20-180
Fractions
AT THE TOP OF THE
COLUMN
AT THE BOTTOM OF
THE COLUMN
•Short carbon chains
•Long carbon chains
•Light molecules
•Heavy molecules
•Low boiling points
•High boiling points
•Gases & very runny liquids
•Thick, viscous liquids
•Very volatile
•Low volatility
•Highly flammable
•Not very flammable
•Light colour
•Dark colour
Word Bank
• Viscosity describes how well a liquid pours e.g.
treacle is very viscous, it is thick and pours very
slowly. Large hydrocarbons are more viscous than
small hydrocarbons.
• Flammability is how easily a substance will catch
fire. Small hydrocarbons are more flammable than
large hydrocarbons.
• Boiling point is a change of state from liquid to gas.
• Fraction is a group of hydrocarbons with boiling
points within a given range.
Catalytic Cracking
Changing large hydrocarbons into smaller
more useful hydrocarbons.
e.g. C10H22
Catalyst
C8H18 + C2H4
During this type of reaction a smaller alkane
(hydrocarbon) is always produced.
Catalysts
• A catalyst is a substance which:
– Increases rate of reaction
– or
– Allows a reaction to occur at a lower
temperature
N.B. A catalysts does not get used up during
a chemical reaction. Therefore it is never
included in a chemical reaction.
North Sea oil – How much left
for Scotland/UK?
• Are there alternatives? And what are the pros and
cons? Remember to analyse where your source
information comes from [is it without bias?]
What is Fracking and why is it controversial?
http://www.bbc.co.uk/news/uk-14432401
Fracking in the news!
Alternatives in the news
•Hydrocarbon
•families
Aims:
Discuss the Alkanes, which are a family of
hydrocarbons.
The name for the first eight alkanes.
The structural formula of the first eight
alkanes.
The molecular formula of the first eight
alkanes.
The general formula for the alkane family.
Hydrocarbons
• There are 3 hydrocarbon sub-groups:
– Alkanes
– Alkenes
– Cycloalkanes
Homologous
series
Same second
name
Look similar
Alkanes
•
•
•
•
•
Homologous series
Look similar
Contain covalent bonds.
All alkanes end in ‘ane’.
Remember each carbon must form 4 bonds.
Each hydrogen must form 1 bond only.
Methane
CH4
H
H
C
H
H
Ethane
C2H6
H
H
H
C
C
H
H
H
Propane
H
C3H8
H
H
H
C
C
C
H
H
H
H
Name of
Alkane
Methane
Molecular
Formula
Full Structural
Formula
Shortened
Structural Formula
H
H
C
CH4
H
CH4
H
Ethane
H
Propane
H
Butane
H
Hexane
H
Octane
H
C
C
H
H
H
H
H
H
C
C
C
H
H
H
H
H
H
H
C
C
C
C
H
H
H
H
H
H
H
H
H
H
C
C
C
C
C
H
H
H
H
H
H
H
H
H
H
H
H
C
C
C
C
C
C
H
H
H
H
H
H
H
H
H
C
C
C
C
C
C
C
H
H
H
H
H
H
H
H
Heptane
H
H
H
Pentane
H
H
H
H
H
C2H6
CH3CH3
C3H8
CH3CH2CH3
C4H10
CH3CH2CH2CH3
C5H12
CH3CH2CH2CH2CH3
C6H14
CH3CH2CH2CH2CH2CH3
H
C7H16
H
H
H
H
H
H
H
H
H
C
C
C
C
C
C
C
C
H
H
H
H
H
H
H
H
H
C8H18
CH3CH2CH2CH2CH2CH2CH3
CH3CH2CH2CH2CH2CH2CH2
CH3
• The molecular formula tells you the number
of carbon and hydrogen atoms in each
molecule.
• The full structural formula shows relative
position of the atoms in the molecule and
the bonds holding the atoms together.
• The shortened structural formula does not
show all of the bonds in the molecule.
Alkanes – Homologous series
The alkanes are a homologous series. This
means that:
1. All alkanes can be represented by a general
formula which is:
CnH2n+2
2.They all have similar chemical properties
3.There is a link between their physical
properties and their melting and boiling
points.
The following rhyme might help you to remember
the order of the hydrocarbons.
