Fuels Revision Notes

Fossil fuels are formed from the remains of plants and animals that lived millions of years ago. Over geological time, these organic materials were buried under layers of sediment and subjected to heat and pressure, transforming them into energy-rich substances.

Coal, natural gas, and petroleum are the three main fossil fuels. They are classified based on their physical state and chemical composition: coal is solid, natural gas is gaseous, and petroleum is liquid at room temperature.

These fuels are primarily composed of hydrocarbons, compounds made of hydrogen and carbon atoms, which release energy when combusted.

• Formed from ancient organic matter under heat and pressure over millions of years.
• Non-renewable energy sources due to their slow formation rate.
• Main fossil fuels: coal (solid), natural gas (gas), petroleum (liquid).

Coal is a black or dark brown solid fossil fuel mainly composed of carbon, with small amounts of hydrogen, oxygen, nitrogen, and sulfur. It is formed from plant remains that accumulated in swampy areas and were compressed over millions of years.

Coal is primarily used for electricity generation in power stations and for industrial heating processes.

When coal burns, it reacts with oxygen to produce carbon dioxide and water vapour, releasing energy. The combustion reaction can be represented as: C + O₂ → CO₂ (where C represents carbon in coal).

• Solid, black or dark brown in appearance.
• Mainly carbon with impurities.
• Used for electricity generation and industrial heating.
• Combustion: C + O₂ → CO₂ (balanced equation).

Natural gas is a mixture of gases, predominantly methane (CH₄), with smaller amounts of other hydrocarbons such as ethane and propane.

It is colourless and odourless in its pure form but an odourant is added for leak detection.

Natural gas is widely used for domestic heating, cooking, and electricity generation due to its cleaner combustion compared to coal and petroleum.

• Gaseous at room temperature, mainly methane (CH₄).
• Colourless and odourless; odourant added for safety.
• Used for heating, cooking, and electricity generation.
• Combustion reaction: CH₄ + 2O₂ → CO₂ + 2H₂O (balanced).

Petroleum, also known as crude oil, is a complex mixture of hydrocarbons in liquid form at room temperature.

It is extracted from underground reservoirs and refined to produce fuels such as petrol, diesel, kerosene, and other products.

Petroleum is mainly used as a fuel for transportation and as a raw material in the chemical industry.

• Liquid mixture of hydrocarbons.
• Extracted by drilling and refined by fractional distillation.
• Used for vehicle fuels and industrial feedstocks.
• Combustion releases energy and produces CO₂ and H₂O.

Each fossil fuel has distinct physical properties and uses based on its state and composition.

Coal is solid and mainly used for electricity generation.

Natural gas is gaseous and used for heating and electricity with cleaner combustion than coal or petroleum fuels, producing less soot and sulfur dioxide emissions when burnt properly.

• Coal: solid, electricity generation.
• Natural gas: gas, heating and electricity.
• Petroleum: liquid, transportation fuels and industry.

Methane is the simplest alkane hydrocarbon with the molecular formula CH₄. It consists of one carbon atom covalently bonded to four hydrogen atoms in a tetrahedral arrangement. The structural formula is:

H | H–C–H | H Methane is a colourless, odourless gas under standard conditions. It is highly flammable and burns with a clean blue flame, producing carbon dioxide and water when combusted completely.

• Molecular formula: CH₄
• Structural formula: tetrahedral shape with four C–H single bonds
• Physical state: colourless, odourless gas at room temperature
• Highly flammable

Natural gas is a fossil fuel formed from the decomposition of organic matter over millions of years under high pressure and temperature underground. It is primarily composed of methane, which typically makes up about 70% to 90% of natural gas by volume.

Other components of natural gas may include small amounts of ethane, propane, butane, carbon dioxide, nitrogen, and traces of other gases, but methane is the dominant hydrocarbon.

Because methane is the main component, natural gas is often referred to as 'methane-rich' gas.

