Fuels Revision Notes

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

This process results in three main types of fossil fuels: coal, natural gas, and petroleum. Each differs in composition, physical state, and energy content, reflecting the original organic material and formation conditions.

• Originates from ancient biological material.
• Transformation by heat and pressure over millions of years.
• Results in solid, liquid, and gaseous fuels.

Coal is a black or brownish-black sedimentary rock primarily composed of carbon. It forms mainly from ancient plant material in swampy environments where decay was incomplete due to low oxygen levels.

Coal is predominantly used as a solid fuel for electricity generation in power stations and for industrial heating. Its combustion releases energy but also produces pollutants such as sulfur dioxide and carbon dioxide.

Example: A coal-fired power plant burns coal to heat water, producing steam that drives turbines to generate electricity.

• Solid fossil fuel, mainly carbon.
• Formed from plant remains in swampy areas.
• Used mainly for electricity generation and industrial heating.

Natural gas is primarily composed of methane (CH₄), a simple hydrocarbon gas. It forms from the decay of marine microorganisms under anaerobic conditions, often found alongside petroleum deposits.

It is a cleaner-burning fuel compared to coal, used for domestic heating, cooking, and electricity generation. Its gaseous state allows easy transport via pipelines.

Example: Natural gas is commonly used in homes for heating and cooking due to its high energy content and cleaner combustion.

• Gaseous fossil fuel, mainly methane.
• Originates from marine microorganism decay.
• Used for heating, cooking, and electricity generation.

Petroleum, or crude oil, is a complex mixture of hydrocarbons in liquid form. It originates from the remains of tiny marine organisms deposited on the sea floor and transformed under heat and pressure.

Petroleum is refined into various fractions such as gasoline, diesel, and kerosene, which serve as fuels for transportation and as raw materials in the chemical industry.

Example: Gasoline derived from petroleum powers most cars, while diesel fuels trucks and some trains.

• Liquid fossil fuel, mixture of hydrocarbons.
• Formed from marine organism remains.
• Refined into fuels for transport and chemical feedstocks.

Fossil fuels are currently the dominant source of global energy due to their high energy density and established infrastructure.

However, their combustion releases greenhouse gases contributing to climate change and pollutants affecting air quality.

Understanding the types and uses of fossil fuels is essential for managing energy resources and developing sustainable alternatives.

• Major energy source worldwide.
• Combustion releases CO₂ and pollutants.
• Necessitates consideration of environmental impact and alternatives.

Methane (CH₄) is the simplest alkane, consisting of one carbon atom bonded to four hydrogen atoms in a tetrahedral arrangement. This structure makes methane a saturated hydrocarbon, meaning it contains only single bonds between carbon and hydrogen atoms. Methane is a colourless, odourless gas under standard conditions and is less dense than air. Its chemical formula is CH₄, and it is non-polar and relatively unreactive under normal conditions, which contributes to its stability as a fuel.

• Simplest alkane with formula CH₄.
• Tetrahedral molecular shape with four C–H single bonds.
• Colourless and odourless gas at room temperature.
• Saturated hydrocarbon with single bonds only.

Natural gas is a fossil fuel formed over millions of years from the decomposition of organic matter, such as dead plants and marine organisms, buried under sedimentary rock layers. Under high pressure and temperature, this organic material breaks down into hydrocarbons, primarily methane. Natural gas extracted from underground reservoirs typically contains about 70–90% methane, with smaller amounts of other hydrocarbons like ethane, propane, and butane, as well as impurities such as carbon dioxide and nitrogen.

• Formed from organic matter decomposed under high pressure and temperature.
• Mainly composed of methane (70–90%).
• Contains minor hydrocarbons and impurities.
• Extracted from underground reservoirs.