•Monkeys
•Eat
•Peanut
•Butter
•Pieces
•Horses
•Have
•Oats
Branched Alkanes
• Branched: When not all of the carbon atoms are in
a straight chain.
H
H C H
H
H
H
H
H
C
C
C
C
H
H
H
H
butane
H
H
H
H
H
C
C
C
C
H
H
H
H
H
2-methylpropane
Each molecule has:
Same molecular formula but a different structural formula
Different names
Naming Branched Alkanes
Rules
1. Look for the longest carbon atom chain
to give the name of the alkane.
2. Number the carbon atoms from the end
closest to the branch.
3. Identify the branch CH3 is a Methyl group
 C2H5 is an Ethyl group
4. Use prefixes to indicate how many of a
particular branch (di-2, tri-3)
5. Show the position of each branch with a
number placed in front of its name.
6. If more than one branch, names are put
in alphabetical order (ethyl before
methyl).
Example 1
Methyl branch
1
2
3
4
Number carbon atoms from one end closest
to branch 2 Methyl butane
Example 2
Methyl branch
C
1
C
2
C3
C4
C5
C6
C
C
Ethyl branch
 3 Ethyl, 2 Methyl hexane
Example 3
Methyl branch
3
2
1
Methyl branch
 2,2-dimethyl propane
Drawing structures from
systematic names
Example 1
 3-methyl pentane
1
2
3
4
5
Example 2
 4 ethyl, 2,2 dimethyl octane
C
1
C
C
2
C
C
3
C
4
C
C
C
5
C
6
C
7
C
8
Name the following structures
1.
4-ethyl-2-methylhexane
2.
4-ethyl-3,3-dimethylheptane
3.
3-ethyl-4-methylhexane
Draw the following structures
Draw a structural formula for each of the
following alkanes
1. 3-ethylhexane
1. 2, 2, 4-trimethylpentane
1. 2-methyl, 3-ethylheptane
1. 4, 4-dimethyloctane
Lesson Starter
1. State the bonding present in alkanes.
2. State the name of the third member of the
alkanes.
3. State the molecular formula of the sixth member
of the alkanes.
4. Draw the molecular structure of pentane.
5. State the general formula for the alkanes.
Homologous
series
Same second
name
Look similar
Aims:
Discuss the Alkenes, which are a family of
hydrocarbons.
The name for alkenes up to C8.
The structural formula for the alkenes up to C8.
The molecular formula for the alkenes up to C8.
The general formula for the alkene family.
Ethene
C2H4
H
H
H
C
C
H
H
H
Propene
H
C3H6
H
H
H
C
C
C
H
H
H
H
The Alkenes
• The alkenes contain a carbon to carbon double
bond and so they are called unsaturated
hydrocarbons.
• The names of all members end in ‘ene’.
• The alkenes generally have lower melting and
boiling points than their equivalent alkanes. The
smaller alkenes are gases, but as the molecules
increase in size they become liquids and
eventually solids.
• Their melting and boiling points also increase as
we move from ethene  hexene.
Name of Molecular
Alkene
Formula
Full Structural
Formula
Shortened
Structural Formula
Ethene
C2H4
CH2=CH2
Propene
C3H6
CH2=CHCH3
Butene
C4H8
CH2=CHCH2CH3
Pentene
C5H10
CH2=CHCH2CH2CH3
Hexene
C6H12
CH2=CHCH2CH2CH2CH3
Alkenes – Homologous series
1. The alkenes can also be represented by a
general formula which is:
CnH2n
2. They all have similar chemical properties
3. There is a link between their physical
properties and their melting and boiling
points.
Naming Straight Chain Alkenes
Rules
1. Identify the longest carbon chain.
2. Number carbon atoms starting at the end
nearest the double bond.
3. Identify the number of the carbon atom
where the double bond starts, and insert
it into the name.
Examples
1.
2.
 But-2-ene
 But-1-ene
3.
 Hex-3-ene
4.
 2 MethylProp-1-ene
Starter
1. What is the general formula of the
alkanes?
2. What is the general formula of the
alkenes?
3. How can you distinguish between the
alkanes and the alkenes?
4. What is the different between the
bonding in these two families?