• Natural gas composition: 70–90% methane
• Other hydrocarbons and gases present in smaller amounts
• Origin: formed from ancient organic matter under geological conditions

Methane is a clean-burning fuel, producing less carbon dioxide per unit of energy released compared to other fossil fuels such as coal or oil. Its combustion reaction is:

CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g) (balanced equation)

This reaction releases a large amount of energy, making methane an efficient fuel for heating and electricity generation. Methane is also less dense than air and is lighter, which affects its handling and storage safety considerations.

• Combustion reaction: CH₄ + 2O₂ → CO₂ + 2H₂O
• Releases significant energy (exothermic)
• Produces less CO₂ per unit energy than coal or oil
• Lighter than air, colourless, odourless (odorants added for safety)

Methane forms naturally through the anaerobic decomposition of organic matter by microorganisms in environments such as swamps, marshes, and sediments. Over geological time, this methane accumulates underground as natural gas deposits.

This fossil fuel is extracted by drilling wells into gas reservoirs. The methane trapped underground is then processed and distributed for commercial use.

Methane is also produced synthetically and biologically, but natural gas remains the primary commercial source.

• Formed by anaerobic decomposition of organic matter
• Accumulates in underground reservoirs as natural gas
• Extracted by drilling and processing
• Primary commercial source of methane

Due to its high energy content and clean combustion, methane-rich natural gas is widely used for domestic heating, cooking, and electricity generation.

It is also used as a fuel in industry and increasingly as a fuel for vehicles in compressed natural gas (CNG) form.

The predominance of methane in natural gas makes it a valuable and versatile energy source worldwide.

• Domestic heating and cooking fuel
• Electricity generation in power stations
• Industrial fuel
• Compressed natural gas (CNG) for vehicles

Hydrocarbons are chemical compounds that consist solely of hydrogen and carbon atoms. These two elements combine in various ways to form molecules that are the foundation of many organic substances.

The general formula of hydrocarbons depends on their type but always includes only carbon (C) and hydrogen (H). For example, methane (CH₄) is the simplest hydrocarbon, containing one carbon atom bonded to four hydrogen atoms.

Because hydrocarbons contain only carbon and hydrogen, they are non-polar molecules and generally insoluble in water but soluble in organic solvents.

• Hydrocarbons contain only carbon and hydrogen atoms.
• They can be represented by general formulas depending on their class.
• They are the simplest organic compounds and form the basis of many fuels.

Hydrocarbons are classified into two main types based on the types of bonds between carbon atoms: saturated and unsaturated.

Saturated hydrocarbons, called alkanes, contain only single bonds between carbon atoms. Their general formula is CₙH₂ₙ₊₂. For example, methane (CH₄), ethane (C₂H₆), and propane (C₃H₈) are alkanes.

Unsaturated hydrocarbons contain one or more double or triple bonds. Alkenes have at least one carbon-carbon double bond (C=C) with the general formula CₙH₂ₙ, such as ethene (C₂H₄). Alkynes contain at least one carbon-carbon triple bond (C≡C), with the general formula CₙH₂ₙ₋₂, such as ethyne (C₂H₂).

• Alkanes: saturated hydrocarbons with single bonds only.
• Alkenes: unsaturated hydrocarbons with one or more double bonds.
• Alkynes: unsaturated hydrocarbons with one or more triple bonds.

Hydrocarbons can be represented by molecular formulae, structural formulae, and displayed formulae.

For example, methane (CH₄) has a molecular formula showing the number of atoms, a structural formula showing bonds: H–C–H with four single bonds, and a displayed formula showing all atoms and bonds explicitly.

Ethene (C₂H₄) has a double bond between the two carbon atoms, represented as H₂C=CH₂ in structural formula, indicating the unsaturation.

• Molecular formula shows the number of each atom.
• Structural formula shows how atoms are bonded.
• Displayed formula shows all atoms and bonds explicitly.

Hydrocarbons are the primary components of many fuels such as natural gas, petrol, diesel, and kerosene.

Their combustion releases energy, making them valuable as energy sources.

The properties of hydrocarbons, such as volatility and viscosity, depend on their chain length and structure, influencing their use as fuels.