Methane-rich natural gas is a versatile energy source used for heating homes, cooking, and generating electricity. It burns more cleanly than coal or oil, producing mainly carbon dioxide and water vapour with fewer pollutants such as sulfur dioxide or particulates. For example, methane combustion in a gas stove provides efficient heat for cooking with minimal smoke. Additionally, natural gas is used as a fuel in some vehicles and as a raw material in chemical industries to produce hydrogen, methanol, and other compounds.

• Used for heating, cooking, and electricity generation.
• Burns cleaner than coal and oil, producing less pollution.
• Fuel for some vehicles and chemical feedstock.
• Example: methane gas stove for cooking.

Although methane burns cleaner than other fossil fuels, it is a potent greenhouse gas when released unburned into the atmosphere. Methane has a global warming potential approximately 25 times greater than carbon dioxide over a 100-year period. Leaks during extraction, transport, or storage of natural gas contribute to atmospheric methane levels. Therefore, controlling methane emissions is important for mitigating climate change. However, using methane as a fuel instead of coal or oil reduces overall carbon emissions due to its higher hydrogen-to-carbon ratio and cleaner combustion.

• Methane is a potent greenhouse gas with high global warming potential.
• Leaks during natural gas handling increase atmospheric methane.
• Burning methane produces less CO₂ than coal or oil per unit energy.
• Reducing methane emissions is crucial for climate change mitigation.

Hydrocarbons are chemical compounds made up solely of hydrogen and carbon atoms. This exclusive composition means no other elements are present in their molecular structure. The carbon atoms form the backbone of the molecule, while hydrogen atoms attach to the carbon atoms to satisfy their bonding requirements.

Because hydrocarbons contain only two elements, their chemical formulas are straightforward, such as CH4 for methane or C2H6 for ethane. This simplicity allows for a wide variety of structures, from simple molecules to complex chains and rings.

• Hydrocarbons contain only carbon (C) and hydrogen (H).
• They can be represented by general formulas depending on their type, e.g., alkanes have the formula CnH2n+2.

Hydrocarbons exist in different physical states depending on their molecular size and structure. Small hydrocarbons like methane (CH4) and ethane (C2H6) are gases at room temperature. Medium-sized hydrocarbons such as octane (C8H18) are liquids, while very large hydrocarbons can be solids.

They are the primary components of fossil fuels such as natural gas, petroleum (crude oil), and coal. These fuels are mixtures of various hydrocarbons, which can be separated and used for energy and chemical feedstocks.

• Small hydrocarbons tend to be gases; larger ones are liquids or solids.
• Fossil fuels are complex mixtures of hydrocarbons formed from ancient organic matter.

Hydrocarbons are classified based on the types of bonds between carbon atoms. Saturated hydrocarbons, known as alkanes, have only single bonds between carbon atoms. Unsaturated hydrocarbons contain one or more double or triple bonds and are called alkenes and alkynes, respectively.

For example, ethane (C2H6) is an alkane with a single bond, while ethene (C2H4) is an alkene with a double bond. The presence of double or triple bonds affects the chemical reactivity and physical properties of the hydrocarbons.

• Alkanes: only single C–C bonds, saturated hydrocarbons.
• Alkenes: contain at least one C=C double bond, unsaturated.
• Alkynes: contain at least one C≡C triple bond, unsaturated.

Hydrocarbons are crucial as energy sources because their combustion releases significant amounts of energy. For example, methane, the simplest hydrocarbon, burns to produce carbon dioxide and water, releasing heat used for cooking and heating.

Beyond fuels, hydrocarbons serve as raw materials in the chemical industry to produce plastics, solvents, and other chemicals. Their structure allows for chemical modifications to create a wide range of useful products.

• Combustion of hydrocarbons releases energy used in heating, transport, and electricity.
• Hydrocarbons are feedstocks for producing polymers and chemicals.

Petroleum, often called crude oil, is a naturally occurring liquid found underground. It is not a single chemical compound but a complex mixture of many different substances. The main components of petroleum are hydrocarbons, which are molecules made up exclusively of hydrogen and carbon atoms. These hydrocarbons vary in size, structure, and properties, making petroleum a diverse and complex material.