Aims:
1. Discuss the cycloalkane family.
2. Discuss isomers.
Same first
name
All ring
structures
Homologous
series
Look similar
Cycloalkanes
• The cycloalkanes are another series of
hydrocarbons.
• The carbon atoms form a ring and all
carbon to carbon bonds are saturated
(carbon to carbon single bonds).
Cyclopropane
H
H
C3 H6
C
H
H
C
H
C
H
Cyclobutane
H
H
H
C
C
H
H
C
C
H
H
H
C4 H8
Name of
Cycloalkane
Cyclopropane
Cyclobutane
Cyclopentane
Molecular
Formula
C3H6
C4H8
C5H10
Full Structural
Formula
Shortened
Structural Formula
H2
C
H2C
CH2
H2
C
H2C
C
H2
CH2
H2
C
CH2
H2C
C
H2
CH2
H2
C
Cyclohexane
C6H12
H2C
CH2
H2C
CH2
C
H2
Cycloalkanes – Homologous series
1. All alkanes can be represented by a
general formula which is:
CnH2n
2.They all have similar chemical properties
3.There is a link between their physical
properties and their melting and boiling
points.
Isomers
-Same molecular formula but
different structural formula.
e.g. C3H6 is the molecular formula for …
Propene
Cyclopropane
H
H
H
C
H
C
H
C
and
C
H
H
H
H
H
H
C
H
C
H
Butene and cyclobutane are isomers because
they both have the molecular formula C4H8
but their structures are different.
There are two isomers of butane, both
with the molecular formula C4H10.
Butane
2-methylpropane
The larger the hydrocarbon, the more isomers
are possible.
Can you try to draw 3 isomers for pentane?
Quick Quiz
1. What is the molecular formula for
cyclopentane?
2. Draw the structure of cyclobutane.
3. What is an isomer?
4. Draw three isomers of pentane.
Starter
1.
2.
3.
4.
Name the 6th member of the alkanes.
Name the 1st member of the alkenes.
Name the 2nd member of the cycloalkanes.
What is the general formula for the
cycloalkanes?
5. What bonding is present in the
cycloalkanes?
6. What is a homologous series?
Saturated or Unsaturated
• Saturated hydrocarbon:
• a hydrocarbon that contains carbon to
carbon single covalent bonds only.
• Alkanes and cycloalkanes are saturated
compounds
• Unsaturated hydrocarbon:
• a hydrocarbon that contains at least one
carbon to carbon double covalent bond.
• Alkenes are unsaturated compounds
Testing for Unsaturation
• Bromine water is used in order to
determine whether an unknown
hydrocarbon is saturated or unsaturated.
Unsaturated hydrocarbons rapidly turn
orange/brown bromine water colourless.
• Saturated hydrocarbons will have no
effect on bromine water.
Addition Reactions
• A C=C is very reactive and will easily break
to give a carbon-to-carbon single covalent
bond.
When Addition occurs…
alkene + hydrogen  alkane
e.g.
ethene + hydrogen  ethane
+ H2 
H
H
H
C
C
H
H
H
Carbon to carbon double bond breaks. Bond
between hydrogen molecule breaks and H
atoms will join across the double bond.
Butene +
H2

+ H2 
Butane
H
H
H
H
H
C
C
C
C
H
H
H
H
 Addition of hydrogen is also called
hydrogenation.
H
Addition of bromine - Bromination
1,2 dibromopropane
(saturated)
The bromine molecule adds on across the double
bond to give a Br atom on each atom either side
of where the double bond used to be.
Addition of Water - Hydration
Adding water to an alkene gives the
corresponding alcohol.
Alkene + water 
H-OH
(H2O)
Alcohol
Making Plastics
• The most important use of alkenes and
alkene derivatives is as feed stocks for
the plastics industry…This is dealt with
in Unit 3 (additional polymerisation)…so
we will leave it till.
Contents
Everyday consumer products
• Alcohols
• Carboxylic acids
• Esters
• Energy from fuels
Alkanols
(Alcohols)
Alcohols
• Look like alkanes but contain a hydroxyl
(OH) group.
