Fuels- CIE iGCSE Chemistry Notes - New Syllabus
Fuels for iGCSE Chemistry Notes
Core Syllabus
- Name the fossil fuels: coal, natural gas and petroleum
- Name methane as the main constituent of natural gas
- State that hydrocarbons are compounds that contain hydrogen and carbon only
- State that petroleum is a mixture of hydrocarbons
- Describe the separation of petroleum into useful fractions by fractional distillation
- Describe how the properties of fractions obtained from petroleum change from the bottom to the top of the fractionating column, limited to:
(a) decreasing chain length
(b) higher volatility
(c) lower boiling points
(d) lower viscosity - Name the uses of the fractions as:
(a) refinery gas fraction for gas used in heating and cooking
(b) gasoline / petrol fraction for fuel used in cars
(c) naphtha fraction as a chemical feedstock
(d) kerosene / paraffin fraction for jet fuel
(e) diesel oil / gas oil fraction for fuel used in diesel engines
(f) fuel oil fraction for fuel used in ships and home heating systems
(g) lubricating oil fraction for lubricants, waxes and polishes
(h) bitumen fraction for making roads
Fossil fuels: coal, natural gas and petroleum
Fossil fuels: coal, natural gas and petroleum
Fossil fuels are energy-rich substances formed from the remains of ancient plants and animals that were buried and subjected to heat and pressure over millions of years.They are called “fossil” fuels because they originate from fossilised organic matter.
The three main fossil fuels are:
Coal
A solid fossil fuel mainly composed of carbon. It was formed from the remains of ancient plants that were buried in swamps and forests. Coal is burned to produce electricity and heat, but it produces large amounts of pollutants such as \( \text{CO}_2 \), \( \text{SO}_2 \), and particulates when combusted.
Natural gas
A gaseous fossil fuel primarily consisting of methane (\( \text{CH}_4 \)). It is often found above petroleum deposits and is used extensively for heating, cooking, and electricity generation. It is considered the “cleanest” fossil fuel because it produces less carbon dioxide per unit of energy compared to coal and petroleum.
Petroleum (crude oil)
A liquid fossil fuel composed of a complex mixture of hydrocarbons. It is extracted from underground reservoirs and then refined by fractional distillation to produce useful fuels and chemicals such as petrol, diesel, kerosene, and lubricants.
These fossil fuels are non-renewable resources, meaning they will eventually run out if used continuously. They are also the primary contributors to greenhouse gas emissions and climate change.
Example
Name the three main fossil fuels and state one use of each.
▶️Answer/Explanation
The three fossil fuels are:
Coal – used in power stations to generate electricity.
Natural gas – used for heating and cooking.
Petroleum – refined into fuels such as petrol and diesel for vehicles.
Methane
Natural gas is a fossil fuel found in underground reservoirs, often trapped with petroleum. Its main component is methane (\( \text{CH}_4 \)), which typically makes up about 70–90% of natural gas
- Small amounts of other hydrocarbons such as ethane (\( \text{C}_2\text{H}_6 \)), propane (\( \text{C}_3\text{H}_8 \)), and butane (\( \text{C}_4\text{H}_{10} \)) may also be present, along with trace gases like carbon dioxide and nitrogen.
- Methane is a simple hydrocarbon and is classified as an alkane.
- It is colourless, odourless (an artificial smell is added for safety), and highly flammable.
- Its combustion is an important source of energy:
\( \text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O} \)
This reaction releases a large amount of heat energy, making methane an excellent fuel for domestic heating, electricity generation, and as a feedstock in chemical industries.
Example
State the main component of natural gas and give one of its uses.
▶️Answer/Explanation
The main component of natural gas is methane (\( \text{CH}_4 \)).
Use: Methane is burned to release heat energy, which can be used for domestic heating, electricity generation, or as a feedstock in chemical industries.
Hydrocarbons
Hydrocarbons
Hydrocarbons are a class of organic compounds made up of only two elements: hydrogen (H) and carbon (C).