• Petroleum is a natural liquid mixture found underground.
• It contains many different molecules, not just one compound.
• Hydrocarbons are the primary constituents of petroleum.

Hydrocarbons are organic compounds consisting only of hydrogen and carbon atoms. In petroleum, these hydrocarbons can be classified into three main types: alkanes, cycloalkanes, and aromatic hydrocarbons. Alkanes are saturated hydrocarbons with single bonds, cycloalkanes have carbon atoms arranged in rings, and aromatic hydrocarbons contain one or more benzene rings, which are stable ring-shaped molecules with alternating double bonds.

• Alkanes: straight or branched chains with single bonds (e.g., methane, ethane).
• Cycloalkanes: ring-shaped saturated hydrocarbons (e.g., cyclohexane).
• Aromatic hydrocarbons: contain benzene rings (e.g., benzene, toluene).

The exact mixture of hydrocarbons in petroleum varies depending on its geographical source and formation conditions. Some petroleum samples may have a higher proportion of lighter hydrocarbons like methane and ethane, while others contain more heavy hydrocarbons with longer carbon chains. This variability affects the properties of the crude oil, such as viscosity, boiling point range, and suitability for different uses.

• Composition depends on the source of petroleum.
• Light hydrocarbons have shorter carbon chains and lower boiling points.
• Heavy hydrocarbons have longer chains and higher boiling points.
• Variability influences refining and end-use applications.

Because petroleum is a mixture, it can be separated into useful fractions by fractional distillation. Each fraction contains hydrocarbons with similar chain lengths and boiling points, which can be used for different purposes such as fuels, lubricants, and chemical feedstocks. Understanding that petroleum is a mixture of hydrocarbons is fundamental to the petroleum industry and the production of fuels and materials.

• Fractional distillation separates petroleum into fractions.
• Each fraction contains hydrocarbons with similar properties.
• Fractions include gases, petrol, diesel, and bitumen.
• This separation relies on petroleum’s nature as a mixture.

Crude oil is a complex mixture of hydrocarbons—compounds made only of hydrogen and carbon atoms. These hydrocarbons vary in chain length and structure, resulting in different physical properties such as boiling points. Because crude oil is not useful in its raw form, it must be separated into simpler mixtures called fractions, each containing hydrocarbons with similar boiling points and chain lengths.

• Crude oil contains many hydrocarbons with varying chain lengths.
• Hydrocarbons with similar boiling points group into fractions.
• Separating crude oil allows extraction of useful products.

Fractional distillation separates a mixture into components by exploiting differences in boiling points. When crude oil is heated, it vaporises. The vapour rises through a tall fractionating column that is cooler at the top and hotter at the bottom. As the vapour ascends, it cools. Hydrocarbons with higher boiling points condense back into liquid lower in the column, while those with lower boiling points travel higher before condensing. This creates distinct layers or fractions that can be drawn off separately.

• Heating vaporises crude oil into a mixture of gases.
• Temperature gradient in the column causes condensation at different heights.
• Fractions separate by boiling point, from heavy (bottom) to light (top).

The fractionating column is a tall vertical tower filled with trays or plates. These trays provide surfaces for vapour to condense and re-evaporate, improving separation. The bottom of the column is heated to vaporise the crude oil. As vapour rises, the temperature decreases. Hydrocarbons with the highest boiling points condense first on lower trays, while lighter hydrocarbons reach higher trays before condensing. The separated liquids are collected from outlets at different heights.

• Column contains trays to increase surface area for condensation.
• Temperature decreases from bottom to top.
• Liquids collected at different heights correspond to different fractions.

Hydrocarbons with longer carbon chains have stronger intermolecular forces, requiring more energy (higher temperature) to vaporise. Therefore, they have higher boiling points and condense lower in the column. Shorter chain hydrocarbons have lower boiling points and condense near the top. For example, gases like methane and propane are collected at the top, while heavy oils and bitumen remain at the bottom.