• names similar to alkanes but ending in ‘OL’,
and each contains the hydroxyl group, OH,
in place of a hydrogen atom.
e.g. second alcohol is ethanol (C2H5OH)
Shortened structural
formula: CH3CH2OH
Name of
alkanol
Molecular
formula
Methanol CH3OH
Ethanol
C2H5OH
Propanol C3H7OH
Butanol
C4H9OH
Full structural
Formula
Shortened structural
formula
CH3OH
CH3CH2OH
CH3CH2CH2OH
CH3CH2CH2CH2OH
Pentanol C5H11OH
CH3CH2CH2CH2CH2OH
Hexanol C6H13OH
CH3CH2CH2CH2CH2CH2O
H
Heptanol C7H15OH
CH3CH2CH2CH2CH2CH2CH2OH
C8H17OH
CH3CH2CH2CH2CH2CH2CH2CH2OH
Octanol
Naming Alkanols
Rules
1. Longest chain containing OH gives alkane
name (replace e with ol)
2. Number carbon atoms in the chain start
from the end closest to the OH.
3. For chains of 3 or more carbons name the
position of the OH must be given.
4. Any branches must be named and
numbered.
Propan-2-ol
Dehydration of Alkanols
Dehydration= removal of water
Alcohol  equivalent Alkene + water
Butan-1-ol


But-1-ene + water
+
O
H
H
Alkanoic Acids
• They look like alkanes but contain a
carboxyl (COOH) group.
• Same names as alkanes but end in “oic”.
e.g. first alkanoic acid is METHANOIC ACID.
e.g.
 Full structural formula
Carboxyl functional group
HCOOH
Or
HCO2H
 Shortened structural formula
Name of
alkanoic
acid
Molecular
formula
Methanoic
acid
HCOOH
Ethanoic
acid
CH3COOH
Propanoic
C2H5COOH
acid
Butanoic
acid
C3H7COOH
Full structural
formula
Shortened
structural
formula
HCOOH
CH3COOH
CH3CH2COOH
CH3CH2CH2COOH
or
CH3(CH2)2COOH
Name of
alkanoic
acid
Molecular
formula
Pentanoic
acid
C4H9COOH
CH3-(CH2)3-COOH
Hexanoic
acid
C5H11COOH
CH3-(CH2)4-COOH
Heptanoic
acid
C6H13COOH
CH3-(CH2)5-COOH
Octanoic
acid
C7H15COOH
CH3-(CH2)6-COOH
Full structural
formula
Shortened
structural
formula
Naming Alkanoic Acids
Rules
1. Count the number of atoms in the
carbon chain to give the name of the
parent alkanol.
2. Remove the –ol ending of the parent
alkanol and replace it by –oic acid.
Esters
Esters
 Are covalent compounds that contain carbon,
hydrogen and oxygen.
 Have a characteristic smells (e.g. pear drops)
 Are insoluble in water.
 names end in –OATE and contain the ester
linkage –COO-, shown below.
O
C
O
Formation of esters
 Esters are the products of reactions
between carboxylic acids (alkanoic acids) and
alcohols (alkanols).
 The alkanol loses an –H and the alkanoic
acid loses the –OH group.
 molecules join together to form an ester
molecule with a water molecule.
 the reaction between ethanoic acid and
methanol can be represented as shown:
O
CH 3
C
OH
HO
CH 3
ethanoic acid
methanol
(alkanoic acid)
(alkanol)
Ester
link
O
CH 3
C
O
ester
CH 3
+
H2O
Ethanoic Acid
Ethanol
Ester link
Ethyl Ethanoate
Water
Naming Esters
1. The names for esters are based on the
alkanol and alkanoic acid from which they are
made.
2. Esters names are usually written as two
words of the type alkyl alkanoate.
3. The first part of the name comes from the
alkanol with the –ol ending removed and –yl
added.
4. The second part of the name comes from
the alkanoic acid with the –oic acid ending
changed to –oate.
Alkanol
Alkanoic acid
Name of ester
Methanol
Methanoic acid
Methyl methanoate
Ethanol
Methanoic acid
Ethyl methanoate
Methanol
Ethanoic acid
Methyl ethanoate
Ethanol
Ethanoic acid
Ethyl ethanoate
Methanol
Propanoic acid
Methyl propanoate
Ethanol
Propanoic acid
Ethyl propanoate
Reactions of Carbon Compounds
Many hydrocarbons take part in chemical
reactions.