- They do not contain any other elements such as oxygen, nitrogen, or sulfur.
- The carbon atoms can join together to form chains, branched structures, or rings.
- The hydrogen atoms bond to the carbon atoms to satisfy carbon’s four bonds (tetravalency).
- Hydrocarbons are the main components of fossil fuels such as coal, natural gas, and petroleum.
- They release energy when burned, which makes them important fuels.
There are three main types of hydrocarbons:
- Alkanes – Saturated hydrocarbons with only single bonds, e.g. methane (\( \text{CH}_4 \)), ethane (\( \text{C}_2\text{H}_6 \)).
- Alkenes – Unsaturated hydrocarbons with at least one C=C double bond, e.g. ethene (\( \text{C}_2\text{H}_4 \)), propene (\( \text{C}_3\text{H}_6 \)).
- Alkynes – Unsaturated hydrocarbons with at least one C≡C triple bond, e.g. ethyne (\( \text{C}_2\text{H}_2 \)).
Hydrocarbons are chemically important because they act as the basis for many other organic compounds such as alcohols, carboxylic acids, and esters.
Example
Define a hydrocarbon and give one example each of an alkane, an alkene, and an alkyne.
▶️Answer/Explanation
A hydrocarbon is a compound made of only carbon and hydrogen atoms.
Example of an alkane: ethane (\( \text{C}_2\text{H}_6 \))
Example of an alkene: butene (\( \text{C}_4\text{H}_8 \))
Example of an alkyne: butyne (\( \text{C}_4\text{H}_4 \))
Petroleum
Petroleum
Petroleum, also called crude oil, is a naturally occurring liquid that is obtained from underground reservoirs.
- It is not a pure substance but a mixture of different hydrocarbons.
- The hydrocarbons present vary in the number of carbon atoms in their chains.
- The mixture includes both alkanes (mainly) and smaller amounts of cycloalkanes and aromatic hydrocarbons.
- These hydrocarbons differ in chain length, boiling point, volatility, and viscosity.
- Because it is a mixture, petroleum cannot be used directly and must be separated into useful fractions by fractional distillation.
- Petroleum is the most important source of fuels like petrol, diesel, kerosene, and also raw materials for plastics and chemicals.
Examples of hydrocarbons found in petroleum:
- Short-chain hydrocarbons (e.g. propane, butane) – more volatile, used as cooking gas and fuel for heating.
- Medium-chain hydrocarbons (e.g. octane in petrol) – used in vehicles.
- Long-chain hydrocarbons (e.g. heavy fuel oils, bitumen) – less volatile, used for ships, road surfacing, and lubricants.
Example
Why is petroleum described as a mixture and not a pure substance?
▶️Answer/Explanation
Petroleum is made up of many different hydrocarbons with varying chain lengths and properties, rather than just one compound. For example, it contains short-chain alkanes such as butane (\( \text{C}_4\text{H}_{10} \)), medium-chain alkanes such as octane (\( \text{C}_8\text{H}_{18} \)), and long-chain hydrocarbons such as bitumen. This is why it is considered a mixture.
Fractional distillation of Petroleum
Crude petroleum (crude oil) is a mixture of many different hydrocarbons, which cannot be used directly. It must be separated into simpler, useful fractions by a process called fractional distillation.
- Fractional distillation makes use of the fact that different hydrocarbons in petroleum have different boiling points.
- The crude oil is first heated until it vaporises (turns into a mixture of gases).
- The vapour is then passed into a tall fractionating column, which is hot at the bottom and cooler at the top (temperature gradient).
- Hydrocarbons with high boiling points condense at the bottom of the column.
- Hydrocarbons with low boiling points rise higher up the column before condensing.
- Each fraction collected contains a mixture of hydrocarbons with similar boiling ranges.
- The fractions collected are useful products such as gases, petrol, kerosene, diesel, fuel oils, lubricants, and bitumen.
Key steps in the process:
- Crude oil is vaporised by heating.
