Crude oil, hydrocarbons and alkanes — AQA GCSE Chemistry
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Crude oil, hydrocarbons and alkanes explained
Crude oil, hydrocarbons and alkanes covers 4 specification points, including Crude oil is a finite resource found in rocks.… and Crude oil is a mixture of a very large number….
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Open a point below for its explanation and revision guidance.
Crude oil is a finite resource found in rocks. Crude oil is the remains of an ancient biomass consisting mainly of plankton that was buried in mud.
Crude oil is a mixture of hydrocarbons found in rocks and is described as a finite resource because it is being used up faster than it forms and cannot be replaced on a human timescale. It formed over millions of years from the remains of ancient biomass, mainly plankton, that was buried in mud. Heat and pressure over geological time converted this biomass into crude oil. Because formation takes millions of years, once extracted and burned, crude oil is effectively gone. This matters because fuels and petrochemical feedstocks depend on it, so alternatives and conservation are important. For example, petrol, diesel and kerosene are fractions obtained from crude oil, and their supply is limited.
Crude oil is a mixture of a very large number of compounds.
Crude oil is a naturally occurring, dark, viscous liquid found in porous rocks underground. It is not a single substance but a mixture of a very large number of different compounds, mostly hydrocarbons, which are not chemically combined and can be separated by physical means such as fractional distillation. Because the components have different boiling points, they can be separated without chemical reaction. For example, when crude oil is heated, the compounds with lower boiling points rise as vapour and condense at different heights in a fractionating column, producing fractions such as petrol and diesel. The mixture has no fixed boiling point and its composition varies depending on its source.
Most of the compounds in crude oil are hydrocarbons, which are molecules made up of hydrogen and carbon atoms only.
A hydrocarbon is a molecule that contains only hydrogen and carbon atoms. In crude oil, the vast majority of the compounds present are hydrocarbons, although small amounts of other compounds containing sulfur, nitrogen or oxygen may also be found. The carbon atoms in a hydrocarbon bond to each other to form chains or rings, and hydrogen atoms are attached to the carbon framework. For example, methane (CH₄) contains one carbon atom bonded to four hydrogen atoms, and ethane (C₂H₆) contains two carbon atoms joined by a single bond with six hydrogen atoms attached. Because hydrocarbons contain only carbon and hydrogen, they burn in oxygen to produce carbon dioxide and water.
Most of the hydrocarbons in crude oil are hydrocarbons called alkanes.
Alkanes are a family of hydrocarbons that contain only single covalent bonds between carbon atoms. They are described as saturated hydrocarbons because each carbon atom is bonded to the maximum number of hydrogen atoms possible. Most of the hydrocarbons found in crude oil are alkanes. The general formula for alkanes is CₙH₂ₙ₊₂, where n is the number of carbon atoms. For example, methane has one carbon atom and four hydrogen atoms (CH₄), ethane has two carbon atoms and six hydrogen atoms (C₂H₆), propane has three carbon atoms and eight hydrogen atoms (C₃H₈), and butane has four carbon atoms and ten hydrogen atoms (C₄H₁₀). Alkanes are relatively unreactive but burn well, making them useful as fuels.
Your focus
- Define crude oil as a finite resource found in rocks.
- Describe the origin of crude oil from ancient biomass, mainly plankton, buried in mud.
- Explain why crude oil is described as finite in terms of its formation time.
Show all 12 objectives
- Define crude oil as a mixture of a very large number of compounds.
- Describe how the components of crude oil can be separated by fractional distillation.
- Explain why crude oil does not have a fixed boiling point.
- Define a hydrocarbon as a molecule containing only carbon and hydrogen atoms.
- Identify hydrocarbons from their molecular formulae.
- Describe the products formed when hydrocarbons undergo complete combustion.
- Define alkanes as saturated hydrocarbons containing only single covalent bonds.
- Use the general formula CₙH₂ₙ₊₂ to determine the molecular formula of an alkane.