We have already discussed 1-3
1. Addition
2. Cracking
3. Production of Ethanol
4. Making and Breaking Esters
Making and Breaking Esters
Making Esters
• Esters are made by a condensation
reaction between carboxylic acid and an
alcohol.
• This can also be called an esterification
reaction.
• In this reaction a water molecule is
eliminated from the functional groups of
the carboxylic acid (COOH) and alcohol
(OH).
An ester link is formed by the reaction of
the hydroxyl (OH) functional group and the
carboxyl (COOH) functional group.
O
CH3
C
+
OH
H
CH3
O
Ester link
O
CH3
C
O
+
CH3
H
OH
• Condensation reaction is slow at room
temperature and yield of the ester is low.
• Rate can be increased by heating reaction
mixture and by using concentrated
sulphuric acid as a catalyst.
• Evidence that an ester is formed is its
typical sweet smell, and that is appears
as a solid/oily liquid on the water.
• The process is reversible i.e it operates
in both directions. This means it is
possible to break the ester down to the
alkanol and alkanoic acid that made it.
Breaking Esters
• An ester can be broken down into its
parent alcohol and carboxylic acid.
• This involves heating the ester with water
and so it is called a hydrolysis reaction.
• Hydrolysis reactions are the reverse of
condensation reactions.
• This is also a reversible reaction.
Remember...
• Condensation reaction makes an ester and
water is formed.
• Hydrolysis reaction breaks an ester and
water is used up.
• These are reverse reactions.
Energy from Fuels
• Experimental Determination of Enthalpy Changes
Many chemical reactions require complicated methods for calculating the
enthalpy changes but a simple determination can be done in the laboratory – the
enthalpy of combustion of fuels can be determined by heating water with
burning alcohol and measuring the temperature rise. The energy from the flame
is that which is absorbed (ignoring heat losses for the purpose of simplifying
the calculations!) and this can be calculated from a formula:
•
Energy absorbed (kJ) = C x m x ∆T
where:
• C = Specific heat of water, 4.18 kJ kg−1 (°C−1)
• m = Mass of water in kg
• ∆T = Temperature rise of water in °C
• Bitesize revision
• Enthalpy experiment
National 5 Chemistry + Higher at the end
Enthalpy of combustion.
The enthalpy of combustion of a substance is the amount of energy given out
when one mole of a substance burns in excess oxygen.
Worked example 1.
0.19 g of methanol, CH3OH, is burned and the heat energy given out increased the
temperature of 100g of water from 22oC to 32oC.
Calculate the enthalpy of combustion of methanol.
( c is specific heat capacity of water,
Use DH = cmDT
m is mass of water in kg, 0.1 kg
DH = -4.18 x 0.1 x 10
DT is change in temperature in oC, 10o)
DH = - 4.18 kJ
Use proportion to calculate the amount of heat given out when
1 mole, 32g, of methanol burns.
0.19 g
So
32 g
4.18 kJ kg-1 oC-1)
Higher Grade Chemistry

-4.18 kJ

32/
0.19
x –4.18
= -704 kJ
Enthalpy of combustion of methanol is –704 kJ mol-1.
Calculation using enthalpy of combustion
Calculations for you to try.
1.
0.25g of ethanol, C2H5OH, was burned and the heat given out raised the
temperature of 500 cm3 of water from 20.1oC to 23.4oC.
Use DH = cmDT
DH = 4.18 x 0.5 x 3.3
= - 6.897kJ
Use proportion to calculate the enthalpy change when 1 mole, 46g, of ethanol burns.
Higher Grade Chemistry
0.25 g

-6.897 kJ
So 46g

46/
0.25
x -6.897
= -1269 kJ mol-1.
2. 0.1 moles of methane was burned and the energy given out raised the temperature of
200cm3 of water from 18oC to 28.6oC. Calculate the enthalpy of combustion of methane.
Use DH = -cmDT
DH = -4.18 x 0.2 x 10.6
= - 8.86 kJ
Higher Grade Chemistry
Use proportion to calculate the enthalpy change when 1 mole
of methane burns.
0.1 mol 
-8.86 kJ
So 1mol

1/
0..1
x -8.86
= -88.6 kJ mol-1.