• Longer chains → higher boiling points → condense lower.
• Shorter chains → lower boiling points → condense higher.
• This pattern allows predictable separation of fractions.

Each fraction contains hydrocarbons suited for specific uses based on their properties. For instance, refinery gas (top fraction) is used for heating and cooking. Petrol (gasoline) fuels vehicles. Diesel oil powers diesel engines. Bitumen (bottom fraction) is used in road construction. Understanding the separation process helps industries produce fuels and materials tailored to needs.

• Refinery gas: heating and cooking.
• Petrol: fuel for cars.
• Diesel oil: fuel for diesel engines.
• Bitumen: road surfacing and roofing.

Petroleum is a complex mixture of hydrocarbons that can be separated into different fractions by fractional distillation. Each fraction contains hydrocarbons with similar chain lengths and boiling points, which determine their physical properties and suitability for various applications. Understanding these uses helps explain why petroleum is a vital resource in energy, transport, industry, and construction.

• Fractional distillation separates petroleum into fractions based on boiling points.
• Each fraction has characteristic chain lengths and properties.
• Uses depend on physical and chemical properties of each fraction.

Refinery gas is the lightest fraction obtained from petroleum, consisting mainly of gases like methane, ethane, propane, and butane. Because these gases are easily combustible and have low boiling points, they are ideal for domestic heating and cooking applications. For example, bottled LPG (liquefied petroleum gas) used in homes is mainly propane and butane from refinery gas.

Refinery gas burns cleanly and efficiently, making it a convenient fuel for stoves and heaters. Its gaseous state at room temperature allows easy transport and storage under pressure in cylinders.

• Contains light hydrocarbons such as propane and butane.
• Used as liquefied petroleum gas (LPG) for cooking and heating.
• Burns efficiently with a clean flame.
• Stored under pressure in cylinders for domestic use.

Gasoline, also called petrol, is a volatile liquid fraction with hydrocarbons typically containing 5 to 10 carbon atoms per molecule. It has a relatively low boiling point and high volatility, which makes it suitable for use as a fuel in spark-ignition internal combustion engines found in most cars and motorcycles.

Gasoline vaporises easily to mix with air and combust efficiently in engines, providing the energy needed to power vehicles. Its controlled combustion produces mechanical work, propelling vehicles forward. Additives are often included to improve performance and reduce engine knocking.

• Hydrocarbons with 5–10 carbon atoms.
• Used as fuel in petrol engines of vehicles.
• Volatile and easily vaporises for combustion.
• Provides energy to power spark-ignition engines.

Naphtha is a fraction with hydrocarbons generally containing 7 to 11 carbon atoms. Unlike gasoline, naphtha is primarily used as a chemical feedstock rather than a direct fuel. It serves as a raw material in the petrochemical industry for producing chemicals such as solvents, plastics, and synthetic fibres.

Through processes like cracking and reforming, naphtha is converted into smaller molecules or aromatic compounds that form the building blocks for polymers and other industrial chemicals. This distinguishes naphtha from other fractions used mainly for combustion or construction.

• Hydrocarbons with about 7–11 carbon atoms.
• Used as a raw material in chemical manufacturing.
• Feeds petrochemical processes to produce plastics and solvents.
• Not primarily used as a fuel.

Diesel oil is a heavier fraction than gasoline, containing hydrocarbons with approximately 12 to 20 carbon atoms. It has a higher boiling point and is less volatile than gasoline, making it suitable for compression-ignition engines, commonly called diesel engines.

Diesel engines compress air to a high temperature before fuel injection, causing diesel oil to ignite spontaneously. This makes diesel oil an efficient fuel for trucks, buses, and some cars, especially where high torque and fuel economy are required.

• Hydrocarbons with 12–20 carbon atoms.
• Used as fuel in diesel engines.
• Less volatile and higher boiling point than gasoline.
• Ignites under compression in diesel engines.