- The vapour enters the base of the fractionating column.
- As the vapour rises, the column temperature decreases from bottom to top.
- Different hydrocarbons condense at different heights depending on their boiling points.
- Fractions are collected at trays placed at different levels in the column.
Important point:
Fractional distillation does not produce pure compounds, but fractions containing a mixture of hydrocarbons with similar chain lengths and properties.
Example
Why do shorter-chain hydrocarbons condense at the top of the fractionating column, while longer-chain hydrocarbons condense at the bottom?
▶️Answer/Explanation
Shorter-chain hydrocarbons have lower boiling points because they have weaker intermolecular forces between their molecules.
They only condense at the cooler top of the column. Longer-chain hydrocarbons have stronger intermolecular forces, so they have higher boiling points and condense lower down in the hotter region of the column.
Properties of fractions obtained from petroleum
As we move from the bottom to the top of the fractionating column, the fractions show a clear trend in their physical properties. These differences arise because of the variation in chain length of the hydrocarbons present.
- Decreasing chain length: Fractions collected near the bottom contain hydrocarbons with long carbon chains. Fractions collected near the top contain hydrocarbons with shorter carbon chains.
- Higher volatility: Hydrocarbons at the top are more volatile, meaning they evaporate more easily. Hydrocarbons at the bottom are less volatile and remain as liquids or semi-solid substances.
- Lower boiling points: Short-chain hydrocarbons at the top have weaker intermolecular forces and therefore boil at lower temperatures. Long-chain hydrocarbons at the bottom have stronger intermolecular forces and boil at much higher temperatures.
- Lower viscosity: Hydrocarbons at the top flow easily (runny liquids or gases). Hydrocarbons at the bottom are thick, sticky, and viscous (e.g., bitumen).
Overall trend:
From bottom to top, hydrocarbons change from long-chain, high boiling, viscous, less volatile liquids → to short-chain, low boiling, runny, highly volatile liquids and gases.
Example
Why does petrol (gasoline) evaporate much faster than diesel oil, even though both come from crude oil?
▶️Answer/Explanation
Petrol contains shorter-chain hydrocarbons, which have weaker intermolecular forces and therefore lower boiling points.
This makes petrol more volatile, so it evaporates easily. Diesel oil contains longer-chain hydrocarbons with stronger intermolecular forces, so it has a higher boiling point and is less volatile.
Uses of fractional distillation of petroleum products
Uses of fractional distillation of petroleum products
Each fraction obtained from fractional distillation has different uses, depending on its chain length and physical properties.
- Refinery gas fraction: Used mainly as a fuel for heating and cooking. It is stored in cylinders as liquefied petroleum gas (LPG).
- Gasoline / petrol fraction: Used as fuel in cars and other vehicles with internal combustion engines. It burns easily and releases a large amount of energy.
- Naphtha fraction: Used as a chemical feedstock in the petrochemical industry. It is the starting material for producing plastics, detergents, medicines, and synthetic fibres.
- Kerosene / paraffin fraction: Used as jet fuel in airplanes. Also used in lamps and domestic heating in some regions.
- Diesel oil / gas oil fraction: Used as fuel in diesel engines (cars, trucks, buses). Provides more energy per litre than petrol, making it suitable for heavy vehicles.
- Fuel oil fraction: Used as fuel in ships. Also used for home heating systems in some countries.
- Lubricating oil fraction: Used to make lubricants for reducing friction in engines and machinery. Also used in producing waxes and polishes.
- Bitumen fraction: Used for making road surfaces. Also used in roofing materials and waterproofing.
Example
Explain why kerosene is suitable for use as jet fuel while bitumen is not.
▶️Answer/Explanation
Kerosene contains hydrocarbons of medium chain length, giving it a moderate boiling point and good volatility, so it burns efficiently in jet engines.
Bitumen, however, contains very long-chain hydrocarbons with high boiling points. It is thick, viscous, and does not vaporise easily, making it unsuitable as a fuel but ideal for road surfacing.