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Curriculum: Irish Leaving Certificate (ILC)
Level: Ordinary Level (OL)
Subject: Chemistry
Chapter 5: Fuels and Heats of Reaction
Version: Complete Compiled Chapter
Prepared By: ExamsLogic Academic Team
Chapter 5 Overview
Learning Objectives
Describe the main sources of hydrocarbons: coal, natural gas and petroleum.
Explain methane formation by decomposition and its hazards.
Compare alkanes, alkenes and alkynes using structure, formula and properties.
Recognise aromatic hydrocarbons: benzene, methylbenzene and ethylbenzene.
Explain fractional distillation, petrol quality, knocking and octane number.
Describe cracking, reforming, lead alternatives and oxygenates.
Explain exothermic and endothermic reactions and simple heat of combustion ideas.
Describe ethyne and hydrogen as other chemical fuels.
OL Focus: Keep the chapter organised in this order: source → structure → refining → fuel quality → energy changes → other fuels. That makes exam answers much cleaner.
5.1 Sources of Hydrocarbons
Hydrocarbons are found mainly in coal, natural gas and petroleum. They are important because they burn to release useful energy.
Source
Main Idea
Examples / Notes
Coal
Solid fossil fuel formed from ancient plant material.
Used as a fuel and industrial raw material.
Natural gas
Mainly methane, with small amounts of other gases.
Used for heating, cooking and electricity generation.
Petroleum (crude oil)
Liquid mixture of hydrocarbons with different chain lengths.
Separated into useful fractions by fractional distillation.
Methane from Decomposition
Methane can be formed when animal waste and plant material decompose in the absence of oxygen. This happens in places such as slurry pits, marshes, refuse dumps and biogas systems.
Where methane forms
Slurry pits on farms
Coal mines
Refuse dumps / landfill sites
Marshes and swamps
Hazards of methane
Very flammable
Can form explosive mixtures with air
Danger in enclosed places such as slurry pits and mines
Greenhouse Effect Link: Methane is a greenhouse gas. It traps heat in the atmosphere and contributes to global warming.
Coal, natural gas and petroleum are the main syllabus sources of hydrocarbons.
5.2 Structure of Aliphatic Hydrocarbons
Aliphatic hydrocarbons are non-aromatic hydrocarbons. At Ordinary Level you need the main families: alkanes, alkenes and alkynes.
Family
Bonding
General Formula
Example
Saturated?
Alkanes
C–C single bonds only
CnH2n+2
Methane CH₄
Yes
Alkenes
At least one C=C double bond
CnH2n
Ethene C₂H₄
No
Alkynes
At least one C≡C triple bond
CnH2n−2
Ethyne C₂H₂
No
Alkanes: CₙH₂ₙ₊₂
Alkenes: CₙH₂ₙ
Alkynes: C₂H₂ is the key OL example
Required Structural Examples
Name
Condensed Structural Formula
Why It Matters
Hexane
CH₃(CH₂)₄CH₃
straight-chain alkane
Heptane
CH₃(CH₂)₅CH₃
octane scale reference fuel (0)
Octane
CH₃(CH₂)₆CH₃
petrol-range hydrocarbon
Cyclohexane
C₆H₁₂ ring
cyclic hydrocarbon
2,2,4-trimethylpentane
branched isomer of octane
isooctane, octane rating reference (100)
Examiner Tip: For Ordinary Level, you do not need every possible isomer. You do need to recognise that branched hydrocarbons can behave differently from straight-chain ones.
Physical Properties
Property
Trend / Rule
Physical state
Smaller hydrocarbons are gases; larger ones are liquids, then solids.
Boiling point
Usually increases as chain length increases.
Water solubility
Hydrocarbons are generally insoluble in water.
Solubility in non-polar solvents
Hydrocarbons dissolve better in non-polar solvents.
Practical Link: Methane, ethene and ethyne are all only very slightly soluble in water. This is why gases can be collected over water in many school experiments.
Test for Unsaturation
Bromine water is used to test for unsaturation.
Alkene / alkyne + bromine water → decolourised
Alkane + bromine water → no immediate colour change
Examiner Trap: Ethyne is also unsaturated, so it decolourises bromine water. Do not write that only alkenes do this.
5.3 Aromatic Hydrocarbons
Aromatic hydrocarbons contain the benzene ring. At Ordinary Level the key examples are benzene, methylbenzene and ethylbenzene.
Compound
Formula
Simple Description
Benzene
C₆H₆
A six-carbon ring aromatic hydrocarbon.
Methylbenzene
C₆H₅CH₃
Benzene ring with a methyl group attached.
Ethylbenzene
C₆H₅C₂H₅
Benzene ring with an ethyl group attached.
Physical Properties
Usually colourless liquids at room temperature.
Burn with smoky flames because of their relatively high carbon content.
Poorly soluble in water.
More soluble in organic / non-polar solvents.
These are the three aromatic hydrocarbons named directly in the OL syllabus.
Methylbenzene Solubility Demo: Methylbenzene does not mix with water, but it dissolves better in non-polar substances. This matches the “like dissolves like” rule.
5.4 Exothermic and Endothermic Reactions
Exothermic = heat released
Endothermic = heat absorbed
Type
What Happens?
Examples
Exothermic
Energy is released to the surroundings.
Combustion, neutralisation, respiration.
Endothermic
Energy is absorbed from the surroundings.