Bitumen is the heaviest fraction obtained from petroleum, consisting of very long-chain hydrocarbons with over 35 carbon atoms. It is a thick, sticky, black substance that is solid or semi-solid at room temperature.

Bitumen is primarily used in road construction as a binder mixed with aggregate to form asphalt. Its waterproofing and adhesive properties make it ideal for creating durable road surfaces that withstand traffic and weather conditions. Bitumen is also used in roofing and waterproofing materials.

• Very long-chain hydrocarbons (over 35 carbons).
• Thick, sticky, black substance at room temperature.
• Used as a binder in road construction (asphalt).
• Provides waterproofing and durability to roads and roofs.

Petroleum is a complex mixture of hydrocarbons with varying chain lengths and boiling points. To separate these into useful fractions, fractional distillation is employed. This process uses a fractionating column where petroleum is heated, and components separate based on their boiling points.

Inside the fractionating column, temperature decreases from bottom to top. Fractions with higher boiling points condense lower down, while those with lower boiling points rise higher before condensing. This arrangement allows the collection of fractions with distinct properties at different heights.

• Petroleum is a mixture of hydrocarbons with different chain lengths.
• Fractional distillation separates these based on boiling points.
• Temperature gradient exists in the fractionating column: hottest at bottom, coolest at top.

Hydrocarbons in petroleum fractions vary in the number of carbon atoms they contain, known as chain length. At the bottom of the fractionating column, fractions contain hydrocarbons with longer carbon chains, often more than 20 carbon atoms. These long chains form heavier molecules.

As you move up the column, the chain length decreases. Near the top, fractions consist of hydrocarbons with short chains, sometimes as few as 1 to 4 carbon atoms. Shorter chains produce lighter molecules.

This variation occurs because longer chains have higher boiling points and condense lower in the column, while shorter chains have lower boiling points and rise higher before condensing.

• Bottom fractions have long hydrocarbon chains (e.g., >20 carbons).
• Top fractions have short hydrocarbon chains (e.g., 1–4 carbons).
• Chain length influences molecular weight and physical properties.

Boiling point is the temperature at which a liquid turns into vapour. In petroleum fractions, boiling points decrease from bottom to top of the fractionating column.

Longer hydrocarbon chains have stronger intermolecular forces (van der Waals forces), requiring more energy (higher temperature) to separate molecules into vapour. Hence, these fractions have higher boiling points and condense near the bottom.

Shorter chains have weaker intermolecular forces, so they boil at lower temperatures and condense near the top. For example, methane (CH4) boils at −161 °C, while heavy hydrocarbons boil above 300 °C.

• Boiling points decrease from bottom to top of the column.
• Stronger intermolecular forces in longer chains increase boiling points.
• Short chains boil at lower temperatures due to weaker intermolecular forces.

The key reason for the variation in boiling points is the strength of intermolecular forces between hydrocarbon molecules. Longer chains have larger surface areas, leading to stronger London dispersion forces.

Stronger intermolecular forces mean more heat energy is needed to overcome attractions and convert liquid to vapour, resulting in higher boiling points.

Conversely, shorter chains have smaller surface areas and weaker forces, so they vaporise at lower temperatures. This explains why fractions with shorter chains are found at the top of the column.

• Longer chains → stronger London dispersion forces → higher boiling points.
• Shorter chains → weaker forces → lower boiling points.
• Intermolecular forces govern boiling point differences in fractions.

Understanding these variations helps in selecting appropriate fractions for different uses. For instance, gasoline, used as fuel for cars, is collected near the top and contains short-chain hydrocarbons with low boiling points.

Diesel oil, used in diesel engines, is collected lower down and contains longer chains with higher boiling points.

Bitumen, used for road surfacing, is found at the bottom with very long chains and very high boiling points, making it thick and viscous at room temperature.

• Gasoline: short chains, low boiling point, collected near top.
• Diesel: medium-long chains, higher boiling point, collected mid-column.
• Bitumen: very long chains, very high boiling point, collected at bottom.