• Hydrocarbons are major constituents of fossil fuels.
• Combustion of hydrocarbons releases energy.
• Chain length affects physical properties and fuel applications.

Petroleum, also known as crude oil, is a naturally occurring liquid found beneath the Earth's surface. It is not a pure substance but a complex mixture of many different compounds.

The primary constituents of petroleum are hydrocarbons, which are molecules made exclusively of hydrogen and carbon atoms. These hydrocarbons differ in chain length, structure, and bonding, resulting in a wide variety of compounds within petroleum.

Because petroleum is a mixture, it can be separated into fractions with different properties by physical methods such as fractional distillation.

• Petroleum is a mixture, not a pure compound.
• Mainly composed of hydrocarbons (hydrogen + carbon).
• Contains a range of hydrocarbon types and sizes.

The hydrocarbons in petroleum can be classified into three main types based on their structure: alkanes, cycloalkanes, and aromatic hydrocarbons.

Alkanes are saturated hydrocarbons with single bonds only, having the general formula CₙH₂ₙ₊₂. They can be straight-chain or branched.

Cycloalkanes are saturated hydrocarbons arranged in ring structures, with the general formula CₙH₂ₙ. They have similar chemical properties to alkanes but different physical properties due to their ring shape, which affects boiling points and reactivity slightly compared to straight chains.

• Alkanes: saturated, single bonds, formula CₙH₂ₙ₊₂.
• Cycloalkanes: saturated ring structures, formula CₙH₂ₙ.
• Aromatic hydrocarbons: contain benzene rings with delocalised electrons.

Aromatic hydrocarbons contain one or more benzene rings, which are planar ring structures with alternating double bonds and a system of delocalised electrons.

These compounds have distinct chemical properties, such as undergoing substitution reactions rather than addition reactions typical of alkanes.

Examples include benzene (C₆H₆) and its derivatives, which contribute to the chemical diversity of petroleum and influence its refining and use.

• Contain benzene rings with delocalised π electrons.
• Chemically distinct from alkanes and cycloalkanes.
• Important in chemical industry as feedstocks.

The mixture of hydrocarbons in petroleum allows it to be processed into various useful products, such as fuels (gasoline, diesel, kerosene) and raw materials for petrochemicals.

The different hydrocarbons have varying boiling points and chemical properties, which enable separation by fractional distillation and further chemical processing.

Understanding that petroleum is a mixture of hydrocarbons is fundamental to studying its refining and applications in industry.

• Mixture enables separation into fractions by boiling point.
• Hydrocarbon variety determines fuel properties.
• Basis for refining and chemical production.

Petroleum, also known as crude oil, is a complex mixture of hydrocarbons with varying chain lengths and molecular masses. These hydrocarbons have different boiling points, which allows them to be separated by fractional distillation into useful fractions. Each fraction contains hydrocarbons with similar boiling points and properties, making them suitable for specific applications.

• Petroleum is a mixture of hydrocarbons with different boiling points.
• Fractional distillation separates these hydrocarbons into fractions.
• Each fraction has a range of boiling points and uses.

Fractional distillation separates components of a liquid mixture by heating it to vaporise the components and then cooling the vapour to condense them at different temperatures. Hydrocarbons with lower boiling points vaporise first and condense higher up the fractionating column where the temperature is lower. Those with higher boiling points condense lower down where the temperature is higher. This separation is possible because boiling point depends on molecular size and intermolecular forces.

The process relies on the continuous temperature gradient in the fractionating column, which is hottest at the bottom and coolest at the top. Vapours rise through trays or packing material, undergoing repeated condensation and vaporisation, which improves separation efficiency.

• Components vaporise according to boiling points.
• Temperature decreases up the column.
• Vapours condense at different heights.
• Repeated condensation and vaporisation improve separation.

Crude oil is first heated strongly in a furnace until it vaporises. The hot vapour enters the base of the fractionating column. The column contains trays or packing to increase surface area for vapour-liquid contact. As vapour rises, it cools and hydrocarbons with higher boiling points condense on trays lower down and are collected as liquids. Hydrocarbons with lower boiling points reach higher trays before condensing.