Thermal decomposition, photosynthesis.
Heat of Reaction and Heat of Combustion
The heat of reaction is the heat change during a chemical reaction. The heat of combustion is the heat released when one mole of a substance is completely burned in oxygen.
Examiner Tip: If you see the word combustion, oxygen must be involved.
Exothermic reactions end lower; endothermic reactions end higher.
Simple Fuel Comparison
Fuels are compared partly by how much energy they release per mole or per gram. A useful fuel should release plenty of energy and burn reasonably efficiently.
Common Practical Errors in Calorimetry: heat loss to surroundings, incomplete combustion, evaporation of fuel, and heat absorbed by the apparatus.
5.5 Oil Refining and its Products
Fractional Distillation
Crude oil is a mixture of hydrocarbons. It is separated by fractional distillation. Different hydrocarbons condense at different heights in the column because they have different boiling points.
Fraction
Typical Use
Relative Boiling Point
Refinery gases
bottled gas / heating
lowest
Petrol
cars
low
Naphtha
chemical feedstock
moderate-low
Kerosene
aircraft fuel
moderate
Diesel
diesel engines
moderately high
Fuel oil / bitumen
ships / roads
highest
Natural Gas and LPG
Natural gas is mainly methane, with smaller amounts of ethane, propane and butane.
LPG means liquefied petroleum gas and is mainly propane and butane.
Safety Point: Mercaptans are added to natural gas because natural gas is nearly odourless. Mercaptans give it a strong smell so leaks can be detected quickly.
Petrol, Knocking and Octane Number
Knocking is uneven or premature combustion in a petrol engine. The octane number tells how resistant petrol is to knocking.
Higher octane number = better resistance to knocking
Isooctane = 100, heptane = 0
Lead in Petrol and Alternatives
Lead compounds were once added to petrol to improve octane number, but this caused environmental and health problems. Alternatives include:
isomerisation – converting straight-chain hydrocarbons to branched-chain ones
dehydrocyclisation / reforming – producing ring compounds and aromatics with better fuel quality
catalytic cracking – converting long-chain hydrocarbons into shorter, more useful molecules
oxygenates such as MTBE – help cleaner burning and improved fuel quality
Common Mistake: Fractional distillation is a physical separation, but cracking and reforming are chemical changes.
5.6 Other Chemical Fuels
Ethyne
Ethyne (acetylene) is an alkyne fuel. It burns with a very hot flame and is used in welding and cutting metals.
2C₂H₂ + 5O₂ → 4CO₂ + 2H₂O
Hydrogen
Hydrogen is a possible clean fuel because burning hydrogen forms water.
2H₂ + O₂ → 2H₂O
How hydrogen is made
Explanation
Electrolysis of water
Electric current splits water into hydrogen and oxygen.
Steam reforming
Methane reacts with steam to produce hydrogen and carbon monoxide / carbon dioxide.
Industrial Uses and Fuel Potential
Hydrogen is used in chemical industry, for example in ammonia production.
It can be used as a fuel in rockets and in some fuel-cell technologies.
Its advantage is clean combustion to water, but storage and transport are difficult.
Advantage: hydrogen burns cleanly to produce water.
Limitation: hydrogen is difficult to store safely because it is very light and flammable.
5.7 Calorific Value and the Bomb Calorimeter
The syllabus also expects students to understand how fuel energy can be measured. The calorific value of a fuel tells us how much energy is released when a given mass of fuel burns completely.
Term
Meaning
Calorific value
The energy released per gram or per kilogram of fuel when it burns completely.
Bomb calorimeter
A device used to measure the energy released by burning a fuel sample.
Complete combustion
Burning in excess oxygen to form carbon dioxide and water.
A bomb calorimeter is used to compare the energy released by different fuels.
Exam idea: A fuel that releases more heat for the same mass has a higher calorific value.
Examiner Tip: If asked to compare fuels, mention both the amount of heat released and how completely they burn.
Exam Practice and Quick Review
Fast Summary Table
Topic
One-Line Memory Hook
Hydrocarbon sources
coal, natural gas, petroleum
Alkanes
single bonds, saturated
Alkenes
double bond, unsaturated
Ethyne
key OL alkyne and chemical fuel
Aromatic hydrocarbons
benzene ring compounds
Octane number
resistance to knocking
Exothermic
heat released
Endothermic
heat absorbed
Exam-Style Questions
1. Name the three main syllabus sources of hydrocarbons. coal, natural gas, petroleum
2. What is meant by the octane number of a petrol? a measure of the fuel's resistance to knocking
3. State one reason mercaptans are added to natural gas. to give natural gas a smell so leaks can be detected
4. Give one use of ethyne and one possible advantage of hydrogen as a fuel. ethyne: welding / cutting metals; hydrogen: burns to form water / cleaner fuel
Examiner Secret: In short-definition questions, use the exact syllabus phrase. For example, say “resistance to knocking” for octane number and “heat released when one mole burns completely in oxygen” for heat of combustion.
Common Mistakes to Avoid
Calling any carbon compound a hydrocarbon.
Forgetting that methane is a greenhouse gas.
Saying fractional distillation is a chemical reaction.
Forgetting benzene, methylbenzene and ethylbenzene in aromatic hydrocarbons.
Confusing exothermic and endothermic energy diagrams.
Forgetting that hydrogen can be produced by electrolysis and steam reforming.