- Name and write the molecular formulae of methane, ethane, propane and butane.
Crude oil, hydrocarbons and alkanes exam tips
Marking Points
- State that crude oil is a finite resource, meaning it is being used up faster than it is formed.
- State that crude oil is found in rocks.
- Describe crude oil as the remains of ancient biomass.
- Identify plankton as the main component of the biomass from which crude oil formed.
- Explain that the biomass was buried in mud and subjected to heat and pressure over millions of years.
- Link the finite nature of crude oil to the need for conservation or alternative energy sources.
- Crude oil is a mixture, not a compound or a pure substance.
- A mixture contains two or more substances that are not chemically combined.
- Crude oil contains a very large number of different compounds.
- The compounds in crude oil can be separated by physical processes such as fractional distillation.
- Because the components have different boiling points, they can be separated without chemical reaction.
- The composition of crude oil varies depending on where it is extracted.
- A hydrocarbon is a molecule made up of hydrogen and carbon atoms only.
- Most of the compounds in crude oil are hydrocarbons.
- Carbon atoms bond to each other to form chains or rings in hydrocarbon molecules.
- Hydrogen atoms are attached to the carbon framework.
- Examples include methane (CH₄) and ethane (C₂H₆).
- Complete combustion of hydrocarbons produces carbon dioxide and water.
- Alkanes are hydrocarbons that contain only single covalent bonds between carbon atoms.
- Alkanes are described as saturated hydrocarbons.
- Most of the hydrocarbons in crude oil are alkanes.
- The general formula for alkanes is CₙH₂ₙ₊₂.
- Examples include methane (CH₄), ethane (C₂H₆), propane (C₃H₈) and butane (C₄H₁₀).
- Alkanes burn well and are used as fuels.
Examiner Tips
- 💡Use the word 'finite' and explain it in terms of rate of use compared with rate of formation.
- 💡Name plankton as the main source of the biomass when describing the origin of crude oil.
- 💡Mention burial in mud and the action of heat and pressure over millions of years.
- 💡Define 'mixture' clearly before applying it to crude oil.
- 💡Link the separation method to the physical property of boiling point.
- 💡Use the phrase 'very large number of compounds' to match the specification wording.
- 💡Use the word 'only' when defining a hydrocarbon to emphasise the absence of other elements.
- 💡Give a named example such as methane to support your definition.
- 💡Link the composition of hydrocarbons to the products of their complete combustion.
- 💡Use the term 'saturated' and explain that it means only single bonds are present.
- 💡Apply the general formula CₙH₂ₙ₊₂ to work out the formula of an alkane from its carbon number.
- 💡Name the first four alkanes correctly: methane, ethane, propane and butane.
Common Mistakes
- Describing crude oil as a single compound; the correction is that it is a mixture of hydrocarbons.
- Saying crude oil formed from dinosaurs; the correction is that it formed mainly from plankton.
- Stating that crude oil is renewable because it is still forming; the correction is that it is finite because formation takes millions of years.
- Describing crude oil as a compound or element; correct this by stating it is a mixture of many compounds.
- Stating that crude oil has a fixed boiling point; correct this by explaining that a mixture boils over a range of temperatures.
- Claiming that fractional distillation involves chemical reactions; correct this by stating it is a physical separation based on boiling points.
- Stating that hydrocarbons contain carbon and hydrogen and other elements; correct this by emphasising 'only' carbon and hydrogen.
- Confusing hydrocarbons with carbohydrates; correct this by noting carbohydrates contain carbon, hydrogen and oxygen.
- Writing molecular formulae with baseline numbers; correct this by using subscript notation such as CH₄ and C₂H₆.
- Describing alkanes as unsaturated; correct this by stating they are saturated because they contain only single bonds.
- Applying the general formula CₙH₂ₙ to alkanes; correct this by using CₙH₂ₙ₊₂ for alkanes.
- Stating that alkanes contain double bonds; correct this by explaining that alkanes have only single covalent bonds.