The temperature gradient is maintained by heating at the base and cooling at the top. The fractions are drawn off at different heights corresponding to their boiling point ranges.

• Crude oil is heated to vaporise hydrocarbons.
• Fractionating column has a temperature gradient (hot bottom, cool top).
• Trays or packing allow vapour-liquid contact.
• Fractions condense and are collected at different heights.

The main fractions obtained from petroleum fractional distillation, listed from the top (lowest boiling point) to the bottom (highest boiling point) of the column, include:

Refinery gases (boiling point below 20°C) used for heating and cooking.

Petrol (gasoline) (boiling point 40–205°C) used as fuel for vehicles (cars). Naphtha (boiling point 60–120°C) used as a chemical feedstock for producing plastics and chemicals. Kerosene (boiling point 150–275°C) used as jet fuel and for heating. Diesel oil (boiling point 200–350°C) used as fuel for diesel engines. Fuel oil (boiling point 250–400°C) used for ships and industrial heating. Lubricating oil (boiling point above 350°C) used for lubricants and polishes. Bitumen (residue) used for road surfacing and roofing.

• Refinery gases: <20°C, heating and cooking.
• Petrol: 40–205°C, vehicle fuel.
• Naphtha: 60–120°C, chemical feedstock.
• Kerosene: 150–275°C, jet fuel.
• Diesel oil: 200–350°C, diesel engines.
• Fuel oil: 250–400°C, ships and heating.
• Lubricating oil: >350°C, lubricants.
• Bitumen: residue, road construction.

The fractional distillation of petroleum is a physical separation technique that does not involve chemical change. It depends on the differences in boiling points of hydrocarbons. The process allows the efficient extraction of useful fuels and raw materials from crude oil. Each fraction collected has characteristic properties such as viscosity, volatility, and chain length that determine its use.

The process is continuous in large-scale refineries, with crude oil fed continuously and fractions drawn off simultaneously.

• Physical separation based on boiling points.
• No chemical change occurs.
• Fractions have distinct properties and uses.
• Process is continuous in industry.

Petroleum is a mixture of hydrocarbons with varying chain lengths and molecular masses. Fractional distillation separates these hydrocarbons by exploiting differences in their boiling points.

The petroleum mixture is heated until it vaporises and enters a fractionating column. The column is hotter at the bottom and cooler at the top. Hydrocarbon vapours rise and condense at different heights depending on their boiling points, forming distinct fractions.

Fractions collected near the bottom contain hydrocarbons with higher boiling points, while those near the top have lower boiling points.

• Petroleum is heated to vaporise it.
• Vapours rise through the fractionating column.
• Temperature gradient causes condensation at different heights.
• Fractions collected correspond to hydrocarbons with similar boiling points.

Hydrocarbons in petroleum vary from short-chain molecules (e.g., methane, ethane) to very long chains (e.g., heavy oils).

As you move from the bottom to the top of the fractionating column, the average chain length of hydrocarbons decreases.

Longer chains have stronger intermolecular forces and higher boiling points, so they condense lower in the column. Shorter chains have weaker intermolecular forces and lower boiling points, so they condense higher up.

• Bottom fractions: long-chain hydrocarbons (e.g., C20+).
• Top fractions: short-chain hydrocarbons (e.g., C4–C10).
• Chain length inversely proportional to height in column.

Volatility is the tendency of a substance to vaporise. Shorter-chain hydrocarbons are more volatile because they have weaker London dispersion forces.

Boiling point is the temperature at which a liquid turns to vapour. Longer chains have higher boiling points due to stronger intermolecular forces.

Therefore, as chain length decreases up the column, volatility increases and boiling points decrease. This explains why lighter fractions are collected near the top.

• Volatility increases from bottom to top.
• Boiling points decrease from bottom to top.
• Examples: bitumen boils above 350°C; gasoline boils between 40–205°C.

Viscosity measures a fluid's resistance to flow. Longer-chain hydrocarbons have higher viscosity because their molecules entangle and interact more.

Fractions at the bottom of the column (heavy oils, bitumen) are thick and viscous liquids or solids at room temperature.

Fractions at the top (refinery gases, gasoline) are less viscous, flowing easily as liquids or existing as gases.

• Viscosity decreases from bottom to top.
• Heavy fractions are thick and sticky.
• Light fractions are thin and runny or gaseous.

The systematic changes in properties of petroleum fractions are interrelated and result from molecular structure differences.

These changes determine the practical uses of each fraction, such as fuel type and handling requirements.

Understanding these trends is essential for refining and utilising petroleum products effectively.

• Chain length decreases upwards.
• Volatility increases upwards.
• Boiling point decreases upwards.
• Viscosity decreases upwards.

Petroleum is a naturally occurring mixture of hydrocarbons that is separated into fractions by fractional distillation. Each fraction contains hydrocarbons with similar chain lengths and boiling points, resulting in different physical properties and uses. Understanding the applications of these fractions is essential for appreciating their role in energy production, transportation, manufacturing, and construction.

Refinery gas is the lightest fraction obtained from petroleum, consisting mainly of hydrocarbons with 1 to 4 carbon atoms, such as methane (CH₄), ethane (C₂H₆), propane (C₃H₈), and butane (C₄H₁₀). It is highly volatile and easily ignited, making it suitable as a fuel for heating and cooking.

Refinery gas is commonly supplied as liquefied petroleum gas (LPG) for domestic use. Its combustion releases energy efficiently, producing carbon dioxide and water:

CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g) (balanced combustion equation for methane)

• Used as LPG for heating and cooking.
• Burns with a clean flame, producing heat efficiently.
• Stored under pressure as a liquid for ease of transport.

Gasoline, also called petrol, is a fraction containing hydrocarbons with approximately 5 to 12 carbon atoms. It is a volatile liquid with a relatively low boiling point and is the primary fuel for spark-ignition internal combustion engines in cars and motorcycles.

Gasoline vapour mixes with air and ignites in the engine cylinders, releasing energy to power the vehicle. Its high volatility ensures easy vaporisation for efficient combustion.

Typical hydrocarbons in gasoline include branched alkanes and cycloalkanes, which improve engine performance and reduce knocking.

• Used as fuel in petrol engines of vehicles.
• Volatile and easily vaporised for combustion.
• Contains hydrocarbons with 5–12 carbon atoms.

Naphtha is a fraction with hydrocarbons typically containing 6 to 12 carbon atoms, overlapping with gasoline but distinguished by its use rather than fuel properties.

It serves as a chemical feedstock in the petrochemical industry, where it is cracked to produce smaller molecules such as ethene and propene. These smaller alkenes are the building blocks for polymers and other chemicals.

Naphtha is not commonly used as a fuel due to its chemical composition and value as a raw material.

• Used as a feedstock for producing chemicals and plastics.
• Subjected to cracking to yield smaller alkenes.
• Not primarily used as a fuel.

Kerosene consists of hydrocarbons with about 10 to 16 carbon atoms. It is a clear, less volatile liquid than gasoline, with a higher boiling point range.

Kerosene is widely used as jet fuel due to its suitable combustion properties and energy density. It is also used for heating and lighting in some regions.

Its combustion is cleaner than heavier fractions, producing sufficient energy for jet engines and heaters.

• Used as jet fuel in aviation.
• Employed for heating and lighting.
• Hydrocarbons with 10–16 carbon atoms.

Diesel oil contains hydrocarbons with approximately 14 to 20 carbon atoms. It is less volatile than kerosene and gasoline, with a higher boiling point and viscosity.

Diesel oil is used as fuel in compression-ignition engines, commonly found in trucks, buses, and some cars. Diesel engines rely on the fuel's ignition by compression rather than spark.

Its higher energy content per litre compared to gasoline makes it efficient for heavy-duty transport.

• Fuel for diesel engines in vehicles and machinery.
• Higher boiling point and viscosity than gasoline and kerosene.
• Hydrocarbons with 14–20 carbon atoms.